I have one of the 720 cc pistons from Ian I received a year ago and finally have time now to bore the cylinder for my M21. One thing I have forgotten, with all else being factory except for the big piston, what will the compression ratio be? In some of the more rural areas I may be touring on the bike, high octane fuel is not always available so will need to on occasion be using 87 octane. The bike runs fine now on 87 octane without problems. What about at 720 cc as needing to retard the spark under heavy load defeats the purpose of the big piston?
Also, I was looking at 4 1/2 thousand piston clearance or is this too tight?
I can’t directly answer your question about compression with the 720 setup, but I have one of Mr. Wright’s 720 pistons installed along with an M21 crankshaft and have to say that I’ve noticed no issues with detonation under high load. Anecdotally, my experience leads me to conclude that the original WM20 compression was low enough that even with the increased displacement cylinder utilizing the original cylinder head configuration, the compression is not dramatically increased.
And as you suggest, if detonation under high load or overheating is noticed, you have the means at hand to adjust the ignition advance as needed.
Also, .004 piston clearance seems within reason, with a proper break-in of the new top end.
Here’s an interesting link to previous discussions about this topic:
