I am confused about what engine you have. You seem to be talking about a horizontal shaft engine while the model number shows a vertical shaft engine. Horizontal shaft engines did have two crankcase breathers while verticals only had one. And rear seal? Also, I have never seen an aluminum bore 42 series engine, 40 series, yes.

Sounds like excessively worn cylinder bores.

Walt Conner

Whoops 421417. It is a horizontal shaft. It's the PTO side oil seal the keeps blowing out.

The 1980 Briggs update announced the 421400 and the 421700 series of engines. Cast iron sleeves was listed as an option..

A blowdown compression test and a crankcase vacuum check would probably provide clues. Removing the cylinder heads to inspect the cylinders is very easy on a flathead engine.

Alright I tried another oil seal and so far it's holding and hasn't blown out. No more oil out the breather just smoke now. Guess it's tired

Did you check that the engine actually had 2 crankcase breathers, not just one and a dummy? And both working? Don't know why horizontal shaft engines had 2 breathers.

Walt Conner

Disregard everything it blew.