1938 B manifold

Yes, there was a bolt that held the muffler to the manifold. It was a 3/8" x 1" long square head machine bolt that was inserted through the hole from the inside. The muffler tubing has a slot in it that fits down past the bolt. A nut and lockwasher were used on the exterior to tighten and clamp the muffler to the manifold. I have used hex head bolts (with a flat washer) as a replacement for the original square head one. In that case, I would tighten down the nut to force the washer to conform to the ID of the manifold before inserting the muffler. I suggest a brass nut be used. That original design works poorly, IMO, when used with replacement through-the-hood style mufflers. The problem is that rain water can run down the muffler inlet tubing and enter the manifold and then get into the cylinders. The original style under-hood muffler prevented that from happening by virtue of the design. I have gone back to the original under-hood style muffler for that very reason. Later designs had different mufflers and different manifolds that used two attaching bolts inserted into tapped holes in the manifold from the exterior. Those mufflers, even though they were through-the-hood, were designed in such a way as to provide a shielded area where the inlet tube enters the manifold in order to keep water out.

Rod

Notice the type of manifold gasket used in the drawings.

I never saw paint problems on the hoods where any of the old under the hood mufflers were used. If you were to drastically in crease the compression you might see a hotter muffler that would discolor the paint.

I don't know if you know the history of your tractor. Some people with those early manifolds do, in fact, tap the exhaust opening for a length of pipe (1-1/4 dia?) and either use a different muffler or expand the inlet tube to fit down over that length of pipe. That would also solve the water intrusion problem. If you always shed your tractor, the matter may not be a concern to you.

I don't remember, since the last time my B had an original underhood muffler was over 40 years ago. At the time, I replaced it with a through-the-hood style one because those were readily available and cheap. I don't recall seeing any blistered paint but at that point there wasn't that much paint left on the tractor anyway. In any event, to prevent the possibility of any heat affecting new paint, I used an aluminized heat shield on the inside of the hood in the area of the muffler. It's available from Eastwood and attaches by a very strong adhesive. I installed the shield after painting and before installation of the hood. If there was ever a problem originally, it has now been solved since there has been no adverse affect on the paint on my working B. And the water-in-the-cylinders problem has gone away with the underhood muffler. My B is not under cover and water in the cylinders has been an annoying problem for many years. It was never enough to cause a hydraulic lock and prevent the tractor from starting but when it was started, there would be a lot of sooty water droplets blown out the exhaust and all over the hood.

Be prepared for a price shock on a new, underhood style, OEM (Nelson) muffler - over $100! There are cheaper knock-offs available but I am advised that they can have an interference problem with the hood. It's real tight under there and small changes in the dimensions can cause a problem.

Rod