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Old 04-25-2014, 01:29 AM
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?

If your piston comes to the top of the deck on your barrel it's probably a 40mm stroke.
If it sits down in the barrel at TDC a 38mm crank.
But this will depend on deck height, height of barrel and base gasket.
But generally if you want to set your squish to less than a mm on a 38mm stroke motor you will have to shave the barrel.
Again this is why the 38mm stroke motors have an angle fire ('high compression') head, to compensate for the lack of stroke not bringing the piston to deck height.
A 40mm stroke motor should give correct squish of around 1mm with a base gasket of around 0.4mm and a head gasket of around 0.5mm with a decent quality barrel. Barrel heights also differ, but are generally suited better to the 40mm crank.
A motor with properly set squish is likely to find the standard angle fire head to have a little too much compression.
A 38mm stroke motor with a standard barrel and straight fire head is likely not to give enough compression.
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