Ted,

This formula should make things easier for you:

Chord Width = (Bore) x [(Arc Width x 180) / (Pi x Bore)]

Arc width means the size of the port as measured along its circumference.

Chord width means the size of the port if you were to take the liner alone and measure it with calipers.

The formula for 75% is for Chord width, NOT arc width.

Anyway on these small engines there is a very small difference between the two.

I suggest you start with 65% if you are not confident of getting a perfect oval shape.



On a different note, I wonder if you have a CAPS LOCK OFF on your keyboard...