I have heard mention of M20's even bigger than 720cc. One supposedly over 800cc. I am assuming the stroke needs to be longer even with the M21 crankshaft. What is the biggest one out there?
There's one out there and running at 812cc...This was achieved by increasing the stroke beyond the 112mm stroke of the M21 as well as over boring...
720cc is the biggest I've done so far and I've also done a B33 to the same size...I'm currently looking at doing a bigger B33 after completing the 600cc Goldie I'm currently building...I find 'oversizing' engines tends to be a bit addictive...Ian:laughing:
I don't know about performance figures, but you can up gear the bike for lower rev cruising and have much more pulling power for hills and overtaking.
I just built a 720 bike for a mate and gave it standard M21 solo gearing (21T engine sprocket). You have to start with a post war barrel with no liner fitted. The 98mm piston must be as big as you dare go, as my engineer said that after the re-bore, the the wall thickness near the bottom of the barrel on the left side was just about 3mm.
There is a guy in Sweden? who is/was building an 800cc plus engine, but he made his own bespoke barrel.
I'm more than happy with my 600cc (20T sprocket) But I do have a new 95mm piston in stock for 635cc......One day maybe, just for fun!!
Ron
Hi Ron...Don't you mean 90mm for the largest piston and 85mm (B33 'long rod') for the 635cc engine?....
It's worth noting that you won't turn the M20/21 into a fast bike without other modifications whatever capacity you give it...Rather it just pulls much better up hills, will maintain a better, slightly increased cruising speed and top speed and as a bonus, uses less fuel in the larger capacity as it is not being revved so much in lower gears up hills...
The 'Swedish' bike is up and running at 812cc (from memory) He made his own flywheels to get that capacity...I think the barrel remained unaltered apart from overboring...He also made himself a new alloy cylinder head from solid...There's a clip of the bike running on You Tube as I recall...Ian
My indifferent attempt at a larger engine had a bore of 91mm. Still got the barrel, if I do anything with it I'll probably try to get it sleeved to standard size.
