I know lock nuts I think, have nylon in them? I could be wrong - can you use them on exhaust parts, or with they get to hot and melt the nylon or whatever it is?

Yes, the plastic in those locknuts will melt if used on exhaust.

There are some locknuts that grip by being slightly oval - perhaps try those?

Or Rodney Dickman ( www.rodneydickman.com ) sells entire nut and stud kits... perhaps take a look there?

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Patrick W. Heinske -- LZeitgeist@aol.com
1988 Red Fiero Formula Convertible
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There are several different types of self-locking nuts, part of a family that is more formally called "prevailing torque fasteners." They are widely used in aircraft applications.

As you have already figured out, do not use nylon-insert or fiber-insert lock nuts in high-temperature service. There are several types of all-metal lock nuts that you should use instead. All of these designs use a more-or-less standard-looking nut that has had its threads carefully deformed in manufacture, which leads to a slight interference fit when assembled onto a bolt or stud. These all-metal lock nuts are widely available in AN, MS, NAS, and SAE (all inch) standards from aircraft hardware suppliers, but they are harder to find in metric sizes ... at least in the U.S.

An industrial bolt and nut specialist such as Bosco would be one place to start. I know that the V6 exhaust manifold stud kits Rodney Dickman sells includes all-metal locking nuts, so you might e-mail and ask him about his sources.

McMaster-Carr has several different styles

www.mcmaster.com

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1986 SE 350 V8

Why not use lock-washers instead?

and use anti-sezzzzzzzzzzzzzzz!

I have seen split lock washers used with cast iron exhaust manifolds in older automobiles, but lock washers of any kind aren't considered secure enough in a high-vibration environment. Standard aircraft practice requires either prevailing-torque fasteners, castellated nuts and cotter pins, or safety wiring of multiple fasteners. Positive locking with a cotter pin or safety wire is required where the fastener itself is able to rotate in place.

[This message has been edited by Marvin McInnis (edited 02-13-2008).]

quote Originally posted by Marvin McInnis: but lock washers of any kind aren't considered secure enough in a high-vibration environment.

excatly, I have nuts with lock washers on them on my exhaust, with poly urethane engine mounts, and my one of them is missing.

If you look at the exhaust nuts used on todays vehicles, they mostly use the ones that have an oval shape in the threaded area. I have never seen one of those come off in any application. Maybe get some from a dealer for a newer car.

-JOE

I have a 2001 8.1 Litre Vortec that had a problem with one exhaust nut backing off after only one heat cycle it was replaced twice under warranty and still did the same thing. I bought another nut and after torquing the nut up to spec added the second nut as a jamb nut hasn't leaked since. That was six years ago.

Deformed-thread fasteners are better for hot locations. Lockwashers tend to lose their temper and effectiveness over time.

JazzMan