Need help buying the right piston

[Nick228]

I bought a 2002 suzuki rm 85 and the engine seized up when I was riding it so i took off the top end half the piston was shattered inside, I'm buying a new piston and I'm wondering if I should buy the same size piston that's already in the bike or if that's the wrong size. I looked it up and it says the standard size for the cylinder of my bike is 48mm and the current piston in the bike is 47.43mm is that the right size or should I buy a 47.94mm piston

(sorry if I explained this bad this is my first time ever working on a bike)

 

[Okiewan]

You need to measure the cylinder. It's likely it's been bored, but you won't know without measuring.

 

[RM_guy]

Also, do yourself a favor and get a forged piston. They are much less likely to shatter and will last a lot longer. Wiesco makes them.

 

[JSB-PP]

It wise to rebore and install an oversize piston when a relatively catastrophic failure like the one you describe occurs, since the smashed piston bits and flailing Conrad end tend to damage the bore surface. As long as your bike does not have a chrome, nikasil or similar plating inside its cylinder it should be easy to rebore. In this case it would have an iron liner (tubular sleeve) cast into the aluminium cylinder. And the manufacturer (or aftermarket, eg Wiseco as someone suggested) will have oversize pistons available, usually in 0.50mm steps.

If you have a plated cylinder, it is a much easier but more expensive fix - just replace the whole top end ie piston & rings, cylinder, rod bearing, gaskets and seals. Check your power valve and reed block and br sure to check the crankshaft for damage and/or excessive runout, before dropping all that loot on a new top end.