Lapping Model motors always use diamond lapping powders you can get good grades of this from MSC and other machine shop places. DO NOT USE VALVE LAPPING PASTE. It is much to coarse. you want the finest grades of lapping powder you can find. when you are done a mirror finish is what you want. I had heard many times about you want a dull cross hatched finish. "BULL" stuff The big kids the ones turning over 48,000 rpm are using a mirror finish I guess I will do so as well. my preference is for 4000 6000 and 8000 diamond grade grit I know that sound fine but it may take longer but when you are done it will show a reflection of a candle flame that is perfect. I know these grades of Diamond powder seem expensive.  Still this grade and type of powder will work a thousand times better than anything else you could use. One more thing about lapping when you have your cylinder chucked into a lathe or drill or mill. Do NOT use a pair of gloves to work in your powder or the piston in case you are lapping. Should something happen and the lathe catch your glove It can tear off your hand or ARM. From then on we may call you lefty. Now that is a hard way to get a nick name. You have two really great ways to  work lapping compound into a cylinder or piston Use a rod of metal and wrap a thin length of leather around that rod. You may either use oil or water to work the powder to a paste and work that into the leather strap. When you insert this leather into the cylinder it will keep the powder working in the cylinder as it turns. Like I said earlier the old way of doing the cylinder would have you create a dull crosshatched finish in the cylinder or on the piston but after the last ten or so years the current way to lap the cylinder and piston is to put a mirror finish on them. The super fine polish helps by actually pushing the lead of the iron and the copper of the plating into the cylinder and the piston surface this gives the diamond polish something to us in creating a super smooth surface. All the rest of the Story dray