There's little information about rebuilding these transmissions out there on the internet so I figured I'd take a bunch of pictures and post them for anyone who cares. We're getting to the point that some of these cars are getting up there in miles and might need it sooner or later, and some people just want better gearing so I hope this information helps someone else. Honda manual transmissions from the 90s are all fairly similar, this one is just a bit bigger and heavier and more annoying.
This thread is intended to be used as a reference in addition to the diagrams and instructions found in the factory service manual. If you don't have one, go download it and read the chapter on tearing down the transmission.
In an ideal world you'd have access to a big press, various pullers, and other special tools. But I don't have that stuff and most DIYers don't either. Part of being a good DIY garage hack with limited tools is knowing when you've reached the limits of your capability and to call a friend/pro/machine shop/etc. If something is moving like you think it should, stop and reevaluate, whipping out the sledge should never cross your mind.
Also, if you're having issues with your transmission, change the fluid before you rip it open. Old fluid can cause all sorts of strange issues and you might get lucky.
Parts List
Below is a list of parts one might use in a "typical" transmission rebuild. This is a good baseline for what to buy, but do your own research and make sure you order everything you need, there is a very good chance you will find other things that need to be replaced and find things that you probably didn't need to replace. Examples include clearance adjustment shims, needle bearings, worn hub/sleeve (especially on 5th), etc. I've found that parts are much cheaper if you order from Japanese suppliers like Amayama, but for some things such as clearance shims that you can't predict ahead of time I find it easiest to order them from local Acura or Honda dealerships since they can usually get them in much faster than you could order them and don't cost all that much more.
To minimize the cost of a rebuild, if you have a grinding issue, stop using that gear immediately or figure out how to make it not grind when you select it. The more the gear grinds, the more likely you'll have to replace parts that aren't normal wear items.
If you find anything strange with the part numbers or list items, please let me know, I'm only human and there is a non-zero chance that I might have screwed up when copying stuff over from my spreadsheet.
Synchronizers
23641-PR8-010 qty 1 1st gear synchro
23646-PR8-020 qty 1 2nd gear synchro
23641-PR8-911 qty 2 3rd/4th gear synchro
23642-PG1-912 qty 1 5th gear synchro
Bearings
91002-PR8-008 qty 1 Mainshaft bearing (bottom)
91004-PR8-008 qty 1 Mainshaft bearing (top)
91103-PR8-018 qty 1 Countershaft bearing (bottom, the one with no inner race)
91102-PR8-018 qty 1 Countershaft bearing (middle)
91003-PR8-008 qty 1 Countershaft bearing (top, the one with the snap ring groove)
91105-PR8-008 qty 1 1st gear needle bearing
91108-PR8-008 qty 1 2nd gear needle bearing
91104-PR8-008 qty 2 3rd/4th gear needle bearings
91107-PR8-008 qty 1 5th gear needle bearing
91121-PR8-008 qty 1 Differential taper bearing (bottom)
91122-PR8-008 qty 1 Differential taper bearing (top)
Oil Seals
91216-PR8-005 qty 1 Input shaft seal
91207-PR8-005 qty 1 Driver side driveshaft seal
91206-PR8-005 qty 1 Passenger side intermediate shaft seal.
91215-PR8-005 qty 2 Shift lever oil seals
Other
90030-PG2-000 qty 2 Countershaft bearing retainer bolts (these are staked so must be replaced if removed)
90201-PR8-000 qty 1 Countershaft lock nut
23926-PR8-000 qty 1 Countershaft lock nut spring washer
90602-PR8-000 qty 1 Snap ring (I wouldn't replace this unless your transmission is in snap ring range, it's difficult to remove without damaging the transmission case)
22841-PR8-010 qty 1 Clutch fork boot
21180-PR8-016 qty 1 Oil strainer
21181-PR8-000 qty 1 Oil strainer spring
91307-PR8-005 qty 1 Oil strainer O-ring
91309-PX4-003 qty 1 O-ring for the cover plate above differential in clutch housing
OPTIONAL JDM Gearing Upgrade
23210-PR8-020 qty 1 JDM mainshaft
23441-PR8-020 qty 1 JDM mainshaft 3rd
23451-PR8-010 qty 1 JDM mainshaft 4th
23431-PR8-010 qty 1 JDM countershaft 2nd
23471-PR8-000 qty 1 JDM countershaft 3rd
23481-PR8-000 qty 1 JDM countershaft 4th
OPTIONAL NSX-R Differential Upgrade
41696-PR8-000 qty 2 Preload spring washer
41581-PR8-J00 qty 1 NSX-R Preload spring retainer
OPTIONAL NSX-R 4.23 Final Drive
23220-PR8-J00 qty 1 NSX-R countershaft
41233-PR8-J00 qty 1 NSX-R differential ring gear
21173-PR8-J00 qty 1 NSX-R oil pump drive gear
1) The shafts
Start off by removing the various sensors, wires, and engine mount.
