Classic/Antique Car Repair: exhaust manifold leak, flapper valve, milling machine

Question
My 1950 Buick 50 series (F263 engine) has a slight exhaust manifold leak. Not bad, just a little "rumble" at idle and on deceleration. I have been told that the most likely cause is warpage of the manifold; that the old straight eight manifolds were notorious for this problem. If possible, I would like to correct the problem. How would I go about determining the location/cause of the leak and possibly correcting it short of replacing the manifold? Thanks.
Charlie Cain
North Texas

Answer
Typically, this sort of leak is worst when the engine is cold and lessens or goes away as the engine warms up.   

The leak could be due to a missing or burned gasket, or it could be due to warpage of the manifold.  It is also possible that the noise is coming from the exhaust plumbing - either the heat riser flapper valve or the attachment to the exhaust pipe.

The only way to sort this out is to remove the manifold and inspect the machined surface that was bolted to the block, and also inspect the heat riser valve and the flange that connects to the exhaust pipe.  Before you do that, however, use a stethoscope to snoop around listening for the apparent location of the leak.  If you don't have access to a stethoscope, use a 4 foot or so length of garden hose - put one end up to your ear, plug the other ear with a wad of tissue, and wave the hose around - you'll find you can localize noises very well - just watch out for the FAN!

If you find no problem elsewhere, and no cracks are evident in the manifold itself, you can have the manifold resurfaced to make it perfectly flat.  This takes a machine shop equipped with a milling machine, and a machinist who knows what he is doing.  While the manifold is off, also inspect the mating surface on the head - it may have residue from an old gasket, or it may have some other surface problem.  That can also be resurfaced at a machine shop, if neccesary.   Make sure all the mounting bolts are in good shape.

It is not likely that you will have to replace the manifold, unless it is cracked, which is usually pretty obvious when you inspect it.

Good luck,

Dick