Single rings pistons have less friction, but the trade-off is ring life. The top ring always wears the fastest. and if there are two, you get alot more life.

The NSR exhaust port can be effectively changed. The top of the exhaust ports are angled down on the sides. If you make them straight across, matching the sides to the height of the middle, it puts the porting much closer to where it should be (more blow-down timing). Dont raise the existing port height more than about .5mm, you may be disappointed in the results.

Also, dont bother polishing the exhaust port. It really does nothing for power. And dont bother with the port where it transfers to the pipe. Alot of guys hog this out to match the pipe diameter, but in my experience it does nothing but hurt low-end / midrange power with no gains on top. It makes the bike feel more peaky, an illusion of more power, but nothing is actually gained.
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Paul Herr
'88 FZR4/GSXR/YZF Frankenbike
MY BIKE PICS