Years ago when I bought my 261 engine from a Junk Yard, I was told it was a 235 and out of a 59 school bus. When I got it home and ran the numbers it turned out to be a 59 261, (lucky Me). It had been rebuilt before (was 0.30) over size but had rusty cylinders from sitting in then yard. But it had a 235 gasket and a 235 head (848 head no steam holes). When I had it bored 0.60 over and reassembled it I drilled the steam holes in the head, and used the correct gasket. I always wondered if it would make a difference not having the right gasket and the head not being the correct head, (steam holes).
