How do you convert a 76 shovel head to 81 CI?
If you have a 74 inch engine to start with, you can install 80"
barrels on it but this would bring down the compression so low you
wouldn't be able to ride it. This would be basically the same as
boring out the 74" barrels .060 (IF the 80" barrels are standard
bore!) But your 74ci engine would only be about 76ci. To convert
your 74" to a true 80" you would need to change the flywheels to
gain the additional stroke length. (4.25 for 80" as compared to 3
31/32 for the 74").