Then use a 1/2" drive breaker bar to expose the snap ring and spread it to release the countershaft from the transmission housing. It should drop down a bit when it releases and the snap ring should stay in the open position. I had to balance on top of the transmission while pushing down on the breaker bar to get enough leverage, since the snout of my impact gun was rounded and started to round out the cover.
Once the countershaft has been released, undo all of the 12mm housing bolts in a criss cross pattern. There are 4 bolts that are longer, I'd keep them in separate baggies for ease in finding them later, the longer bolts go in the holes with the dowel pins. Also pay attention to the bracket that shares a housing bolt as you'll need to put it back in the same place.
Once you've lifted the housing off, there are a couple measurements to take with a feeler gauge to verify clearances.
The manual then states to remove the reverse gear lever, lift up the mainshaft a bit, remove the reverse gear fork, then remove the reverse gear shaft. I was able to get the reverse fork out, but I couldn't get the mainshaft to lift up enough so I strapped all 3 shafts and the 3 shift forks together with some big zip ties and removed them as one unit.
There is another method I've found that can easily be done with 2 people (or 1 if you're handy) and requires no zip ties. Have one person grab both shafts and lift them up 1/2 inch or so, then have another lift up the reverse shaft and pivot it around the reverse fork until it is free, then both can be removed from the clutch housing at once. Once the reverse shaft and fork are removed, grab the two shafts while holding the forks with your thumbs and lift them up and out of the case.
Once the shafts are out you can begin taking them apart. Since I'm replacing a bunch of gears and bearings and synchros I didn't bother to take any measurements since I'm going to do that later once the new parts are on. Honda isn't very consistent with how tight their press-fit parts are, some transmissions come apart easily with no tools and others need a puller and/or press. I used the 7 ton rental puller from Autozone and it worked pretty well.
I wiped the parts off with a microfiber towel then cleaned them with brake cleaner before reassembly. The shafts are hollow so its a good idea to give them a good blast with brake cleaner down the middle to clear out any gunk, as everything will be covered in a layer of black goo.
The service manual lists a couple of shaft diameter measurements you can check now that the shaft is mostly apart. If you have a mic go ahead and check them but it's probably not necessary unless there's lots of obvious wear.
Starting to assemble the JDM mainshaft. The fit of the hubs wasn't very tight so I was able to do it mostly by hand. At the end, I taped up the nose and gave the 5th gear hub a few light taps with a hammer and big socket to make sure everything was properly seated.
I don't really have many pictures here but the countershaft process is essentially the same but it has a REVERSE THREADED staked locknut at the top and almost definitely requires a puller to separate the first couple gears since they are a tighter press fit.
EDIT: So far both [MENTION=35590]Big McLargeHuge[/MENTION] and [MENTION=18194]Honcho[/MENTION] are reporting that a 40mm socket is too small. I guess I mis-measured or made a mistake converting my inches mic reading to metric. Sorry about that. Since 1 5/8" sockets are equivalent to metric 41.28mm and fit and Honda uses metric sizes I think its actually a 41mm nut.
I started off by removing the locknut with my impact gun. Theoretically, you can do this in a vice with a breaker bar but it's bad enough torquing a new locknut on where you only have to get to 112lbft instead of whatever the breakaway torque needed is for a nut thats been on there for 25 years. IMO you'd be better off taking the countershaft to the nearest lube shop and asking one of the techs to zip it off with his impact gun than you would trying to do it in a vice, but you do you.
My puller was only 6" so I couldn't reach 3rd gear, so I hooked it onto the bottom of 4th gear and pulled off 4th and 5th, then turned the shaft upside down and tapped it on a piece of wood to dislodge 3rd gear, you might not be able to get away with this, but if you try, make sure to be ready for 3rd to fall off at any given moment. There are thin sheet metal bits at the ends of the shafts since they are hollow, you can't use a puller against these, you need a socket on the end of the shaft or you will destroy it.
If you turned it upside down and tapped it to remove 3rd gear the rest of the parts probably came off at the same time. If you're using a press, do it the right way mentioned in the manual. But basically 1 and 2 are more of the same stuff as the mainshaft with the exception of the 2nd gear synchro which is a dual cone synchro and you need to make sure the tabs line up with 2nd gear during reassembly.
Again, check the measurements of the shaft as per the service manual if possible. Mine was perfectly within spec at 109k miles.
