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Discussion Starter · #1 ·
Wow. Didn't see THIS one coming. Some time ago, I swapped out my transmission mounts (the two on the driver's side, holding the end of the transmission in place). I remember having some trouble getting one of the bolts in, and when I tightened it, it stripped the threads with what seemed like very little torque. I just put it down to my manly strength, and didn't sweat it because there are two bolts holding that mount to the transmission housing, and another mount just a foot away.

Then some months later, I noticed that the subframe and the two mounts were wet with transmission fluid. I looked around, and it LOOKED like the leak was at the bolt, but "no way" that there could be a leak at a bolt hole, so I cleaned the area thoroughly and drove the car for a couple days so I could better located the REAL source. Pulled the wheel, got under the car and it sure LOOKED like that's where the leak was. Hmmmm. Pulled the mount, leaving the bolt hole open. No leak. OK, started the car, went P-R-N-D-N-R-P on the shifter, and checked again.

WOW. Fluid dribbling out the bolt hole. How can that happen? So I got out my trusty borescope (thank goodness I have one!), looked inside and saw this...


It's clear that what happened was that the casing is paper-thin inside that bolt hole, and that the torque of installing the bolt broke a "chunk" out of the case - you can clearly see the section of threaded material canted off to the side.

I looked at a lot of options. I ruled out drilling and tapping this, since I didn't want metal chips getting into the transmission through the crack. I looked at various types of plugs or helicoils, etc., but really all of them seemed to be either a little too big, or a little too small. I figured I could probably slather an SAE helicoil with red Loctite and get it to "stick", but it seemed risky, and there's no way I'd ever count on it to hold a bolt again for any purpose other than just plugging the hole.

In the end, I opted for some Quiksteel putty. This is a two-part epoxy that you mix by kneading it until it's a uniform color, and it stays workable for a few minutes after that (but ONLY a few minutes). I just rolled a cylinder a little smaller than the bolt hole, and slipped it into the hole, then added a blob on top, and compressed it inward with pressure from my thumb. I did this a few more times to ensure that the putty was well and truly packed into the threads around the hole (you can't tell from the photo, but there were a few threads intact toward the outside of the hole). I'm trusting that this stuff is solid enough that it's not likely to squeeze through the crack and THEN break off. There's no real pressure at that point, so I'm confident that the repair will keep the fluid from escaping through the hole. A marble-size chunk of the putty hardened to a virtual rock within 10-15 minutes, and the product suggests that it can be drilled in one hour. As a result, I'm calling it fixed. I'm not concerned about the tranny mount struggling to keep everything where it should be - there is plenty of redundancy in that system, IMHO. FYI, here's what that front mount looks like (the bolt in question goes through the upper hole, into a hole tapped into the transmission case).


I posted this to create an archive for anyone else who wonders why they have a leak at the transmission mount bolt hole. Who woulda thunk it? ;-)
 

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Yep. Not as uncommon as you may think. When I rebuilt the engine in my Jeep there were a few fasteners that went through the oil passages and water jacket. You had to be careful not only what torque you used for each fastener, but also which length fastener you used, as too long could crack the block and too short would fail to seal properly. A few bolts even required a special two-part sealant.
 

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Discussion Starter · #3 ·
Ouch. That had to be "interesting" to figure out. At least I know that the bolts were the right length (they're all the same). I tend not to over-torque things, though anything is possible. My recollection is that I felt the bolt spin out WAY before I got to anything resembling "too much torque". Maybe just getting the bolt started was all it took to knock that chunk out of the threading (I had to get the bolt inserted and then use a little force to get it lined up by stretching the rubber mount a little. Again, hard to imagine that breaking anything, but from what I can see, these mounts don't fail very often, so there's probably not much in the way of history of this kind of thing happening.
 

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That looks more like a material failure than an over torque failure. Might have been a flaw in the casting process or when it was threaded during manufacturing.

Quick steel is pretty tough. When I was teaching my son to use a torque wrench, he over torqued a bolt on a Miata engine. Filled it ground it flush then drilled and tapped. Never failed.
 
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