A few years ago @Honcho sent me an article in Japanese talking about converting a 5 speed transmission to a 6 speed while reusing some parts such as the bell housing and diff and replacing everything else. I started down that path, then after ordering a bunch of the parts, realized that it would actually be way cheaper to just build a 6 speed from scratch and sell the original R spec 5 speed afterwards. (but lets be real it will just end up mothballed in storage somewhere since I'm a NSX parts hoarder at this point)
The nice thing about a from-scratch build is you can spec out everything to your own preferences, and the cost is actually about the same as just buying a used one, once you wade through the parts lead times and availability issues, and quality issues... I first started ordering parts more than 2 years ago and just now am finally getting all of them, T3 makes a few aftermarket replacements so I could have put it together earlier I'm just lazy.
The late year transmissions came with double cone synchronizers on every gear, so I went with that gear set. I also opted for the factory 4.23 Type R final drive since my car is mostly street driven and I don't see a compelling reason to go for one of the higher ratio more exotic options, and the Type R input shaft for clutch compatibility reasons. For the diff, I started off just sourcing a used diff and converting it to R spec, then ended up selling it here and buying a Wavetrac after @RYU twisted my arm and threatened my family if I didn't agree to it. (he actually just told me it was better)
I was going to just put it all together as Honda intended, then decided that if I'm going to be shoveling money into the fire I might as well toss in a little more and went all in on the surface treatments more as an experiment than anything else. I had the entire gear set and final drive REM polished by New England Gear Polishing, then WPC treated. Then most of the rest of the internal parts I had just WPC'd including oil pump gear, shifter mechanisms, shift forks, etc. Total cost for surface treatments was around $1500 not including shipping oddly shaped chunks of iron across the country multiple times which was probably another $500 on top of that. I sent the parts out in original Honda packaging and received them back in exactly the same way, both NEGP and WPC had quick turnaround times and were very organized so no complaints there.
Its important to note that I did not WPC treat the synchronizers. I was thinking about it and ended up calling Synchrotech first to get their thoughts, since they seem to be the experts on aftermarket Honda transmission development, and they said that they love WPC for gear sets but absolutely do not do it on the synchronizers. Which makes perfect sense to me, the synchros and cone surfaces are specifically engineered to interact with each other in a specific way, and WPC could interfere with that. If not at first, potentially down the road and the last thing I want to do is end up with a grinding 3rd gear or something. The nice thing about the double cone synchros is they are completely separate from the gears, so the gears can be polished and treated without touching the synchronizers at all.
I also didn't treat the ball or needle bearings, because that just seemed pointless.
Cleaning
Run everything that was polished or treated through the ultrasonic cleaner, dry everything, and oil lightly with WD40 to prevent rust. Keep in mind that WD40 is not acceptable for long term storage rust protection, but in the short term works great. Several of the shafts are hollow as well, such as the mainshaft, countershaft, and 5/6 synchro shaft and need to be thoroughly flushed out since I saw trace amounts of blasting media still inside them from the WPC process when I got the parts back.
Shift Forks (and discover that I forgot to have some parts treated)
The 5/6 fork is welded, but the 1/2 and 2/3 forks (shown here) have these very tight double spring pins, presumably for serviceability so the fork can be replaced without the whole shaft? First you put in the big spring pin, then once it's in, you drive a smaller spring pin through the middle of it. The vice with my DIY aluminum L bracket soft jaws works pretty well for this, then I used a punch and a hammer to push it in the last few millimeters that the vice couldn't get just by resting the fork across my legs to support it. The black marks on the shaft are sharpie marks to help line the hole for the pin up.
I forgot to have the parts that bolt onto the 5/6 fork treated so those are sent off and should be back in a few days, and fortunately they aren't blocking any progress until final assembly.
Main Shifter Assembly
This is what physically engages with the shift forks to push them up or down to select each gear. The shaft, arm thingy, and the sideways U shaped part were treated, but the big plate on the bottom was not as it has that roller and I didn't want to risk it getting jammed up with blasting media. I was expecting this to be more difficult to assemble than it was.
When I sent the sideways U shaped thing off for treatment it was still sealed in the Honda bag so I didn't realize that it had a little rubber bumper on it to pad swinging the shifter left to right, and underneath the rubber bumper ring had started to develop surface rust presumably due to moisture being trapped after the part was cleaned so make sure not to make that mistake. Shouldn't hurt anything though.
Reverse Shaft
This is why it is absolutely critical to inspect all parts and try to feel how they interact. This tiny tiny bur was on the reverse gear cone where the synchronizer sits. You can just barely feel it with a fingernail, and I missed it the first few times when I was looking for it. The only indication was that the synchronizer would stick and drag terribly when trying to rotate it without pressure on the cone. The steel this gear is made out of is rather hard so a file wouldn't really even touch it, I had to use a find sandpaper roll on the dremel which cleaned it up nicely, then another trip through the ultrasonic cleaner.
This brings us to the odd point that NSX transmissions have synchronized reverse gears, contrary to nearly every other Honda gearbox (and other manufacturers). Compared to something like a D, B, or K series gearbox where the reverse gear is pushed down between the mainshaft and countershaft by a little hand as seen here. It always seemed to me since this could just result in more parasitic drag since the gears are always engaged but I guess it is negligible.