Putting the JDM gears on my countershaft proved rather difficult, the right way is a press, but I used a hammer. A normal sized hammer with reasonable force, not a 4+ pound sledge. With 1st and 2nd gear on the shaft, slide 3rd down and make sure the splines are lined up and push it on as far as possible by hand. Then slide 4th on and line up the splines, then slide 5th on. 5th won't engage with the splines yet but you need it as a spacer since most sockets aren't deep enough without it.
Here is what the stack looks like when hammering. I put a big washer over 5th to protect it a bit more, and also taped up the nose to keep the socket from rattling against it. It will take a bit of movement before the 5th gear splines engage so you constantly need to be checking. Go slow.
At this point, get out your feeler gauges and do a preliminary check of the clearances covered in the manual. You'll do it again after torquing the nut down but it will save you a bit of time if something is way too loose or way too tight at this point.
Once the gears are all on, put the two bearings on. Put tape over the bearings so you don't get any debris that may be on your hammer/press or socket in the races. Do the bearings one at a time.
Once both bearings are on, clamp the shaft in a vice with blocks of wood like the manual states, put the spring washer back on, and put a new locknut on, torqing to 112lbft. At minimum, you need a 4.5" vice and 2 scraps of 3/4" plywood. A 6" vice and scraps of 2x4s would be better but 4.5" worked for me.
Now do a final check with the feeler gauges.
2) The differential
1991-1994 NSXs as well as the NSX-R use a LSD with clutch disks whereas the 95+ non-R NSXs use a torsen diff which I've never messed with. One fairly cheap and easy mod is to swap out the preload springs in the differential to bump up the breakaway torque to match the NSX-R at 110lbft or so instead of the 40lbft in the non-R models.
Make a mark on the ring gear and on the case with a permanent pen so you can put it back on the same way as it came apart. Opening it up is pretty easy, just use a small impact gun (or a big one with the pressure turned way down) to zip the case bolts off in a criss cross pattern with a 12 point 10mm socket.
Once it's open you'll see this. I didn't mess with the clutch disks but I suppose you can inspect them or replace them if you want to. The part we're concerned with is on the right, there's a big snap ring holding the spring washer in, you can pop it out with a screwdriver by working your way around. Keep a hand over the case while you work so it doesn't pop up and hit you. There's not enough force to actually send anything flying across the room, but the snap ring itself is pretty light and springy so wear safety glasses. Or you can do it the right way with a press and compress the spring washers before removing the snap ring.
Once you've popped the snap ring out you can take out the retaining ring and spring washer, and replace them with the NSX-R retaining ring and 2 new spring washers. Some people reuse the original spring washer and only buy one additional one but the parts are a couple bucks so might as well replace it.
Getting the snap ring back in is a bit trickier, again, the right way is with a press. Once you have the spring washers and retaining ring in the case, slide the snap ring down as far as it can go. Instead of a press I used a big C clamp, various large washers, a couple blocks of wood, and a socket. Cranking down on the clamp compressed the washers enough to easily seat the snap ring back in it's groove.
After that, put the case back on in the same place it came off using the mark you made, thread all the bolts in hand tight, then go in a criss cross pattern with a ratchet snugging them down. I did 1/2 turn at a time and made a couple passes to get all the bolts snug, then I held on tight with my left hand (you'll want gloves) and used the torque wrench in my right hand to torque them back to 29lbft in a criss cross pattern. A vice with wooden blocks would be ideal, but if the taper bearings are still on the diff then that's not really an option. If you're having trouble getting the bolts to 29lbft have a friend wear thick gloves and hold the diff while you operate the torque wrench, or if they won't go for that, let them hold the wrench.
Getting the transmission housing diff bearing out the Honda way is terrible, they either tell you to pry it out with a screwdriver (seriously WTF how does that even work) or heat the housing to 212F and remove it that way (again WTF). Pop the oil seal out, flip the housing over, and tap it from the back with anything that fits, in my case, a PVC pipe fitting I use for whacking various things. In my case it took almost no effort to get it out, and no fire or prying on aluminum was involved.
Removing the clutch housing taper bearing race was about the same process, pop the oil seal out, flip it over, and tap on the metal oil control ring or whatever it is that's behind the bearing race. It came out fairly easily with just light tapping.
The bearing race came out before the oil control ring thing so I flipped the housing back over and lightly tapped it back in.
Getting the differential bearings off is beyond the capability of the tools I have in my garage. It might be done with a good bearing splitter, but I don't really care to buy one so I had a local machine shop remove the bearings for me. They ended up cutting them off since there isn't really a good way to latch onto them with any sort of puller. Machine shops tend to have metal shards flying everywhere so wad up paper towels and stuff them in the axle holes before you hand it off to anyone.
