I agree with ian, i came to this conclusion many years ago. I played around with a unlined cylinder and bored it out as much as i could to fit either a VW or Jag piston i cannot remember which. I do remember that i had to bore the little end bush till it was very thin. I also had to take a couple of mm off the crown. I played around with all sorts of goldstar cams. Same as Ian said not much more top speed but loads of pulling power with a 24 engine and 19 gearbox sprocket. 4 off us went over to watch the TT on our M20s i had the big bore engine fitted to mine. I thrashed it around all week but half way through the week i noticed oil on the dynamo and the mag. The cylinder was so thin and porous at the back that oil was working its way through the cylinder casting. Back home i just change it to M21 flywheels and piston and fitted the standard cams. Engine is still like that now but its sat on the garage floor as i have fitted a special long stroke M33 engine to the bike. Big bore cylinder and piston is still on the shelf, cams went a long time ago.

I guess it's the M21 crank that does the trick. Lots of difference between an M20 and an M21 engine and there are just standard M20 cams in that engine.

Henk.

email (option): ahum@quicknet.nl

It's why I fitted an M21 crank to those crankcases I got from you Henk. It's still going strong and I still have that new B33 piston for maybe another day. I still don't ride much over 50mph, but the extra acceleration and pulling power is just nice to have. Ron

email (option): ronpier@talk21.com

Oil trough the cyl casting....have you even heard of that (!)
Changing to an OHV engine seems indeed a much better idea.

The OHV motors are better in both performance and fuel economy...Even the B31 produces 30% more power than the M20 and will do over 50% more to the gallon....However, doing some work on the SV engine helps a lot, particularly if you travel any distance on your WD bike yet want it to retain it's original appearance and general feel...Ian

email (option): ian@wright52.plus.com