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Old 04-24-2014, 11:25 PM
Theon Theon is offline
Motorized Bicycle Elite Member
Join Date: Jan 2014
Location: FNQ Australia
Posts: 1,440
Default Re: Bought bottom end from Boy Go Fast, should I be worried?

your right MAP 110mm is from end to end.
Approx 90mm is from center to center.
And your right KC.
I believe the 40mm stroke gives approx. 69.5cc
and the 38mm gives approx. 66cc.

I know not everyone wants to build a race motor, however most of us want the most we can get out of our China girls with in our capabilities.
A 38mm stroke is possibly able to rev higher than a 40mm stroke motor, except the port timing is usually wrong on the 38's, the crank balance isn't good enough, the rings are too wide...
You can all have your 38mm stroke motors.
And your angle fire heads.
But not me.
And it's not all about the highest reving motor, only one of my 4 motors is going to be doin over 9000 RPM, It's about torque.
With only one gear and wanting to be able to do minimal peddling, I want my bikes to pull well from just off idle all the way to 8500 RPM. I don't ride my bikes around at 80 K all the time, however there aint a lot of intersections, other traffic or cops were I live. The traffic we do have is doing 80 K + so if I'm doin' 75K I feel that I'm not holding anyone up that might try and do an illegal pass on a blind corner and end up pushing me off the road.
I think a lot of us want our bikes to do over 40MPH and I'm only trying to help.

And larger displacement as well as longer stroke both give more torque.
As does higher compression and correct squish which are easier to get with a 40mm stroke motor.
I've only had one 38mm stroke motor, my 40mm stroke motors piss all over it.
The 38mm crank is a paper weight for me.
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