Putting the new taper bearings on was more of a pain than I expected it to be. The bearings have wildly different inner diameters so you can't use the same tool for both, so after thinking for a bit I decided to make my own tool. You can't use a socket because any socket big enough to clear the shaft of the differential will smash the roller cages of the bearings since they stick up, so what is needed is a skinny tube. I used a piece of 1/8" thick aluminum stock I had laying round and rolled it around a socket using vice grips to hold the end to the socket and a hammer to beat it into shape. I did the bigger bearing first and used a socket slightly smaller than needed so after it sprang back a bit it would be close enough to bend into shape. After doing the large bearing, I cut the end off the aluminum piece to make it smaller and saved myself a lot of work since I didn't have to start over. The bearings went on fairly easily with just light taps from a hammer and a block of wood wrapped in tape to keep splinters from getting in the bearings.
To check the preload on the bearings you have to lubricate the bearings with transmission fluid, bolt the case halves together, and see how much force it takes to spin the diff. Honda sells a special tool to stick in the diff to allow you to attach a torque wrench but rather than buy another tool thats probably been discontinued I just got a 10" long bolt, some nuts, and some washers and made my own. This also makes the diff easier to install and remove in the clutch housing since it's fairly heavy and hard to get your fingers around and sharp so definitely something I'd recommend doing before you even separate the case halves in the first place. I put the two nuts on the end so I could easily put a torque wrench on it without worrying too much about how long the bolt was.
Finally, bolt up the case halves, give the diff a good spin with a ratchet to seat the bearings, and attach a torque wrench and see what it takes to spin it. In my case it was about 20 inch pounds, the factory spec is 17-26 so IMO 20 is perfect.
3) The housings
If you want to remove the countershaft bearing, start by getting your favorite flat blade screwdriver and pry the roller retainer like so. Honda has a special slide hammer attachment for this but why would you want to spend money on that when you could just use the free one from Autozone?
Once the bearing is adequately destroyed, put the slide hammer in like this and tap the bearing out of the clutch housing.
Put the new one in using a socket and the outer race of the old one to tap it in. But I prefer to leave reassembly until the very end after I've had a chance to clean everything.
Undo the 3 screws at the bottom of the housing to expose the tiny little baby oil pump so you can check clearances with a feeler gauge here. Behind the spring is a ball, so make sure not to lose it.
Removing the shifter arms from the housings is easy, just remove the 12mm bolt with a low power (low pressure) impact gun and slide it out, then use a screwdriver to pop the oil seal out.
Here is the clutch housing with the guts removed.
4) Mainshaft Thrust Shim
Unfortunately, there is no ghetto cheap way to properly set the mainshaft thrust shim thickness, you need a dial indicator and a micrometer/caliper. To put the bottom mainshaft bearing on I just stacked up a bunch of circular objects of roughly the correct diameter and tapped it on with a hammer. The important thing here is that whatever is in contact with the bearing only be touching the inner race, you don't want to apply force to the outer race. I used the 3/4 hub from an Integra transmission since it fit. It's fairly tight but doesn't take too much force.
I didn't have a dial indicator holder that would work for this, so I made one out of this billet battery tiedown from a Subaru WRX I had laying around. (long story, I've never owned a Subaru)
Install the mainshaft with the shims at the top but NOT the conical spring washer at the base of the mainshaft and bolt the case halves together, then set up your dial indicator at the end of the tip of the mainshaft. Once everything is set up, push in on the mainshaft and zero the dial indicator while holding force on it, then pull out on it and take the reading. In my case it was 0.0380" give or take a few tenths, a 0.0001" precision dial indicator is absolutely overkill for this but it's what I have and it works. This measurement is too tight by 0.001" so in the interest of perfectionism I'll be ordering a shim one step looser to compensate. Honda sells them in increments of 0.05mm (0.002") so that should put my clearance right at 0.0400 which is perfect. (factory specification is 0.039-0.041") The only way to know which shim(s) your transmission has from the factory is to measure them with a micrometer or caliper.
The FSM process for this first uses feeler gauges to find the play in the mainshaft without the spring washer installed (which is what we measured above with the dial indicator since the FSM method is terrible), then you install the shims and spring washer and check the final clearance using a special mainshaft holder tool to compress the spring washer. You don't need this special tool, you can get around this by measuring the thickness of the spring washer using a ball end attachment on a micrometer. In my case it was 0.0334", so subtracting that from 0.0380 gives us a final mainshaft play of 0.0046" which is just shy of the factory specified 0.006-0.008" clearance. This is just a way to confirm that our previous measurements are correct since the only additional work it requires is measuring the thickness of the spring washer, the measurements differ by 0.0004" which is essentially nothing in this case.
