What is the curent thinking on exhaust stud nuts, Copper or regular? Options

[thomasotten]

Are the copper coated nuts the way to go, or is using regular bolts, double bolting the way. I read something yesterday that said the copper nuts should only be used once...why?? Should I coat the studs with anything?

[Jake Raby]

The Milk of Magnesia is the worlds best super high temperature anti-seize, I learned the trick from using it on ignitor plugs and exhaust flanges on Turbo haft jet engines. Our exhaust flanges saw temps of over 3,000F for hours at a time.

The most odd part of that was the fact that a stainless steel double locking nut was used on a mild steel bolt for those flanges and they didn't rust or seize into place when the MOM trick was used. I also use MOM on spark plugs. MOM has a high concentration of pure magnesium in it's composition, with it comes a non lubricant based anti-seize that doesn't burn off or go away with time.

Sometimes on test engines we can see EGT of 1400F for hours at a time, and then change exhaust systems as much as 6 times in one day. After you have tried everything in the book to avoid issues when doing this and one thing repeatedly works over and over again you tend to note the trends and give the engine what it wants. There is nothing worse during development than a pesky exhaust system change that takes more than 15 minutes to do, the whole time you are losing the weather and inviting variables- I avoid this at all costs.

There is also a difference in staniless nuits, some are pure stainless (what I use) and others are just stainless coated nuts, with temperature these will rust.

The double nutting keeps the nuts from relaxing on the studs after heat cycles and falling off because of lost torque from heat cycling. The double nuts also keep thread protrusion from occuring where rust will form and then jam between threads when the fastener is attempted to be loosened. If you have ever loosened an exhaust nut, broke torque on it and then it got tighter you have experienced the effects of crusty threads that cause more snapped studs than anything else combined.

The second nut serves dual purpose as a loking device for the primary nut as well as avoiding thread protrusion, resulting in nice clean threads that are easy to remove years later.

The 911 nuts also avoid thread protrusion with their design.

Remember, at the port exit (flange) temperatures stay over 1000F most all the time with spikes to over 1200F on most all engines daily. That heat will cause issues over time if given the opportunity.

BTW- I have tried full stainless studs in this area coupled to stainless nuts. What was the result? The studs EXPLODED as soon as the car got 3 laps under it's belt and the exhaust system damn near fell off on the track before we could get it back to the pits.

So, whats the significance of this picture? This is a picture of the missing fence bolt that I robbed at the track in July when during a test day with the FP car a stainless steel flange bolt that Len used to fasten the stub pipe to the header also exploded. I robbed this crappy galvanized grade two bolt from the fence along with it's nut and we finished our test work. Len came off the track, we heard a small metallic pop and looked under the car to find the bolt and the nut shattered in 3 pieces laying under the car.


Improvise, Adapt and Overcome.