This is an old article from our original website that someone found on our “classic” site and then sent us an e-mail letting us know it wasn’t on our new site. Thanks Mike!
There are few things more frustrating than finding damaged threads in a captive sub-frame nut, except perhaps causing the damaged threads yourself. Murphy’s Law dictates the discovery will usually take place around 3:00 on a Friday as you are are finishing up a clutch or engine repair, giving you little time to do anything but call your customer and a rental car agency. We repair about 8,000 cars a year, and we usually see this problem at least once in a year, so we’ve developed several techniques for dealing with this issue. One of those techniques is outlined below.
First, a little background on why repairing captive nut threads is such a big deal. A captive nut is a nut that is installed inside the frame, sub-frame, or unibody with a tac welds or a holding cage before the assembly is welded together in the factory. There is no way to touch the nut, so there is no easy way to replace it. The tac welds are only strong enough to hold the nut in place over the hole in the frame. Attempts to re-tap the threads to a larger size or install a Time-Sert usually result in the captive nut breaking free of the tac welds, then the nut spins freely, making it impossible to drill or tap.
We’ve found the best thing to is to spend a very short time planning, then just start working. It’s very easy to stand around staring at the underside of the car thinking, “Wow, this is really screwed up. My day was going fine and now it’s ruined. Isn’t there some easy way to fix this”. It’s best to figure out what needs to be done quickly, then start doing it. Many hands make heavy work light, so here at Art’s, when one mechanic is having a hard time, others will pitch in to help get the job done, knowing that when they are having a hard time, they’ll get the same treatment.
This car came in for a clutch job, and was scheduled to leave on the same day it came in. When we removed the crossmember, we found the rear crossmember bolts did not spin freely once they were loosened. Someone had cross-threaded them into position. Some shops might make their customer buy a new special order sub-frame, adding lots of money and time to the job. Others might find their customer a used part. And a few might just force the bolt back into position and let the next guy worry about it — as seems to be the case with this car.