A crankcase leak might be a valid guess, but probably not it. I’d sooner say the slow speed circuit in the carburetor is blocked. Or the carb butterfly is opening way too soon. Or the main bearings are worn out–but you would have noticed that, right?

OK, first let me say that there were two 12hp motors for 1957, the “Standard” and “Deluxe”. Most obvious difference was the Standard has an integral gas tank, Deluxe has a remote gas tank. There are many other differences also. Give me a model number and I can post the parts list.

Back to the seals question, the Standard doesn’t even have any seals. It relies on the long cast-in bronze bushings to create enough seal–and does it quite well unless they are worn out. The Deluxe has a different exhaust housing, which necessitated adding a seal on the lower end of the crankshaft to keep water out of the crankcase (not to keep pressure in).

About that seal: As I said, it is to keep water out. The motor will run very well with a bad seal, which is a problem because water is getting in un-noticed, which will cause it to throw a rod. Worse, the 303804 seal was garbage and failed often. It is what trashed many a motor in the day.

Waiting for a model number.

Surprise, 9016A is a 1954-55 12D11 Deluxe. The Deluxe became the Standard in later years when the remote tank came into being and became the new Deluxe. Confused yet?

Anyhoo, here ya go—-

• This reply was modified 5 years, 10 months ago by frankr.

Lastly—-