Re: WHAT MOTOR DO I HAVE and IS 38mm or 40mm stroke better ?
The stroke can be measured with the head on, remove the sparkplug and measure the distance from the piston to the top of the head with the piston at tdc, then move the piston to bottom dead center and measure there. Subtract the first measurement from the 2nd and you have the stroke. A unsharpened pencil will work as the depth tool.
Your results may not be the same,Greg