this is from my site:
Here are the measured distances from the top of the stock cylinder:
engine exhaust transfers intake* piston skirt**
48cc 28mm 32mm 55.5mm 44.8mm
60cc 28.7mm 33.1mm 56.3mm 45mm
66cc 29.3mm 33.1mm 57mm 44.7mm
* measured to bottom of intake port
** measured from top edge of piston
(66cc is the cylinder the factory calls 80cc. It is 66cc when measured the most common way.)
Assuming a .7mm deck height (distance from top edge of piston to top of cylinder at TDC) here are the port durations in degrees:
engine exhaust transfers intake blowdown*
48cc 140.7 110.3 113 15.2
60cc 135.7 100.6 116.8 17.6
66cc 141.6 114.7 117.6 13.5
* blowdown is the number of degrees that the exhaust gas has to exit the exhaust port before the transfers open. For this engine 20 degrees is perfect if ported per my recommendations.
To have perfect 157/117/120 degree port durations and 20 degree blowdown change the distances to these:
exhaust transfers intake or piston
25.6mm 31.2mm 56.8mm 44.5mm (48cc, 60cc)
26.9mm 32.8mm 57.1mm 44.6mm (66cc)[/FONT]