You should be able to get 0.6mm, 0.5mm and 0.4mm gaskets though the NSR-World market place or ask Andy.

I'm sure you will get a good response as to what people think a good squish is. I'd say bigger is better than smaller on an NSR motor. Tighten it up too much and you get a peaky (fast feeling) motor but that is otherwise worse off.

It's more important that what ever the squish is set to that they are the same on both cylinders. I like a relativity large squish.
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If I have to take the carbs off once more...