Hello all, I’ve a 1985 300 cu in engine and having issues keeping exhaust manifold to exhaust pipe studs tight. I loosen lock nuts, tighten stud nuts and retighten lock nuts. After a short time, maybe <200 miles it starts exhaust leak again. Not a great deal but a noticeable amount of noise. Thought about removing studs and replacing with new. Before I attempt removing the studs, thought it’d be good to have a new st on hand as the old studs will probably break. What is the size needed and has anyone else had this happen? I believe the “donut” looks to be made of a fiber material? Many thanks.

 

I don't know if this is a good idea or a bad idea but can you put more than one nut on the stud?

 

I don't think that the 300 six uses a gasket of any kind, isn't the seal a metal on metal rounded bell shape on the exhaust pipe and a rounded shape on the cast iron manifold? When these are tightened together the fit makes the seal. If the original studs are in good shape, don't mess with them. They likely will break off when you attempt to remove them, I have removed hundreds of broken manifold studs from cylinder heads and exhaust manifolds. So it there is a gasket involved I think that is the problem, look at the shape of the pipe and the manifold.

 

I put a new exhaust pipe on 5 yrs ago and it definatly has a donut between them, just can’t remember what type.

 

So your saying the studs are getting loose. Locktite should help. ?

 

Are you positive the nuts are somehow backing off? I can't see the studs themselves backing out, as it's pretty common for them to be seized. And I'm 100% in agreement with the previous comment not to try replacing the studs, at least not in situ. Your situation will quickly go from a minor recurring exhaust leak to broken studs leaving the truck out of commission.

Reading between the lines, what are the symptoms pointing towards the nuts and/or studs coming loose? Is it simply due to the exhaust leak, you notice the fasteners are loose, and you extrapolate that to mean the fasteners have worked themselve loose? My hunch is the donut itself is collapsing, and the fasteners are no longer clamped down properly but not because they turned loose. Once the gasket has collapsed, the fasteners could be free to spin, and may have indeed unthreaded themselves a little bit, but that is not the root cause.

To check: Tighten the fasteners per spec and make a little mark with a file. Reinspect when the leak returns. Or use a double nut as previously suggested. Or get special nuts with holes for safety wire and wire them together to make sure they don't spin. I highly recommend using brass nuts and lots of antiseize. The brass is sacrificial in case the nuts seize. You can still unthread them without damaging the studs. And brass would be easy to drill for safety wire:




This is one example I found online, not the specific nut you could use. Wrong material, too.

One last thing: Be sure to clean up the threads on the stud. Best if you can run a die to clean up any burrs or corrosion. At the very least, get in there with a wire brush in a Dremel. If the threads are not clean, you may get a false torque when installing the nuts. They may seem tight, but that is only resistance to torque you are feeling. Meanwhile, the joint is not clamped together adequately, causing the gasket to fail prematurely.

 

I am with Yendra that there is not a donut gasket used it is metal on metal and to double check I ccould walk out to my hot garage and dig out the log manifold from the van my motor came from.
Why I say this is the PO cut the head pipe so was going to replace it with a Walker replacement for a 81 F100 4x2 truck and the pipe would not fit, wrong bends and a different power steering pump and why I say motor came from a van.
Any way I dont remember getting a gasket with the pipe or the manifold needing one.
Here is a picture of the van exhaust manifold and you can see the donut is built in to it.

Dave ----

 

Oh just a thought use brass nuts made just for the exhaust manifold studs.
The nuts are longer and that may have something to do with them staying put?
I know it is easier to get them off when needed.
Dave ----

 

All good ideas ya’ll and thanks! Last time I removed stud nuts & cleaned stud threads with wire wheel & PB Blaster maybe 2 yrs ago.
retightened nuts to torque and added lock nuts (3rd time last 5 yrs doing this). I’m almost afraid to loosen & tighten nuts again[pushing my luck]. You might be correct in the “donut” looking part is actually made onto manifold. Remember cleaning it as well with wire wheel come to think about it, thinking it would make better seal to pipe flange.
The entire reason I started this was too stop the exhaust leak coming from this joint. Last few times I did. This process it stopped thee exhaust leak for a few months then slowly gets louder. Again, loosen stud nuts, clean everything with wire wheel, retighten nuts to torque and double nut both. Week ago last Wednesday loosened double nut and again tightened main nuts (both tightened @ 1/2 turn). Retightened double nuts very tight. Now exhaust leak has started again, slightly. It the main stud nuts aren’t becoming loose what is allowing exhaust leak/noise to happen?
note: the exhaust pipe from this joint back thru muffler & tail pipe was replaced 5 1/2 rears ago. Exhaust pipe, muffler, etc… was trashed.. wanted to replace without catalytic converter, could not find exhaust system without one for this year I6. Ordered one from NPD for 1967 F100 300 cu in that came without cat. converter. The pipe from 1967 fit perfectly, go figure. That’s what I’m running on my 85’. Didn’t want to be this wordy but thought it’d help with questions.
Still would like stud thread size to have on-hand.

 

Thought about locktite but keep reading it will burn off quickly since manifold get very hot very quickly.

 

Good point about special wire nuts! My Army MOS when I 1st joined was 67N, helicopter repair mechanic and used this type nut almost exclusively.
I can’t remember the exact standard for “twists per inch” that was required LoL. Might be a good fix if I can get the stud specs and try to find correct nuts to fit and supplier for the “wire nuts”. Thanks!

 

You do have the heat riser in place?

Just trying to understand why the two nuts keep coming loose. Most never have to problem

do you have a picture

 

At this point I would pull the manifolds as 1 unit to get a good look at the flange and the pipe.
It might be the flange is starting to fall apart?

I had 76 AMC 258 six that leaked and that did use a donut. I replaced it and it helped but also came back.
When I got it in the air on a lift and running I found the cast exhaust manifold had burned a hole in it. I think the heat riser valve got stuck and it overheated the manifold and blew it out.
I know if I was to pull it off I would have see it as it was to the block side and hard to see.
Dave ----