MTFCA Forum

The engine in my 23 runabout has an elusive engine knock. Cutting out cylinders 1,2, and 4 makes no change in the sound. When I kill cylinder 3 the noise goes away. I thought the center main may still be loose even though cutting out cylinder 2 doesnt change the sound. So, I dropped the pan and checked it with plasti gauge. It measured out at .0015."
The noise can be heard at idle and at all times when driving the car. Retarding the spark also makes the noise disappear. Any ideas?

Stephen

Wrist pin?

We had a similar knock in a fresh engine for a customer several years ago, it sounded like a diesel engine with the knock. We had the engine in and out 4 times with a complete tear down each time. Turned out to be a rod problem. Herm Koenke was the one that finally diagnosed the problem and corrected it. It took about 1 hour of his time to straighten the problem rod. He checked the other three and only tweeked them a bit. We put the engine back together and it purred like a kitten in the creamery.

Herm was very specific about his engine building technique and insisted the rods be checked AFTER the babbit was poured and the crank ends were sized. He checked the rods for twist, bend and offset. Moral of the story: I always have the rods checked before assembly.

Always check those rods. Trust but verify. Same with wrist pin fit.
The last 2 re-rebuilds I did both had slightly twisted rods.

I am making assumptions of a fresh motor. Poster doesnt indicate if the problem developed in an old or new motor.

Spark knock?

A twisted rod if run long enough will barrel the crank journal, a tell tail sign.