2002 Yamaha YFM660 Grizzly factory service and repair manual, Page 102Get this manual
MEASURING THE COMPRESSION PRESSURE
6.Measure: Compression pressure Above the maximum pressure: Inspect the cylinder head, valve surfaces, and piston crown for carbon depositsBelow the minimum pressure: Squirt few drops of oil into the affected cylinder and measure againRefer to the table belowCompression pressure (with oil introduced into cylinder) Reading Higher than without oil Same as without oil Diagnosis Worn or damaged pistons Defective ring(s), valves, cylinder head gasket or piston is possible