hi guys,

Would there be any issues in using what I know to be the largest size mc21 pistons(13102 kv3 830 B pistons) in the originally plated mc18 C cylinders?

I have read about the recommended clearances however without going into too much detail, are there any easy checks to determine a good/safe fit?

In day-to-day use, I doubt you will ever even notice any difference.

Basically, due to some pistons being discontinued, it's a case now of using what you have/can get, unless you have barrels replated to spec.
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Andy.
NSR-WORLD.COM

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Once upon a time I had the privilege of being able to cherry pick cylinder and pistons for a particular circuit.

For relatively slow circuits we'd use the smallest cylinder with the biggest piston. High compression and ignition advance.

Fast and "flat out forever" tracks, we'd choose a loose setup. Low comp and maybe a wee bit of retard so that we could get into top and hold it flat with no fears of over heating or nipping.

These days, especially for street use, as Andy says there's very little difference, so I wouldn't worry about it too much. As long as you have the clearances within spec, good two stroke oil and a reliable carb set up then you should be OK.