PORTING THE GRUBEE CYLINDER

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
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 
48cc      140.7      110.3       113        
66cc      141.6      114.7       117.6     


To have better power for street use I recommend you change the distances from the port edge to the TDC line (deck) on the cylinder* to these for exhaust durations of 150 for the 48cc and of 165 for the 66cc, and for 120 intake duration:
exhaust   intake 
25.9mm   55.9mm (48cc)
24.9mm   56.4mm (66cc)

* TDC line is the point on the cylinder where the top of the piston is at the top of its movement.

If you use a porting durations calculator such as Jaaps then you'll need to know these specs also:
engine  stroke    con rod
48cc      38mm    85mm
66cc      40mm    89mm

Here is my method for measuring deck height accurately which takes into account expansion by heat:
Measure from the cylinder top to the top ring markings on the cylinder. That is A. Measure from top of piston ring to top edge of piston. That is B. Subtract B from A to get C, the deck height.


HOME