Slapping noise in top of engine

[Guest docmiller13]

I have a 1927 Pontiac that I picked up from a museum in which I was told the car sat for some 20 years. Got her running last week and noticed a slapping sound on the top of the engine...took it to one mechanic and he said those engines are very noisy and thats the way it is.

It just doesn't sound right but this is the first 1927 I've ever had...any suggestions of what to do ?

[Richard Lichtfel]

It is hard to trouble shoot this without having the engine and hearing it. It could be any number of things and is not normal. My best advice would be to take it to an older mechanic that knows engines.

[Guest Jim_Edwards]

I'd first take some sort of a stethoscope (as simple as a length of rubber hose ir tubing) and determine exactly what area of the engine the noise seems to be originating from. A "slapping" sound doesn't particularly sound like any thing internal to the engine which would likely have a metallic sound, though I have encountered piston failures on flat head engines from crystallization at the uppermost ring that would give a bit of a slapping sound as the dome of the piston hit the head but if that should be the case there should be quite a bit of grayish smoke from the exhaust pipe.

[Guest]

The first six cylinder Pontiac engines (the ones with two heads) have four piston rings, the bottom oil ring is so far below the wrist pin that it comes part way out of the cylinder. If these rings have ever been replaces with the three part oil rings it is possible for the bottom piece to come out past the cylinder wall and break the bottom of the piston off at the oil ring land.

This makes a very interesting slapping sound to a running engine. Also quite a bit of smoke as there is no oil control on the piston.

I have owned 9 of these cars and three of the engines had this problem.

I wouldn't consider rebuilding one of these engines as original. I would always use a modern aluminum piston with three narrow rings all above the wrist pin. It makes a much superior running engine.

[Steve29]

When pistons are loose in the cylinders, the skirt can slap against the cylinder wall causing a soft but very noticeable noise. When I was a teenager in the late 50’s I had a car that when it idled, you could hear soft, dull, almost nonmetallic sounds coming from the engine. I anticipated the worse but I learned that it was the piston skirts slapping the cylinder walls. The problem didn't get any worse in the two years I had the car, even with a lead footed teenager behind the wheel.