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I'm installing a passenger seat in a brand new vehicle that was manufactured without the passenger seat installed ( cargo van ). The holes were already present, but no threads cut. They are painted just like the entire unibody chassis.

As part of the install, I will drill out the holes with the correct drill bit for the thread cutting. Next, cut the threads, then install the bolts.

I have read that anytime you cut a hole in the steel of the car's body, you must take measures to prevent corrosion.

What would be the appropriate measures in my case? Is there any concern about corrosion in the bolt/hole interface? By this I mean the mated threads inside the hole; seems fairly well protected from air and water in there but ...

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I wouldn't worry too much about the threads themselves - as you note they'll be protected by the threads of the bolt. If you remove a bolt that's been in place for a long time you'll notive that the bit of thread that was in contact with the nut is usually fine.

The bit to worry about is the bit of bolt that's sticking out underneath - that will corrode, and make it difficult to remove should you need to. I'd suggest paining over that with a bit of waxoyl or underseal to protect it - which will also seal the gap and stop water getting up into any gaps in the threads.

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There are a number of anti-corrosion products which you may wish to apply to the affected area. I've been using one such product called Boeshield T-9, which is a liquid solvent containing a waxy material in suspension. When the solvent evaporates, the waxy coating remains.

In Florida, high humidity and occasional salt content corrodes almost overnight anything exposed to the air. I've installed hurricane protection using zinc plated header bolts and zinc plated anchor bolts. The act of applying a nut to these bolts will scrape away the plating and promote rust.

The hurricane of 2017 (September 11) resulted in heavy use of our protection. I had applied the aforementioned coating and was pleased to see that more than one year later, the bolts are rust free and the nuts can be spun on by hand.

I'm confident there are other products of this nature. Your application is likely to be a low risk area, but it doesn't hurt to be careful as you note. The underside of the area you are drilling should be protected as well. You may be able to drip some of the protection you select into the hole and have sufficient coverage by capillary action if you are unable to access the underside.

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  • I'm a little concerned that putting something that kind of sounds like a lubricant (wax, easy to turn the bolt) will interfere with the dynamics of tightening the bolt; I'm referring to the idea that the torque specifications are all about stretching the steel of the threads to get the correct seating. Is this an issue?
    – chad
    Sep 13, 2018 at 0:20
  • I've had no problems getting fasteners to tighten properly. One of the uses I've performed was on parts of a sports trailer. No indication of loosening, but even better, no corrosion making those fasteners difficult to remove. The act of tightening displaces the coating, I believe.
    – fred_dot_u
    Sep 13, 2018 at 1:02

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