Need help with a stripped-out threaded hole
 

Hi everyone - I'm a long-time lurker, first time poster here. I ran into a major snag while wrenching on my car today, and I’d like to see if anyone here has ever run into something similar, and if so, I'm curious about how you addressed the problem.

At some point during my car’s life (’83 944 that is now a NASA 944-Spec racer), someone has done some combination of cross-threading/stripping/over-torquing/clumsily re-drilling and tapping the outboard hole that holds the casterblock and rear part of the driver’s-side front control arm to the car. (Actually, they also managed to munge up the inboard hole on this side, too, but that has turned out to be much easier to fix than the outboard hole.)

This particular hole is a threaded hole in the body of the car, in a fully closed-off section of the chassis (i.e. there’s no way to just drill out the top of the section and put a nut on the end of the bolt as it comes through). Cleaning out the thread did not provide enough bite for the bolt to take any torque (the bolt just spins and spins once the head has contacted the bottom of the control arm.)

My current, best option to fix this problem is to drill out the hole, and re-tap it to accept an M12 * 1.5 bolt (the original hole as delivered from the factory was for an M10 bolt, so this basically makes it one size bigger.)

If the drill/re-tap solution doesn’t work, then much more drastic measures will need to be taken to essentially fabricate a new mount for this. This would be specialized enough work that the timing of having this done will put the biggest race weekend of the season (Miller Motorsports Park in two weeks) in jeopardy, which is not something that I really want to think about at this point.

So, I’m curious if anyone has ever dealt with a stripped-out threaded hole in a chassis (whether this specific hole on the 944, or any similar hole on any other car). If so, how did you fix the problem, and how well did your fix hold up? The process of drilling and tapping a larger hole is destructive enough that I’d like to find out if there are other less-destructive options that have worked for those in a similar situation in the past.

Thanks for any help - I always knew something like this would occur during the build…I just hoped it wouldn’t happen so soon before the biggest race of the year, on such a critical component of the car!

Hmmm, I'm not familiar with the bolt you're refering to, however, here are the options I see:
1) Go with the M12 bolt
2) Helicoil, is there alot of strain on this bolt? A helicoil may not work if that's the case
3) Weld it up, redrill and tap it

Good luck and let us know how it pans out.

pics?

Unfortunately I don't have any good pics of the hole in question, but the bolts that we're dealing with are #11 in the attached parts diagram (this isn't 100% the same control arm, but it's close enough to give an idea of the part of the car I'm dealing with).

http://forums.pelicanparts.com/uploa...1212410342.jpg

Thanks for your suggestions. I would guess that there would be a lot of shear stress on the bolt, but probably not as much tension stress, so I'm not sure how much that would affect the Helicoil solution (though several others have recommended that same solution, so it might be worth trying).

Yes, I'll definitely report back with an update on whatever I end up doing, and how it worked out.

Thanks again!

Someone on Rennlist just had this happen to them as well. Make sure that you don't overtighten these bolts, as it will lead to a lot of heartache.

The solution that some have done involved cutting the chassis in that area to expose the stripped nut, then re-tacking a new one, then rewelding it all back together.

Don't think I would trust a heli-coil for such an important suspension location.

Personally, I like your strategy of trying to drill it out to the next size, although I might consider going to an American thread, if that is closer to the original.

Good luck!

Keith

The Helicoil has been suggested by several as a potential solution, but I’ve also received some responses that indicate that it might not be strong enough for this particular application. Does anyone have info on Helicoil failures in high-stress locations, or any info on what the weak point of a Helicoil is (and therefore what applications it can be considered for, and which ones it can’t)?

I agree that this is an important joint that sees a lot of shear stress (at the very least), and thus is one that should be kept as strong as possible. Though, from what I’ve been able to learn about this joint, it seems to be a relatively common point of failure on the car, anyway, even in its original factory condition.

Thanks for all the help!

Helicoil
 

I used helicoil on my race car. A helicoil if done right will be stronger then the original thread. It has stood up to many miles of very hard racing with a few off track excursions. I check the torque after every race weekend and have yet to find they have backed off. Going to the next size of bolt will leave less wall thickness in the original weld nuts and the thread will be much weaker than the helicoil thread.

Get someone who has done helicoiling before to help you. The hole needs to be clean and round after drilling and the tap has to go in very straight.

One other option, not necessarily pretty, but effective, is to weld in a stud (M10) and use a nut on the lower end. I have had to do that to a similar bolt on my 83 at the rear transmission mount. It's working fine, although not subject to the same stresses as the front control arm.

^ that sounds like a pretty good idea to me. only concern is will the bead around the bolt interfere with other parts, or will you have to grind it flush, and then how well will it hold if you have to grind it flush. I don't really have the answers to those questions, I'm just asking them LOL.

As for the helicoil, what size is the head studs on the 911's? I know they have something sort of similar that they use on the head studs and that obviously would take a pounding, just not sure of the size/logistics there.

I have that same problem in my old Jetta! the holes for the bolts that hold the control arms are completely stripped out. I was doubting the helicoil, But Vfourfast says he used it on his race car, so i haven't decided what to use yet. Maybe a slightly larger American bold will do the trick.