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1940 248 Exhaust Leak


Daves1940Buick56S
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Well, I noticed on the way home yesterday that I had what sounded like a pretty good exhaust leak coming from under the hood somewhere. I had jsut finished with the engine rebuild before I sent it off for the underneath work, so I had no chance to retorque anything. I retorqued the manifold this morning, and it needed it - each nut abt 1/4 turn to 25 lb-ft.

But the leak turned out to be coming from the hot box to exhaust pipe connection (I did the old rubber tube and funnel trick and ran along the manifold/cyl head connection to confirm no leaks there). I had put in a new doughnut when reassembling last June. The bolts were pretty loose this morning so I tightened them up, but the leak is still there. Any suggestions here? Do I need another new doughnut, IOW once it blows through it is shot? Or can I use something like the Permatex copper hi temp sealant?

Cheers, Dave

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Yes, it is possible that the donut might have been damaged by the hot gases and might not be capable of forming a tight seal.

I would disconnect the exhaust pipe and check the donut and all the mating surfaces.

Also, make sure that there is no carbon build-up or corrosion that is preventing the donut and all the mating surfaces (on the manifold and the exhaust pipe) and the exhaust pipe from coming together properly.

I can't remember if the end of the exhaust pipe has an insert to protect the inner diemater of the donut. If it does, make sure the insert is in good condition, not badly corroded or crusted up with carbon. Also verify that the insert is the proper length (not too short or too long).

Also, verify that the flared edges of the exhaust pipe are not contacting the mating flange area on the manifold or hot box. Sometimes, the flare can be a little too tall or a little too wide and it can come in contact with the mating part, preventing the pipe from sealing tightly against the donut.

Good luck!

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Thanks!

Update - got the flange off and dropped the pipe enough to get the donut out. Roached. Pic attached. Blew out on 2 sides. No carbon built up (it was a "new" manifold and thoroughly cleaned up). I ordered new donuts - after looking it may take 2 for a good seal. I also unhooked the front of the muffler to give me some play in the pipe so I can ensure it is properly aligned going back in.

Cheers, Dave

post-87161-143142803489_thumb.jpg

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  • 2 weeks later...

Update - got the new donuts. Using a small floor jack to press up the exhaust pipe into the hot box I did a fit check on the flange clearance. It is OK for 1 donut, so I put it in using plenty of Permatex Ultra Copper. It seals nice now, and I will retorque every couple of months for a while to make sure it doesn't get loose on me again.

Cheers, Dave

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