What size of piston?

[Todd5774]

Have just taken my 6 yr old piston out more for peace of mind than anything else and want to buy a new one but am not sure what size I need.

I believe the old piston was a wossner as it's got a 'W' stamped on the bottom, so am thinking of getting the same make again.

The barrel measures 71.35mm.

So then what size piston do you think I need?

[stainlesscycle]

script 'w' is wiseco. i just installed a wossner - i didn't check how they mark their pistons....

check outer back side of cylinder base for a letter -a/b/c - that will get you in size range. if the bore looks excellent, there's no need to resize..

[Todd5774]

Cheers Stainless, have just ordered a wiseco 'A' piston as the barrel looked excellent full piston kit including all gaskets and head orings and the little bearing ?150 from gas gas uk, should be rebuilding in 2 days!

[Todd5774]

Well they ended up sending me a s3 racing piston as the other was out of stock, after abit of research it looks like vertex make the s3 pistons.

Put it all together today, and then did my 3 heat cycles progressively putting more load on the engine, and then took it for a proper ride.

Holy crap, this thing is an animal now!!!!!

I lowered the squish by just putting in one .50 base gasket so the piston at bdc is level with the bottom of the exhaust port whereas before there was a stack of 3mm!

Had to lean my jetting right out, but now the plug is a perfect colour, will idle all day long, whereas before it was very erratic, and It absolutely rips..
I'm a very happy camper now and will in future replace the piston once a year instead of letting it go 3 years..