This thread is intended to be used as a reference in addition to the diagrams and instructions found in the factory service manual. If you don't have one, go download it and read the chapter on tearing down the transmission.
In an ideal world you'd have access to a big press, various pullers, and other special tools. But I don't have that stuff and most DIYers don't either. Part of being a good DIY garage hack with limited tools is knowing when you've reached the limits of your capability and to call a friend/pro/machine shop/etc. If something is moving like you think it should, stop and reevaluate, whipping out the sledge should never cross your mind.
Also, if you're having issues with your transmission, change the fluid before you rip it open. Old fluid can cause all sorts of strange issues and you might get lucky.
Parts List
Below is a list of parts one might use in a "typical" transmission rebuild. This is a good baseline for what to buy, but do your own research and make sure you order everything you need, there is a very good chance you will find other things that need to be replaced and find things that you probably didn't need to replace. Examples include clearance adjustment shims, needle bearings, worn hub/sleeve (especially on 5th), etc. I've found that parts are much cheaper if you order from Japanese suppliers like Amayama, but for some things such as clearance shims that you can't predict ahead of time I find it easiest to order them from local Acura or Honda dealerships since they can usually get them in much faster than you could order them and don't cost all that much more.
To minimize the cost of a rebuild, if you have a grinding issue, stop using that gear immediately or figure out how to make it not grind when you select it. The more the gear grinds, the more likely you'll have to replace parts that aren't normal wear items.
If you find anything strange with the part numbers or list items, please let me know, I'm only human and there is a non-zero chance that I might have screwed up when copying stuff over from my spreadsheet.
Synchronizers
23641-PR8-010 qty 1 1st gear synchro
23646-PR8-020 qty 1 2nd gear synchro
23641-PR8-911 qty 2 3rd/4th gear synchro
23642-PG1-912 qty 1 5th gear synchro
Bearings
91002-PR8-008 qty 1 Mainshaft bearing (bottom)
91004-PR8-008 qty 1 Mainshaft bearing (top)
91103-PR8-018 qty 1 Countershaft bearing (bottom, the one with no inner race)
91102-PR8-018 qty 1 Countershaft bearing (middle)
91003-PR8-008 qty 1 Countershaft bearing (top, the one with the snap ring groove)
91105-PR8-008 qty 1 1st gear needle bearing
91108-PR8-008 qty 1 2nd gear needle bearing
91104-PR8-008 qty 2 3rd/4th gear needle bearings
91107-PR8-008 qty 1 5th gear needle bearing
91121-PR8-008 qty 1 Differential taper bearing (bottom)
91122-PR8-008 qty 1 Differential taper bearing (top)
Oil Seals
91216-PR8-005 qty 1 Input shaft seal
91207-PR8-005 qty 1 Driver side driveshaft seal
91206-PR8-005 qty 1 Passenger side intermediate shaft seal.
91215-PR8-005 qty 2 Shift lever oil seals
Other
90030-PG2-000 qty 2 Countershaft bearing retainer bolts (these are staked so must be replaced if removed)
90201-PR8-000 qty 1 Countershaft lock nut
23926-PR8-000 qty 1 Countershaft lock nut spring washer
90602-PR8-000 qty 1 Snap ring (I wouldn't replace this unless your transmission is in snap ring range, it's difficult to remove without damaging the transmission case)
22841-PR8-010 qty 1 Clutch fork boot
21180-PR8-016 qty 1 Oil strainer
21181-PR8-000 qty 1 Oil strainer spring
91307-PR8-005 qty 1 Oil strainer O-ring
91309-PX4-003 qty 1 O-ring for the cover plate above differential in clutch housing
OPTIONAL JDM Gearing Upgrade
23210-PR8-020 qty 1 JDM mainshaft
23441-PR8-020 qty 1 JDM mainshaft 3rd
23451-PR8-010 qty 1 JDM mainshaft 4th
23431-PR8-010 qty 1 JDM countershaft 2nd
23471-PR8-000 qty 1 JDM countershaft 3rd
23481-PR8-000 qty 1 JDM countershaft 4th
OPTIONAL NSX-R Differential Upgrade
41696-PR8-000 qty 2 Preload spring washer
41581-PR8-J00 qty 1 NSX-R Preload spring retainer
OPTIONAL NSX-R 4.23 Final Drive
23220-PR8-J00 qty 1 NSX-R countershaft
41233-PR8-J00 qty 1 NSX-R differential ring gear
21173-PR8-J00 qty 1 NSX-R oil pump drive gear
1) The shafts
Start off by removing the various sensors, wires, and engine mount.
