Cross threaded trans bolt

[Jprather]

Forum,

I cross threaded 1 of the 2 large bolts on my framehorn that hold in the gearbox. Any ideas how to repair? I took it to my mechanic and it is too large for him. He suggested finding a large equipment repair service.

Are there any tools available for a garage mechanic like me? Any recommendations on a shop that could fix it? I'm here in Southern Califirnia and will tow anywhere. I just need it repaired.

Thanks

[Marc]

The thread's m18x1.5 - same as the O2 sensors on newer cars. If the threads aren't buggered too badly a "chaser" should be enough to fix them. http://www.walmart.com/ip/17013375?wmls ... 91&veh=sem
For more serious damage a legit tap could be needed, they're more costly (but I've seen 'em on eBay for <10).
Hopefully you'll never need it again, so you might also ask your mechanic if he'd buy one for his shop (he really should keep one around) if you chip in a few bucks for the rights to use it.

In a pinch you can also make your own chaser by grinding/filing 3 or 4 notches up the threads of one of the bolts, ~1/8" deep at the end and tapering off towards the head. The bolt should still be serviceable afterwards. If you buy a new bolt, note that the pre`73 ones with the 27mm (1-1/16") head have a shorter shank than the late ones which have to pass through a thicker cradle...they also went from a wavy washer to a thicker split lockwasher on the `73-up bolts. Using the longer bolt with the early cradle may reach more threads if they're weak, just be sure to use enough washer thickness that it torques up before bottoming out at the shank.

Sometimes a chassis escaped the factory with a little weld bead slightly occluding the opening of the horn, making it difficult to get the tap/chaser/bolt started straight. If that's an issue, use a Dremel or similar tool to grind it back some. If the holes in the cradle aren't perfectly concentric that can fight you too, so it may need to be removed until you get the threads restored. Even if it doesn't stop the bolt threads from starting, when you tighten the bolt fully it places a side load on the the threads which can damage them, so make sure you can run the bolt in all the way by hand (or very light wrenching) before you go for final assembly.

Thread-cutting oil's best but any lube, or even WD-40, is better than dry. If you've got compressed air, blow the hole clean/dry; if not sacrifice an old toothbrush to make sure all the grunge is removed before threading in the bolt.

[Ol'fogasaurus]

"J", To add some to what Marc said. A thread chasing bolt looks similar to a regular bolt but it has several groves parallel to the shank that go through the threads; they are for cleaning the threads and collecting debris. They are often used for straightening the threads assuming you didn't cross-thread down to far.

Tap and dies cut threads and can thin out or even destroy the threads if you are using either the tap or die to straighten the threads.

Going way back time wise I seem to remember seeing a weird tool used for fixing threads. It kind of looked like a bolt but had a hole bored through it full length. The threads were cut similar to a thread chasing bolt but the cut was complete into the bored shaft in the center of the tool. The tool was designed to slide into the threads deeply then a tapered rod was inserted into the bore and lightly pushed in until the threads of the tool slipped into the threads you were trying to fix; a lot of twisting and fitting was done to be sure you were fully into the threads. Once the threads were properly seated you drove the tapered rod in for a tight fit and then backed the bolt out. The idea was to match the damaged threads to the still good deep threads in one long pass. You then hand threaded in a bolt to see if the fix worked.

Getting a bit basic: the bolt does the heavy lifting but the threads do the holding and you never put threads in shear; they are not designed for this kind of work for several reasons. When you booger up the threads or thin them out re-tapping you do reduce their strength which can be a lot. When thinking of threads and how they work (most of us don't or don't care enough to consider them) on a nut or bolt think of it this way: curve your fingers in both hands and with one hand facing up and the other hand facing down. Locking your fingers together and hooking one hand into the other pull opposing; this is how threads work. As you crank down on the fasteners you are also stretching the material of both objects which is why some fasteners you measure not by applied torque but by how much things have stretched.

As Marc said you want to be able to get down to the good threads and use them even if it means you have to get a new longer bolt of the same grade. Remember, if it doesn't feel right then don't force it. Sounds a bit late but it is a good thing to remember.

Lee

[Jprather]

Thanks Guys. I will give it a try and report back.