How To Get A Captive Nut Repaired In A Car Frame

When a convict bolt breaks inside a subframe, it's a bummer.

There are few things more than frustrating than finding damaged threads in a convict sub-frame nut, except perhaps causing the damaged threads yourself. Murphy's Law dictates the discovery volition usually take place around 3:00 on a Fri as you are are finishing up a clutch or engine repair, giving you lot footling time to do annihilation but call your customer and a rental machine agency. We repair about 8,000 cars a year, and we ordinarily see this problem at least one time in a year, then we've adult several techniques for dealing with this issue. Ane of those techniques is outlined below.

What is a captive nut?

Showtime, a footling background on why repairing convict nut threads is such a big bargain. A captive nut is a nut that is installed inside the frame, sub-frame, or unibody with a tac welds or a property cage before the assembly is welded together in the factory. In that location is no way to bear upon the nut, and so there is no piece of cake mode to replace information technology. The tac welds are only strong enough to hold the nut in identify over the hole in the frame. Attempts to re-tap the threads to a larger size or install a Time-Sert usually effect in the captive nut breaking free of the tac welds, then the nut spins freely, making it impossible to drill or tap.

It's all-time to not spend a lot of time moping

We've institute the best thing to is to spend a very brusque time planning, then just start working. Information technology'southward very easy to stand around staring at the underside of the motorcar thinking, "Wow, this is really screwed up. My solar day was going fine and now it'south ruined. Isn't there some easy way to ready this". It's all-time to effigy out what needs to exist done quickly, then start doing it. Many hands make heavy work light, so here at Art'south, when ane mechanic is having a hard time, others will pitch in to assist become the job done, knowing that when they are having a hard time, they'll become the same treatment.

Let'south get to information technology!

This car came in for a clutch job, and was scheduled to get out on the aforementioned twenty-four hour period information technology 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 brand their customer buy a new special order sub-frame, calculation lots of money and time to the job. Others might find their customer a used function. And a few might but force the bolt back into position and permit the next guy worry almost it — every bit seems to be the example with this automobile.

subframe removed from car sitting on a cart

Nosotros'll need some room to work

First we remove the sub-frame from the automobile. Sometimes this isn't necessary, but this time it was.

No door? Brand one

Since there is no access to the captive nut, we accept to make our ain. Using lube on your hole saw will keep it sharp a lot longer. Information technology will also make a lot of foul smelling smoke.

a mechanic cutting through metal with a hole saw with clouds of oil smoke

And here the trouble makers are

Here are the captive basics. The photos misconstrue the depth quite a bit. The nuts are a picayune over five inches past the pigsty, and the hole is two inches in diameter.

A view through a hole left by a hole saw

Out with the quondam

The damaged nuts must first exist removed. This is easy since the tac welds holding the nuts or the nut cage are always pretty weak. A quick pop with an air hammer is all it takes in about cases.

using an air hammer to knock captive nut off

A niggling prep work

Once the old captive basics have been removed, I stuff a dice grinder in the hole with a stiff wire brush to remove the paint for better quality welds and easier welding.

Paint cleaned from subframe before welding

Welding in an awkward space

This is not some of my prettiest welding, just considering I had to stuff the MIG gun into the hole past the trigger, so wedge an index finger into the hole to press it while peeking through a 1/2″ gap unoccupied by my finger or the MIG gun, it ain't too shabby.

Two nuts welded to the inside of a subframe viewed through a hole

Using a "special tool", borrowed from one of our parts room shelves, I tac weld the disc left in the hole saw dorsum in place.

A shelving bracket, nut, bolt, and washer holding parts in alignment for welding

Make it await squeamish

Side by side the residual of the gap is filled in and the bold used for the jig is welded in place. I weld the disc back in is mainly for cosmetic reasons. The sub-frame would withal be audio with the hole still in it, but in full general, we similar to practice repairs that leave the auto looking "normal".

Getting prepare for paint

A piffling work with a flap disc and the subframe is ready for some paint.

Back on the road, aforementioned 24-hour interval!

Less than an 60 minutes later, the subframe is set for re-installation. The car will exit the same solar day it came in. No waiting for a very expensive special lodge part, or calling effectually to wrecking yards to try to discover one. It's rare mechanics get to employ their skills to really fix anything. We mostly figure things out and replace parts. However, having some old-schoolhouse skills tin can sometimes salvage the day.

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