Then use a 1/2" drive breaker bar to expose the snap ring and spread it to release the countershaft from the transmission housing. It should drop down a bit when it releases and the snap ring should stay in the open position. I had to balance on top of the transmission while pushing down on the breaker bar to get enough leverage, since the snout of my impact gun was rounded and started to round out the cover.
Once the countershaft has been released, undo all of the 12mm housing bolts in a criss cross pattern. There are 4 bolts that are longer, I'd keep them in separate baggies for ease in finding them later, the longer bolts go in the holes with the dowel pins. Also pay attention to the bracket that shares a housing bolt as you'll need to put it back in the same place.
Once you've lifted the housing off, there are a couple measurements to take with a feeler gauge to verify clearances.
The manual then states to remove the reverse gear lever, lift up the mainshaft a bit, remove the reverse gear fork, then remove the reverse gear shaft. I was able to get the reverse fork out, but I couldn't get the mainshaft to lift up enough so I strapped all 3 shafts and the 3 shift forks together with some big zip ties and removed them as one unit.
There is another method I've found that can easily be done with 2 people (or 1 if you're handy) and requires no zip ties. Have one person grab both shafts and lift them up 1/2 inch or so, then have another lift up the reverse shaft and pivot it around the reverse fork until it is free, then both can be removed from the clutch housing at once. Once the reverse shaft and fork are removed, grab the two shafts while holding the forks with your thumbs and lift them up and out of the case.
Once the shafts are out you can begin taking them apart. Since I'm replacing a bunch of gears and bearings and synchros I didn't bother to take any measurements since I'm going to do that later once the new parts are on. Honda isn't very consistent with how tight their press-fit parts are, some transmissions come apart easily with no tools and others need a puller and/or press. I used the 7 ton rental puller from Autozone and it worked pretty well.
I wiped the parts off with a microfiber towel then cleaned them with brake cleaner before reassembly. The shafts are hollow so its a good idea to give them a good blast with brake cleaner down the middle to clear out any gunk, as everything will be covered in a layer of black goo.
The service manual lists a couple of shaft diameter measurements you can check now that the shaft is mostly apart. If you have a mic go ahead and check them but it's probably not necessary unless there's lots of obvious wear.
Starting to assemble the JDM mainshaft. The fit of the hubs wasn't very tight so I was able to do it mostly by hand. At the end, I taped up the nose and gave the 5th gear hub a few light taps with a hammer and big socket to make sure everything was properly seated.
I don't really have many pictures here but the countershaft process is essentially the same but it has a REVERSE THREADED staked locknut at the top and almost definitely requires a puller to separate the first couple gears since they are a tighter press fit.
EDIT: So far both [MENTION=35590]Big McLargeHuge[/MENTION] and [MENTION=18194]Honcho[/MENTION] are reporting that a 40mm socket is too small. I guess I mis-measured or made a mistake converting my inches mic reading to metric. Sorry about that. Since 1 5/8" sockets are equivalent to metric 41.28mm and fit and Honda uses metric sizes I think its actually a 41mm nut.
I started off by removing the locknut with my impact gun. Theoretically, you can do this in a vice with a breaker bar but it's bad enough torquing a new locknut on where you only have to get to 112lbft instead of whatever the breakaway torque needed is for a nut thats been on there for 25 years. IMO you'd be better off taking the countershaft to the nearest lube shop and asking one of the techs to zip it off with his impact gun than you would trying to do it in a vice, but you do you.
My puller was only 6" so I couldn't reach 3rd gear, so I hooked it onto the bottom of 4th gear and pulled off 4th and 5th, then turned the shaft upside down and tapped it on a piece of wood to dislodge 3rd gear, you might not be able to get away with this, but if you try, make sure to be ready for 3rd to fall off at any given moment. There are thin sheet metal bits at the ends of the shafts since they are hollow, you can't use a puller against these, you need a socket on the end of the shaft or you will destroy it.
If you turned it upside down and tapped it to remove 3rd gear the rest of the parts probably came off at the same time. If you're using a press, do it the right way mentioned in the manual. But basically 1 and 2 are more of the same stuff as the mainshaft with the exception of the 2nd gear synchro which is a dual cone synchro and you need to make sure the tabs line up with 2nd gear during reassembly.
Again, check the measurements of the shaft as per the service manual if possible. Mine was perfectly within spec at 109k miles.
