Displacement question.

[ratdog]

this is something I was wondering about.  I was under the understanding these little guys had a 39 mm stroke.  Now it’s been a very long time since I studied this stuff in school but I thought displacement was equal to pi* r squared * h

so if we have a big bore kit with a 50 mm piston.  r=25. And h would equal 39mm.  So about 76.6 cc

stock would be 39mm piston so r= 19.5 or about 46.5 cc

am I missing something here?

[gouvis]

You are missing that the stock 139qmb stroke is 41.4, not 39mm

[ratdog]

Well that explains it then. Stupid me reading the adds for stroke cranks!

So a 50 mm BBK is really about 81 cc

And stock is about 49.5 cc

Makes more sense when yo have the correct values!

Thanks!

[gouvis]

You're welcome