Here are a few pics of some work that I've just finished on a couple Nsr250 mc21/28 cylinders along with some notes on crankcase and cylinder flowing.
There were some issues while taking a view of the cylinder temporarily installed to the crankcase.
Sharp edges on exhaust port side are not looking that great and making a small radius in there will do no harm. Biggest issue that really catches the eye, is the mismatch between the crankcases and the boost port of the cylinder.
It helps a lot to mark the area that you are going to work on. Proper stuff is machinist's blue or red dye such as Dykem but if the area is small, a permanent red marker will work just fine.
Now that you have painted the area of the cylinder, use a machinist's metal scriber to make a constant and pronounced outline. After this, you will end up with a nice and contrasting background to create your art...
For working on aluminum, double cut carbide burrs are the way to go but feel free to improvise, even wet & dry sandpapers and metal files will work with some patience.
Back to the aforementioned example, I didn't really followed the outline literally. Instead, I have only used it as a limiter on how far to go but kept the oval shape of the port by making a generous radius to the LH & RH side.
Have fun and use at your own risk.
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