Broken motor-mount bolts

Well I was looking for an electrical issue that came up over bumpy roads. The CEL light indicated low MAP voltage, and the car stumbled at times, triggered by a bump each time. That pretty strongly suggested a bad connection somewhere.

As my car is modified, I suspected the AEM F/IC that I use for fueling/timing adjustment. I started a thread here: http://www.nsxprime.com/forum/showt...OBD2-piggyback-(potential-issue-with-my-F-IC)

I finally dig into the car this weekend. First I took out the harness bar then dismantled all of the ECU connections and used contact cleaner and enhancer (DeOxit products) and looked for anything suspicious. Everything looked fine.

I took apart and cleaned any ground connection I could see in the engine compartment. And cleaned the MAP sensor connections. Everything seemed okay.

Then later I went back to the garage to poke around some more and look at ground points in the electrical diagnostic manual. It seems like a main ECU ground is by the alternator, so I looked around there and found two broken ground wires!




That made me pretty happy, as at least I found something that wasn’t as it should be. So then I’m poking around that area some more and see a big bracket with two holes in it but only one bolt, and that bolt secured nothing. It was my right side motor mount. Both bolts had broken flush with the surface they thread into (looks like a bracket itself).

It’s a little hard to see but:



The first picture shows the mount with its open holes. The second picture is after I removed the alternator and some air pipes to see better. You can see the two stubs of thread.

So now I need to get those broken pieces out. I do have an ez-out and have a left-hand tap and cap screw on order. It seems like a tough place to weld a nut to the stub, but I may try that if the ez-out and LH screw don’t work. I’ve soaked with kroil a few times.

Not looking forward to this.

Use left hand drills for this.

Never use an ezy out, only for "you over torqued the fastener and it broke". Not for a corroded (galvanic or oxidized) fastener.

Get a set of LH cobalt drills and go from there.

drew said:

Use left hand drills for this. …
Get a set of LH cobalt drills and go from there.

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I did order some LH drills with the LH tap and screw.

Are you advising to drill all the way up to the thread minor diameter? I’ve done that when I can align things on the mill table but it seems unlikely I can align the hole well enough drilling freehand. Also - why cobalt? Those should be more wear resistant but more brittle too, so not great for hand drilling.

Also - curious why you say the fastener is corroded? It may be I suppose but it didn’t break by me trying to remove it. It broke either when installed or maybe was over tightened when installed and then broke later. Or one of the other engine mounts failing put undue stress on this one. Either way, I’m not sure I have reason to think this fastener is particularly stuck.

I appreciate your advise though. I’ve removed some broken bolts but this one seems tricky if only for the restricted access.

I also found an almost stripped upper control arm bolt in the rear. I'll have to time sert it but luckily the bolt free'd itself.

I'll take a look at this section on my car this wknd. Maybe I'll have some ideas

Well, I had a big success and found an even bigger problem.

I got a left-hand drill and silver soldered it into a rod as an extension:


That let me drill the broken bolts and they screwed right out. Fortunately they weren’t tight:


But when I went to install the engine mount, the large bolt into the body didn’t tighten. Upon further inspection, there is no nut there. I assume there should be a nut affixed somehow on the back side of the body:

That’s the far (lower) end of the engine mount bolt location. You can see the two little openings below it, which are the only way to access the back side.

I’m at a bit of a loss as to what to do. It seems pretty much impossible to get a nut back there. I could consider a riv-nut of sorts perhaps. Even that will be a massive pain to get in place and doesn’t seem as strong as I’d want. And definitely not available in M12-1.25, but I’m okay with changing the bolt to something else.

I’d love some ideas here.

You can definitely get M12-1.75 riv nuts is you are not hung up on the 1.25 pitch. 1.75 appears to be the norm for M12, never seen 1.25 pitch. I know you can get them in steel / zinc plated which would be fairly good compatibility with the aluminum body. Might be available in aluminum from an aircraft specialist.

Old Guy said:

You can definitely get M12-1.75 riv nuts is you are not hung up on the 1.25 pitch. 1.75 appears to be the norm for M12, never seen 1.25 pitch. I know you can get them in steel / zinc plated which would be fairly good compatibility with the aluminum body. Might be available in aluminum from an aircraft specialist.

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Yea it’s weird how fine the pitch is on all three of these engine mount bolts.

I’m liking the riv nut idea okay, but it seems really hard to install one there. Just room to turn wrenches is super tight. But I’m not sure I see much option. I’d like to figure out a little better how the original nut is affixed.

Here’s what the backside looks like; I’m not sure how the captive nut was supposed to affix but I’m still likely to try rivnut from the front or a press-in nut from the back. Slightly tempted to try gluing a flange but on the back.

Bonding a nut back there was my idea. Not ideal but considering the lack of access sand how good the aluminum bonding agents are these days, that's a consideration.

RYU said:

Bonding a nut back there was my idea. Not ideal but considering the lack of access sand how good the aluminum bonding agents are these days, that's a consideration.

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Getting the back side surface prepped for the bonding agent and then getting the glue and then the nut positioned would be an 'F bomb' experience for me. You need to use an endoscopic inspection camera to see the back side of the hole and I can just see me getting a big blob of adhesive on the camera lens bringing the whole process to a halt. If you can get a flanged nut positioned on the back side of the hole the flange would give you some pretty good surface area for bonding. The biggest problem I see is that when bonding you want the bonding agent to be as thin as possible (glue itself is generally weak) between the surfaces. That requires somehow clamping the nut against the back of the body panel while the bonding agent sets up which could be a challenge.