Putting the JDM gears on my countershaft proved rather difficult, the right way is a press, but I used a hammer. A normal sized hammer with reasonable force, not a 4+ pound sledge. With 1st and 2nd gear on the shaft, slide 3rd down and make sure the splines are lined up and push it on as far as possible by hand. Then slide 4th on and line up the splines, then slide 5th on. 5th won't engage with the splines yet but you need it as a spacer since most sockets aren't deep enough without it.
Here is what the stack looks like when hammering. I put a big washer over 5th to protect it a bit more, and also taped up the nose to keep the socket from rattling against it. It will take a bit of movement before the 5th gear splines engage so you constantly need to be checking. Go slow.
At this point, get out your feeler gauges and do a preliminary check of the clearances covered in the manual. You'll do it again after torquing the nut down but it will save you a bit of time if something is way too loose or way too tight at this point.
Once the gears are all on, put the two bearings on. Put tape over the bearings so you don't get any debris that may be on your hammer/press or socket in the races. Do the bearings one at a time.
Once both bearings are on, clamp the shaft in a vice with blocks of wood like the manual states, put the spring washer back on, and put a new locknut on, torqing to 112lbft. At minimum, you need a 4.5" vice and 2 scraps of 3/4" plywood. A 6" vice and scraps of 2x4s would be better but 4.5" worked for me.
Now do a final check with the feeler gauges.
2) The differential
1991-1994 NSXs as well as the NSX-R use a LSD with clutch disks whereas the 95+ non-R NSXs use a torsen diff which I've never messed with. One fairly cheap and easy mod is to swap out the preload springs in the differential to bump up the breakaway torque to match the NSX-R at 110lbft or so instead of the 40lbft in the non-R models.
Make a mark on the ring gear and on the case with a permanent pen so you can put it back on the same way as it came apart. Opening it up is pretty easy, just use a small impact gun (or a big one with the pressure turned way down) to zip the case bolts off in a criss cross pattern with a 12 point 10mm socket.
Once it's open you'll see this. I didn't mess with the clutch disks but I suppose you can inspect them or replace them if you want to. The part we're concerned with is on the right, there's a big snap ring holding the spring washer in, you can pop it out with a screwdriver by working your way around. Keep a hand over the case while you work so it doesn't pop up and hit you. There's not enough force to actually send anything flying across the room, but the snap ring itself is pretty light and springy so wear safety glasses. Or you can do it the right way with a press and compress the spring washers before removing the snap ring.
Once you've popped the snap ring out you can take out the retaining ring and spring washer, and replace them with the NSX-R retaining ring and 2 new spring washers. Some people reuse the original spring washer and only buy one additional one but the parts are a couple bucks so might as well replace it.
Getting the snap ring back in is a bit trickier, again, the right way is with a press. Once you have the spring washers and retaining ring in the case, slide the snap ring down as far as it can go. Instead of a press I used a big C clamp, various large washers, a couple blocks of wood, and a socket. Cranking down on the clamp compressed the washers enough to easily seat the snap ring back in it's groove.
After that, put the case back on in the same place it came off using the mark you made, thread all the bolts in hand tight, then go in a criss cross pattern with a ratchet snugging them down. I did 1/2 turn at a time and made a couple passes to get all the bolts snug, then I held on tight with my left hand (you'll want gloves) and used the torque wrench in my right hand to torque them back to 29lbft in a criss cross pattern. A vice with wooden blocks would be ideal, but if the taper bearings are still on the diff then that's not really an option. If you're having trouble getting the bolts to 29lbft have a friend wear thick gloves and hold the diff while you operate the torque wrench, or if they won't go for that, let them hold the wrench.
Getting the transmission housing diff bearing out the Honda way is terrible, they either tell you to pry it out with a screwdriver (seriously WTF how does that even work) or heat the housing to 212F and remove it that way (again WTF). Pop the oil seal out, flip the housing over, and tap it from the back with anything that fits, in my case, a PVC pipe fitting I use for whacking various things. In my case it took almost no effort to get it out, and no fire or prying on aluminum was involved.
Removing the clutch housing taper bearing race was about the same process, pop the oil seal out, flip it over, and tap on the metal oil control ring or whatever it is that's behind the bearing race. It came out fairly easily with just light tapping.
The bearing race came out before the oil control ring thing so I flipped the housing back over and lightly tapped it back in.
Getting the differential bearings off is beyond the capability of the tools I have in my garage. It might be done with a good bearing splitter, but I don't really care to buy one so I had a local machine shop remove the bearings for me. They ended up cutting them off since there isn't really a good way to latch onto them with any sort of puller. Machine shops tend to have metal shards flying everywhere so wad up paper towels and stuff them in the axle holes before you hand it off to anyone.
