Tips for servicing a RM250 top end
Cleaning the valves: Remove the outer covers and head. Take the cylinder to a local auto parts store/machine shop. Ask for the service of "aqueous power-wash". Its about $10. That is a standard machine with hot, high pressure water mixed with something like caustic soda and de-ruster that is safe to clean the various materials of the valves and bore coating.
Don't take the valves completely apart unless they are carbon-seized after cleaning. If you have to disassemble and clean them don't remove the spring center cartridge from the right side of the cylinder where the link-rod attaches. Just remove the 4mm allen bolt from the valve-yoke and the 8mm hex-head bolt from the right side of the cylinder. Then just disassemble and arrange the parts on a clean sheet in the logical geometric order. The drum valves are marked Left and Right, arrange the parts according to them.
Clean with a soft wire brush or soak in a "Carb Degreaser" can & basket. Use white lith grease as an assembly lube.
Honing: A Flex-Hone with fine ribbons of silicon-carbide is now becoming the factory recommeded standard for deglazing all brands of nickel composite bore coatings. A Flex-Hone from Brush Research in Los Angeles costs about $30
If you can see the hone marks after the cylinder is cleaned, don't mess with honing. If you see deep scour marks on the intake bridges, and gray spots that look like aluminum, then the cylinder needs to be replated. There is an inherant design flaw in the Suzuki RM250 cylinders from 1999 to 2002, with the dimensions of the intake port and bridges. The way to repair the intake port is to heli-arc weld between the top of the intake bridges and the outer intake ports, to increase the top inside corner radius from 2mm to 20mm. Of course the cylinder would need to be bored, faced, plated and honed after welding.
Piston sizes: Suzuki probably offers two different sizes and Pro-X 4-8 sizes in .001 inch increments. Before choosing a piston size, you have to use a dial bore gauge to measure the bore, and chart the sizes from top to bottom, front to back, and side to side. A total of six measurements. Then you choose the piston size that is .002 to .003 inch smaller than the smallest measurement of the bore. Or you can just order the same piston as you had before. A stock cylinder will have an A or B stamped or imprinted on the back corner of the cylinder and piston crown.
Good luck, Eric