RepairOnan exhaust manifold & elbow

Eric Freedman

I was helping a fellow Amel owner with a very similar problem.

They wanted to replace both the manifold and elbow.


For you old time hot rodders, cleaning this up is very similar to porting and polishing the head of an engine.

I used a heavy-duty flex shaft grinder with a flexible carbide bit.


With a little patience the manifold and elbow looked better than new except the pitting.  I sent the elbow, and manifold

to a local machine shop. I had them fill in the pitting by welding , and then had them surface grind both items.

There were a few spots where metal was built up inside the ports due to the welding. Back to the flex shaft motor and the carbide bit. I changed the bits to a finer grit to do a super fine finish inside the ports.

He now has a head and elbow that is ported and polished like an old hot rod head.

Once I tightened down the manifold and elbow there was a tolerance of .42mm.

Just for my OCD , I added some never sees to both sides of the gasket.


I also use

To prevent the bolts from becoming rusted in place. 


It is a great product and works great on the shaft of the mainsail furling shaft.

With this product the pin just drops out with just a few light taps.


I ran into my friend about 2 years later . I took off the elbow , the bolts came out with very little effort. The repair looked perfect. While I had the elbow off used the grinder to remove the glass like carbon as our generators are never run under any great load.

Fair Winds


Kimberlite Amel Super Maramu #376,




From: On Behalf Of CW Bill Rouse
Sent: Saturday, February 18, 2023 10:26 AM
Subject: Re: [AmelYachtOwners] Onan exhaust manifold & elbow




I believe the inside of the exhaust manifold will clean up. It will not be smooth because it is a casted part. I believe what caused this is using the generator with a partially clogged exhaust system.

The challenge will be getting a good seal between the junction of the elbow and the manifold. This has deteriorated because of age and exhaust leakage. I have two suggestions with the first being the best:

  1. Take the exhaust manifold to a machine shop and have them mill the face that joins with the elbow. (This is what I would do)
  2. Try using Permatex® Ultra Copper® Maximum Temperature RTV Silicone Gasket Maker. Be sure and replace this material the next time you inspect for carbon build-up which should be every 2 years. Also, monitor the junction of the exhaust manifold and elbow for any leakage.

You should replace the exhaust elbow and its gasket. The gaskets circled in green below are made for "one-time use" and you should change them if/when the part is removed.


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CW Bill Rouse Amel Owners Yacht School

720 Winnie, Galveston Island, Texas 77550 

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On Sat, Feb 18, 2023 at 4:46 AM Neil Meyrick <Nmeyrick@...> wrote:

We are currently having work done on our Onan MDKAV and the workshop have told me that the exhaust manifold and elbow are both beyond salvage.

After spending a few hours yesterday afternoon we have managed to clean both of them up a lot, removing quite a lot of carbon and rust from the air passages of both parts, and while the passages look like they are still slightly constricted from their original diameter they are a big improvement over what was there before. 


My main concern at the moment is that there is quite a lot of lost material from rust damage at the point where the two parts join, particularly on the manifold side where the mating surface in places is eroded by a good 5mm or more into the metal casting. On the elbow side the loss of material is less, but there are a couple of spots where I can see what looks like a breach between the inner pipe where the exhaust air goes, and the outer jacket where the cooling water goes. I've tried to add a couple of photos, not sure if that will work or not. 


Has anyone had damage similar to this repaired successfully? Or more generally does this seem like something a decent machine shop should be able to fix? I’m guessing they would need to replace the lost material by welding, then grind back to produce a clean mating surface on both parts. 


If the parts do seem salvageable does anyone have any tips on how to clean the interior passages further? I wonder about using either an acid or a caustic oven cleaner to remove the stubborn carbon but don’t want to use something too aggressive and end up creating more problems.

Any suggestions would be apprecaited

Neil Meyrick
SM390 Revelation
Valencia, Spain