Putting the new taper bearings on was more of a pain than I expected it to be. The bearings have wildly different inner diameters so you can't use the same tool for both, so after thinking for a bit I decided to make my own tool. You can't use a socket because any socket big enough to clear the shaft of the differential will smash the roller cages of the bearings since they stick up, so what is needed is a skinny tube. I used a piece of 1/8" thick aluminum stock I had laying round and rolled it around a socket using vice grips to hold the end to the socket and a hammer to beat it into shape. I did the bigger bearing first and used a socket slightly smaller than needed so after it sprang back a bit it would be close enough to bend into shape. After doing the large bearing, I cut the end off the aluminum piece to make it smaller and saved myself a lot of work since I didn't have to start over. The bearings went on fairly easily with just light taps from a hammer and a block of wood wrapped in tape to keep splinters from getting in the bearings.
To check the preload on the bearings you have to lubricate the bearings with transmission fluid, bolt the case halves together, and see how much force it takes to spin the diff. Honda sells a special tool to stick in the diff to allow you to attach a torque wrench but rather than buy another tool thats probably been discontinued I just got a 10" long bolt, some nuts, and some washers and made my own. This also makes the diff easier to install and remove in the clutch housing since it's fairly heavy and hard to get your fingers around and sharp so definitely something I'd recommend doing before you even separate the case halves in the first place. I put the two nuts on the end so I could easily put a torque wrench on it without worrying too much about how long the bolt was.
Finally, bolt up the case halves, give the diff a good spin with a ratchet to seat the bearings, and attach a torque wrench and see what it takes to spin it. In my case it was about 20 inch pounds, the factory spec is 17-26 so IMO 20 is perfect.
3) The housings
If you want to remove the countershaft bearing, start by getting your favorite flat blade screwdriver and pry the roller retainer like so. Honda has a special slide hammer attachment for this but why would you want to spend money on that when you could just use the free one from Autozone?
Once the bearing is adequately destroyed, put the slide hammer in like this and tap the bearing out of the clutch housing.
Put the new one in using a socket and the outer race of the old one to tap it in. But I prefer to leave reassembly until the very end after I've had a chance to clean everything.
Undo the 3 screws at the bottom of the housing to expose the tiny little baby oil pump so you can check clearances with a feeler gauge here. Behind the spring is a ball, so make sure not to lose it.
Removing the shifter arms from the housings is easy, just remove the 12mm bolt with a low power (low pressure) impact gun and slide it out, then use a screwdriver to pop the oil seal out.
Here is the clutch housing with the guts removed.
4) Mainshaft Thrust Shim
Unfortunately, there is no ghetto cheap way to properly set the mainshaft thrust shim thickness, you need a dial indicator and a micrometer/caliper. To put the bottom mainshaft bearing on I just stacked up a bunch of circular objects of roughly the correct diameter and tapped it on with a hammer. The important thing here is that whatever is in contact with the bearing only be touching the inner race, you don't want to apply force to the outer race. I used the 3/4 hub from an Integra transmission since it fit. It's fairly tight but doesn't take too much force.
I didn't have a dial indicator holder that would work for this, so I made one out of this billet battery tiedown from a Subaru WRX I had laying around. (long story, I've never owned a Subaru)
Install the mainshaft with the shims at the top but NOT the conical spring washer at the base of the mainshaft and bolt the case halves together, then set up your dial indicator at the end of the tip of the mainshaft. Once everything is set up, push in on the mainshaft and zero the dial indicator while holding force on it, then pull out on it and take the reading. In my case it was 0.0380" give or take a few tenths, a 0.0001" precision dial indicator is absolutely overkill for this but it's what I have and it works. This measurement is too tight by 0.001" so in the interest of perfectionism I'll be ordering a shim one step looser to compensate. Honda sells them in increments of 0.05mm (0.002") so that should put my clearance right at 0.0400 which is perfect. (factory specification is 0.039-0.041") The only way to know which shim(s) your transmission has from the factory is to measure them with a micrometer or caliper.
The FSM process for this first uses feeler gauges to find the play in the mainshaft without the spring washer installed (which is what we measured above with the dial indicator since the FSM method is terrible), then you install the shims and spring washer and check the final clearance using a special mainshaft holder tool to compress the spring washer. You don't need this special tool, you can get around this by measuring the thickness of the spring washer using a ball end attachment on a micrometer. In my case it was 0.0334", so subtracting that from 0.0380 gives us a final mainshaft play of 0.0046" which is just shy of the factory specified 0.006-0.008" clearance. This is just a way to confirm that our previous measurements are correct since the only additional work it requires is measuring the thickness of the spring washer, the measurements differ by 0.0004" which is essentially nothing in this case.
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