Bore Measurement

[Wbdisco]

I am creeping up on doing a top-end on my 250. I?ve done lots of reading about going up a piston size like a-b or b-c. Having looked in my manual and reading the tolerances I am finding it a little hard to believe that everyone has a freaking $500-$1000 bore gauge lying around. A Mitutoyo 3 axis bore gauge is a grand. Do yall actually measure or take it to a shop or what? I have some feeler gauges and a few mics but nothing like a bore gauge. My original cylinder is an A, the piston in it is a Wiseco, so not stock. Not sure what size it is since I haven?t pulled the head off. Any other routes of piston fitment that I could go by when ordering my new piston? I assume that as long as ring end gap is in spec them the piston shouldn?t be way off or am I wrong.

 

[(F5)]

So if it were to wear you'd want to get a reputable company to diamond hone it so it is parallel. They could then advise you what size to buy.

A disreputable shop may just run a beadbreaker up a worn barrel.

On the other hand I've taken mine ( 2 bikes,) and 2 others apart. Gave them a scuff with scotchbrite and fitted a piston same as barrel said.

You will need to take it apart to inspect it anyway.
Clean it well and if it needs replating you will know.

For such an old bike you may be option A anyway.

My 300 was low mileage, but piston was junk. Obviouslyhad bad airfilter and or water/grit.

I had to take it apart twice more over the folkowing decade and a half! (base gasket leak then main bearing). I was stunned that the ring had only worn about 1/3rd allowable with years of use (ok I don't ride this bike every weekend). Scuff and went back together.

The other bikes were the same but we had pistons so we changed them. Starting from a known base helps.

But. Virtually every time I needed to gap the rings with a needle file and very careful vice only exposed tip. They were all too tight to just throw in.

Good light, a viewing aid and I suspect you will see worn smooth areas meaning hone or maybe replate.

 

[Moto7man]

Here is my rebuild from some time ago on my 2011

I am still on the original barrel but I don't do a lot of hours on it.(just go to pic 7 or so for ring end gap and leak down stuff, just very basic info).

http://www.gasgasrider.org/forum/showthread.php?t=19892&highlight=Moto7man+rebuild

 

[Wbdisco]

Did you stay with the A piston or move up to a B?