I do have the spare '74 block.
Let's see if there is any play in your pistons. If it's always the 'right one' ... when you 'rebuilt the bike' did you replace the con-rod bearings?
'85 R80RT G/Sified
<|> '91 R100GS Bumblebee (101K miles and climbing) <|> '73 R75/5 LWB
Airhead Zen: Ride - Repair - Ride - Maintain - Ride - Repeat