Which piston size?

Need a new piston for my SMR 510. Online store sells A,B,C,D sizes. Bike currently has 16000km/370h, with no previous rebuilds, so I would assume current piston is A
Is the piston size carved in to the cylinder on these huskys?

Can I just pop in a A size?
Or do I have to take the cylinder to a shop to be measured?

Does the cylinder-piston gap affect the bikes power or reliability in any way?

They are supposed to be matched to a measurement of the bore size at manufacture.
So the original piston might be A, B or C.
Frankly, I doubt that they took the time to match the pistons in the factory, but maybe they did.

The difference is very very small. Really any piston should work given a proper break in.

I'd either have the cylinder measured or just put a "B" size piston in.

I think only the piston is sized. I bet the rings are all the same.
The ring end gap for a 510 is like .030" the piston size differences is is like .002".

Check the manual for specifics.

The size letter is stamped on the bottom of the cylinder on the gasket surface. The rings are all the same. I generally like the B pistons because the size difference between pistons is .00039 or in different terms less the four ten thousandths of an inch. Really you could run any piston in any bore with out problem.

Wow! that is not much of a difference at all.

.00039" is like the margin of error in a quality micrometer.
It certainly is within the margin of error of my eyes reading the micrometer!

I'll check the letter on the cylinder. If it's A then I'll buy B size and slap that in, if not --> shop. I'll measure the ring gap and check manual if it's with-in spec.

Most likely going to buy a wossner piston over the prox one.

Also how often should I change the oil during break-in?
I was thinking 50-100km oil + filter, then again at 400-500km and just to be sure ~1000km again (streetriding) while keeping the wheelies at a minium

Sure that sounds like a good interval for an initial oil change.

I typically just warm up the bike once after i put the piston in. Maybe ride it around the road easy just to warm it up. Let it cool overnight.
Run it the next day. Warm it up good then hit the gas a little. help seat the rings.
Next time warm it up good and take it in the dirt and ride.

Just pay attention that it revs and runs normally without any trace of seizing.