1978 6 HP Johnson Low Compression

[BRUPP79]

Thanks guys, I got the pistons back in but I'll take them out tomorrow and check the clearance of the rings before I put it back together. The clearance between the pistons and the cylinder walls was very close. I did place the head on a sheet of heavy glass and sanded the head using 220 then 500 then 1200 and finally 2000 grit paper and it appears to be flat. I did use a new Sierra head gasket and I will use another new Sierra head gasket when I put it back together again.

Good that you flattened the head, but I would have stopped at 220 grit, maybe even 150 with a "figure 8" pattern. The gasket needs a surface with a bite to it for the gasket to grip, so to speak. Smooth, but not too smooth if that doesn't sound to vague. Some prefer a more fine finish, but that is close to what I have always used.
I did some searching today as I have a 76 Evin-Johnson 6hp combo from a bucket-O-parts, I did finally find STD pistons and rings incase I rebuild mine instead of a new 4 stroke.

 

[bigjohnson]

I also have a 6r78b johnson 6hp and it starts and runs great, especially
Since I just did a carb rebuild fuel pump and impeller.