Reverse gear stack and tiny little shift fork done.
The nice thing about a from-scratch build is you can spec out everything to your own preferences, and the cost is actually about the same as just buying a used one, once you wade through the parts lead times and availability issues, and quality issues... I first started ordering parts more than 2 years ago and just now am finally getting all of them, T3 makes a few aftermarket replacements so I could have put it together earlier I'm just lazy.
The late year transmissions came with double cone synchronizers on every gear, so I went with that gear set. I also opted for the factory 4.23 Type R final drive since my car is mostly street driven and I don't see a compelling reason to go for one of the higher ratio more exotic options, and the Type R input shaft for clutch compatibility reasons. For the diff, I started off just sourcing a used diff and converting it to R spec, then ended up selling it here and buying a Wavetrac after @RYU twisted my arm and threatened my family if I didn't agree to it. (he actually just told me it was better)
I was going to just put it all together as Honda intended, then decided that if I'm going to be shoveling money into the fire I might as well toss in a little more and went all in on the surface treatments more as an experiment than anything else. I had the entire gear set and final drive REM polished by New England Gear Polishing, then WPC treated. Then most of the rest of the internal parts I had just WPC'd including oil pump gear, shifter mechanisms, shift forks, etc. Total cost for surface treatments was around $1500 not including shipping oddly shaped chunks of iron across the country multiple times which was probably another $500 on top of that. I sent the parts out in original Honda packaging and received them back in exactly the same way, both NEGP and WPC had quick turnaround times and were very organized so no complaints there.
Its important to note that I did not WPC treat the synchronizers. I was thinking about it and ended up calling Synchrotech first to get their thoughts, since they seem to be the experts on aftermarket Honda transmission development, and they said that they love WPC for gear sets but absolutely do not do it on the synchronizers. Which makes perfect sense to me, the synchros and cone surfaces are specifically engineered to interact with each other in a specific way, and WPC could interfere with that. If not at first, potentially down the road and the last thing I want to do is end up with a grinding 3rd gear or something. The nice thing about the double cone synchros is they are completely separate from the gears, so the gears can be polished and treated without touching the synchronizers at all.
I also didn't treat the ball or needle bearings, because that just seemed pointless.
Cleaning
Run everything that was polished or treated through the ultrasonic cleaner, dry everything, and oil lightly with WD40 to prevent rust. Keep in mind that WD40 is not acceptable for long term storage rust protection, but in the short term works great. Several of the shafts are hollow as well, such as the mainshaft, countershaft, and 5/6 synchro shaft and need to be thoroughly flushed out since I saw trace amounts of blasting media still inside them from the WPC process when I got the parts back.
Shift Forks (and discover that I forgot to have some parts treated)
The 5/6 fork is welded, but the 1/2 and 2/3 forks (shown here) have these very tight double spring pins, presumably for serviceability so the fork can be replaced without the whole shaft? First you put in the big spring pin, then once it's in, you drive a smaller spring pin through the middle of it. The vice with my DIY aluminum L bracket soft jaws works pretty well for this, then I used a punch and a hammer to push it in the last few millimeters that the vice couldn't get just by resting the fork across my legs to support it. The black marks on the shaft are sharpie marks to help line the hole for the pin up.
I forgot to have the parts that bolt onto the 5/6 fork treated so those are sent off and should be back in a few days, and fortunately they aren't blocking any progress until final assembly.
Main Shifter Assembly
This is what physically engages with the shift forks to push them up or down to select each gear. The shaft, arm thingy, and the sideways U shaped part were treated, but the big plate on the bottom was not as it has that roller and I didn't want to risk it getting jammed up with blasting media. I was expecting this to be more difficult to assemble than it was.
When I sent the sideways U shaped thing off for treatment it was still sealed in the Honda bag so I didn't realize that it had a little rubber bumper on it to pad swinging the shifter left to right, and underneath the rubber bumper ring had started to develop surface rust presumably due to moisture being trapped after the part was cleaned so make sure not to make that mistake. Shouldn't hurt anything though.
Reverse Shaft
This is why it is absolutely critical to inspect all parts and try to feel how they interact. This tiny tiny bur was on the reverse gear cone where the synchronizer sits. You can just barely feel it with a fingernail, and I missed it the first few times when I was looking for it. The only indication was that the synchronizer would stick and drag terribly when trying to rotate it without pressure on the cone. The steel this gear is made out of is rather hard so a file wouldn't really even touch it, I had to use a find sandpaper roll on the dremel which cleaned it up nicely, then another trip through the ultrasonic cleaner.
This brings us to the odd point that NSX transmissions have synchronized reverse gears, contrary to nearly every other Honda gearbox (and other manufacturers). Compared to something like a D, B, or K series gearbox where the reverse gear is pushed down between the mainshaft and countershaft by a little hand as seen here. It always seemed to me since this could just result in more parasitic drag since the gears are always engaged but I guess it is negligible.
Reverse gear stack and tiny little shift fork done.
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