KR-1S.co.uk Forum

Hello, looking on Wemoto.com for replacement pistons, there's A,B,C and D of slightly different sizes can anyone advise. The barrels haven't been messed with. Thanks

Your barrels will have their size letter stamped on the top of the exhaust manifold.

500bernie wrote: ↑Sun May 19, 2019 9:08 pm Your barrels will have their size letter stamped on the top of the exhaust manifold.

but you never know if they have already had new nicasil, because if they have the code is wothless

Best measure the bores first, then use the measurements to choose the piston size.

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Cylinder Bore -minus running Clearance = Piston size
for Wiseco, Wossner and VHM where the pistons are forged, 0.07-0.08 mm is specified
for OEM or cast piston the material expands slightly less so are set tighter at 0.04-0.05 mm clearance

so if you cylinder is marked B and new, you need B size pistons
if your cylinder is stamped B and has many hours or rebuilds (worn) on it you will need a C or D size piston

"When a modern forged piston with more initial cold clearance reaches operating temperature, the difference in running clearance is less than might be surmised. For example, Wiseco uses 2618 and 4032 alloys for all its forgings. While the expansion rates are different for each alloy, Wiseco has made pistons from each alloy for the same engine operate successfully at nearly the same running clearance. The higher-expanding 2618 piston may have a larger initial clearance than a 4032 piston, but once the engine reaches operating temperature, both pistons will have similar running clearances."