The 85s and 125s require crankcase modifications for big bore piston clearance. These models include the 1993-2008 Yamaha YZ85 and 1994-2008 YZ125, Suzuki models 2002-2008 RM85 and 1990-2008 RM125.

It is possible to grind away the crankcase with the engine assembled and mounted in the frame, but you have to take care in masking the crankshaft so the grinding material doesn’t contaminate the bearings.

Start by cleaning the oil film from the cases and crank. Rotate the connecting rod to it’s lowest point.

Masking materials include clay, paper and tape. Basically you apply the masking material so only the grinding area is exposed.

The area of the case that needs to be grinded is located above the crankshaft on the exhaust side of the case, You can mark the area by installing the cylinder without the piston and using a marker pen scribing along the bottom exhaust skirt of the cylinder.

A vacuum should be used to collect the metal chips from a die-grinder. Even a small Dremel Mototool can be used to grind the 1-2 millimeters of material from the cases.

To check for clearance, install the piston and cylinder and try to rotate the crank. If the piston stops before BDC (bottom dead center) you will need to grind more from the cases.

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