No worries mate, I’m learning here too. I thought it was done the way you did it a few months ago, but just imagine that a solid mass needs to pass through the hole and you will know exactly how to measure it.Originally Posted by poolieZerUK
I will go so far as to say that “exact” measurements with conventional tools is almost impossible. The best us do-it-yourselfers can do is be consistent in the way we take measurements for comparative purposes.
A perfect example of a fudge factor measurement is the chamfers of the ports. If you stuff a piece of paper folder into the cylinder and work around it with your fingers to leave an impression on the paper (works best if the cylinder is a little dirty) you are going to get a transfer of the port image that is a little fuzzy at best on the edges. Realistically the chamfer is going to appear in that fuzzy section, so I then spread the paper out flat and trace the center of the fuzzy edge with an ink pen (I press hard to leave a groove). I’ve read some people then cut the windows out with a razor and form a circle to measure, but I just wrapped the paper around the piston and let the tips of my caliper drop into the groves that the pen made. I guess if you had to have it more exact you could bore the cylinder and map the ports on paper before chamfering them.
For comparative purposes (before and after) you can do it any way you want as long as it is done the same way each time. The reason you would need to do it “correctly” and remove the chamfer from the numbers is if you are asking someone to build you a pipe, or offer advice on another modification like changing port timing. Lying to your pipe builder about the size of your ports will do you no favors. It’s like buying the Magnum sized condoms to impress the checkout girl and then having them fall off at the worst possible time.
Same thing with checking the port timing. Each action effects the next. You must find TDC and then decide where to determine a port window opens of closes. I first thought it would be after the rings pass the chamfer, but it isn’t. It’s when the dome passes the open window of the port. How can you tell to within 1 degree? Good question, maybe a feeler gauge? I used my nearsighted vision to pick a spot and repeated the process a dozen times till I kept getting the same reading. (It’s so much easier to move the crank around on a 2 stroke as you can turn it back and forth without fear of bending anything) It is entirely possible that someone else doing the same thing on the same engine could come up with results that differ by a couple of degrees.
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