I have a stock main cat that I bought with very low mileage to replace my original.

I installed it with brand new mazda nuts, copper anti-seize, chased the studs with a brand-new M12x1.5 die, torqued to about 50ft-lb, and only drove 3k miles on them. Both threaded on with light consistent torque.

So I removed the cat yesterday to change transmissions and the threads on two fo the studs started ripping up.

Any suggestions how to clean up the threads so they aren't going to gall and tear apart again? Should I try nickel anti-seize? Grind out the locking thread at the top of the mazda nut? Another type of nut?

I'm going crazy with this exhaust stuff, really.

 

In my experience, tearing up threads on the bolt/stud are directly related to three things:
1. Nut being harder than stud,
2. Stud & nut being similar enough in metallurgy (stainless on stainless is the worst) that they can gall, and
3. Nut being an all-metal lock nut

On my F2000 race car, I have had similar problems with "Jet-nuts" ruining the threads on (softer) bolts. Anti-seize does not seem to consistently prevent this. The only thing that consistently seems to work is to use nuts that are softer or of different metal (brass) than the bolts/studs. Of course, brass nuts usually do not have sufficient torque capacity, so that leaves soft nuts, or hardened studs.

An additional factor here is, of course, the exhaust heat, which exacerbates everything involved.

So, IMO, the solution would be a different non-metal lock nut type, preferably softer than the stud. If there is room, double nutting would be the solution.

Dave

 

Chase the threads again, this time run a tap up thru the locking nuts also. Copper anti-sieze should work fine. Add a flat washer to each stud for the nut to ride on. Run it for 50 miles or so then take them loose again - I bet they run off easily.

Later

 

Another solution would be to remove the studs, and use bolts. If something gets messed up, it can be easily replaced.

The simplest solution, of course, would be to quit messing around with the cat . Just bolt it up, and leave it alone .

 

I'll have to mail order for an M12x1.5 tap.

I found I have some M12x1.5 plain nuts in zinc plated steel. They ran onto the studs nicely. I suspect the locking bits on the OEM nut is what tears up the threads.

I'll try the zinc-plated steel with flat washers for a while and see what happens. At least the metals will be dissimilar.

I'd like to press out the studs but I tried it before, then took it to a machine shop who claimed it was a major PITA. What method reliably works for doing it?

 

Pressed in studs, huh? The studs on my stock cats are threaded in--I just looked. I have been using a harder grade of bolt e.g., M10 (or, alternatively, you can use 7/16" grade 8 nuts and bolts, which are readily available and work nicely). Self locking nuts will mess up the threads if they are too tight.

 

I did notice the back of the stud had a pair of flats on it, so perhaps removing them is as simple as screwing them back out.

Right now I have zinc plated steel nuts on. Next week when the Mazda nuts arrive I might try tapping them to remove the self-locking bit and use them instead. I'll also pick up some nickel anti-seize.

Dave

 

The original studs in the cat are very hard to remove. I had the flange glowing red hot with my gas torch and those things still required some effort to turn. I ended up replacing the studs with bolts because my downpipe was too short:

http://metal-matrix.com/rotormafia/downpipe/

I would not use the locking metal nut for the reasons you've described. Instead, use a regular nut with a flat washer + lock washer or use a flat washer and double nut with a pair of jam nuts (they are thinner than normal). It's hard to tell in the above pic, but there is a flat washer + lock washer + regular nut. That setup never came loose. If it had, i'd double nut with with jam nuts.

Sonny

 

I've used a Grade 8 bolt, and then copper nuts from Wurth with nickle high-temp anti-seize. High-temp anti-seize really works!

:-) neil

 

To remove stubborn studs, block plugs, etc., in cast iron or steel: 1) Heat it with a torch, including the area around the stud until it's a dull red heat. 2) Immeadiately upon removing the heat, liberally squirt some penetrating fluid (I use WD-40) onto the stud where it joins the flange. It will smoke like hell, but don't worry about it. As the assy cools, it will draw the fluid into the threads and make the stud much easier to remove 3) using double nuts or even large vise grips.

 

one of the updates to the FD was to change from studs to bolts. the bolts are available separately.

also the factory lock nuts, lock on by having the top threads compressed into an oval, so they kinda do damage the threads.