rebuild? question for scott

[grease monkey]

hi, I hear the norm these days is to put new pistons and rings on after so many hours, unless driven hard, is this true?, then also does this apply to to my 30 year old machines? any suggestions?:?::?:

[SHewelt]

Here's my opinion...Monitoring compression is the best way to determine when you need to rebuild. I remember back in 77, Yamaha recommended replacing the rings on their motocross bikes after 8 hours, then replace piston every 24 hours. I thought that was insane, as many times the compression was nearly as high as when rebuilt! Although, whatever ring material they used, the Yamaha did go through rings more often than most bikes, but it was usually longer than 8 hours of running. Oils, jetting & overall performance (how you run the engine) also plays a part in it. Cylinder bore type & ring type material will also determine wear hours. Monitor compression for rebuild time, typically over 100 psi is fine for most trail sleds. Watch if the 2 sides differ up to 10%, or it falls below 100 psi. Pistons / bores should be replaced / rebored when the clearance exceeds spec's. If there is excessive clearance, compression will be low, even with new rings, skirt slap can happen & you could break a skirt off & really do some lower-end damage with excessive clearence. Measure the piston at the top, mid & lower, at 90* positions, watch for out-of-round...same with the cyl bore. If the piston looks OK (no scoring, burned alum, etc) & is not out of dimensional tolerance, then it can be re-used. Check with any bore-shop on typical clearance for piston to bore...0.005" rings a bell. Use a good-grade oil (I like the synthetics myself) and keep the dust out of the carb's & you'll be amazed how long your pistons & rings will last!