I have never done a rivnut / nutsert as big as 12 mm. Up to 6 mm the insertion method I have used uses a flaring fitting on the top side of the nut with the correct sized bolt screwed into the rivnut. You insert the rivnut into the hole and tighten up the bolt and it presses down on the flaring fitting while the bolt pulls the blind side of the rivnut up. All you need to do is get a box end wrench on the flaring fitting to hold it in place and if you can get a ratchet with extensions onto the tightening bolt the rivnut should be doable (Sez the guy who went out to look for the right hand motor mount on his 2000 and couldn't even find it because of the stuff in the way - the air injection just adds to the mess). If the bolt / flare fitting tool is not an option for 12 mm rivnuts I don't see getting one of those two handled mungo insertion tools into position so that kind of puts you back into the glue option.

Old Guy said:

the air injection just adds to the mess

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It’s crazy, right? Moving all that stuff was early in my process.

I’m going to play with some options today and will see how it goes. The bolt-style rivnut tool would be the only option for that, but I’ll try getting something into place on the backside first.

Old Guy said:

Getting the back side surface prepped for the bonding agent and then getting the glue and then the nut positioned would be an 'F bomb' experience for me. You need to use an endoscopic inspection camera to see the back side of the hole and I can just see me getting a big blob of adhesive on the camera lens bringing the whole process to a halt. If you can get a flanged nut positioned on the back side of the hole the flange would give you some pretty good surface area for bonding. The biggest problem I see is that when bonding you want the bonding agent to be as thin as possible (glue itself is generally weak) between the surfaces. That requires somehow clamping the nut against the back of the body panel while the bonding agent sets up which could be a challenge.

I have never done a rivnut / nutsert as big as 12 mm. Up to 6 mm the insertion method I have used uses a flaring fitting on the top side of the nut with the correct sized bolt screwed into the rivnut. You insert the rivnut into the hole and tighten up the bolt and it presses down on the flaring fitting while the bolt pulls the blind side of the rivnut up. All you need to do is get a box end wrench on the flaring fitting to hold it in place and if you can get a ratchet with extensions onto the tightening bolt the rivnut should be doable (Sez the guy who went out to look for the right hand motor mount on his 2000 and couldn't even find it because of the stuff in the way - the air injection just adds to the mess). If the bolt / flare fitting tool is not an option for 12 mm rivnuts I don't see getting one of those two handled mungo insertion tools into position so that kind of puts you back into the glue option.

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I haven't been near my car all week. I'm not exactly sure where this bolt is. Access sounds like a nightmare! I'll look this weekend.

Either way does not sound like a fun job.
[MENTION=28830]jwmelvin[/MENTION] if the fender is removed a a small 1" hole is drilled on the frame somewhere can that be a possibility? I know that sounds crazy but if the hole can be created for access and it doesn't affect anything structurally a cover could be bonded back in the place. I'm sorry.. I still can't picture exactly where this bolt is until I see it on my car.

Well, I decided if I could get a nut plate in place it would be a good solution. A test convinced me I could. So I made a stainless plate with a lip that extends up a bit to stop fly from seeping into the threads, brazed a nut on it, and glued it in. I think it’s going to work.





I used a string to pull it up into place and rigged a pulley to hold pressure while it cures. Yes it horrifies me to hang a big steel weight over the body but I put some pads down just in case.

And some pics through the scope:

It should work but i sure hope you have that nut "in column" so that the bolt threads in directly. In hindsight, it might have been better to have a bolt screwed into that nut then hang your weight directly off the top of the bolt to keep things in line.

The nut is pretty well centered in the hole so I think it will be okay. It was clamped to the plate when I brazed them together and the ridge on the plate centers it in the body hole. I guess Ill know tomorrow when Input the mount on.

'stainless plate' - as in stainless steel for attachment plate? That would not have been my choice. Most stainless steels and aluminum alloys are at opposite ends of the galvanic potential chart putting you at risk for corrosion of the aluminum.

Galvanic Series (electrochemical series) (structx.com)

If the plates location is always high and dry this may be a non issue, particularly if the bonding agent completely separates the stainless from the aluminum. If there is any possibility of aluminum - stainless contact you might want to protect the joint.

Old Guy said:

If the plates location is always high and dry this may be a non issue, particularly if the bonding agent completely separates the stainless from the aluminum. If there is any possibility of aluminum - stainless contact you might want to protect the joint.

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I hear what you’re saying but I think it’s a non issue here. It does not get wet and there is no metal contact. The aluminum is painted and the glue separates them.

Ha! That did not work out. The epoxy did not bond to the body, which maybe shouldn’t be so surprising as I couldn’t prep it at all.

The nice thing is the epoxy made a nice isolation layer for the stainless plate. I made a wire holder and was able to install, and I have the benefit of no permanent modification.

Just curious how these bolts snapped? Is this a common issue with our cars?

jwmelvin said:

I hear what you’re saying but I think it’s a non issue here. It does not get wet and there is no metal contact. The aluminum is painted and the glue separates them.

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nis350 said:

Just curious how these bolts snapped? Is this a common issue with our cars?

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I’ve never heard of it so it seems pretty strange to me. I think at one point my car did not have the front motor mount connected (after a service) and maybe that stressed the right side mount enough to break the bolts? Or they were overtightened? And maybe once the mount>engine bolts broke, the engine was free to swing around and bang the mount, breaking the mount>body connection? Seems a little far fetched to me but I really have no idea.

I got it all put back together today and went for the first drive in months. Felt great.