Changing Parts - No Wasting Smarts

Maintenance is a funny thing. If it's not done it doesn't cost any money - but it does have a price. When related to two stroke engines the price could be performance or fuel mileage or reliability. It could be many things - including dependability.

Pistons and their associated parts should be replaced at regular timed intervals. I often use 25 to 30 hours of running time for typical high output engines. Race only engines mean more stress and shorter replacement intervals. For that kind of duty cycle 15 to 20 hours is not uncommon. Recreational riders do well with 45 or 50 hours - but many, many riders go much longer than that. Though they may be thinking they're getting away with something, that's simply not true. The longer the parts stay in the engine the more wear they introduce to the parts they interact with. For instance, keeping a piston in an engine too long wears out the bore and crank. It is probably better to change out a piston kit than have to waste a bore size because the time interval was missed. The crank bearings will last much longer if they're not subject to flying debris such as small pieces of aluminum from the piston, hard ring material (or cylinder liner) or bearing cages which could start to come apart.

If a schedule is followed and parts are changed out it would be very rare to have a breakdown due to the material achieving a service life beyond what it can tolerate. The timed parts changing events are scheduled ahead of the expected duty cycle to prevent a disaster. If the schedule is not adhered to sooner or later something will come apart, wear out and/or ruin other parts. Someone once said "If it ain't broke, don't fix it." Some people may have misinterpreted that as wisdom. The statement should be something like "Replace parts before they reach the end of their expected duty cycle. Establish the duty cycle from experience and other peoples mistakes." I'm writing this after inspecting a top end from my own TRX250R. The cast Pro-X piston still has the original machine marks and it's outer diameter is still well within tolerance. It's crown shows a good burn pattern and good color. The inside of its crown is colored medium brown - it did not over heat. The rings were sealing well as evidenced by their clean shiny bottom side and their face still has plenty of plated material. The ring end gap is well within specification. The cylinder bore is still very round and the piston to bore clearance is .003" inches - only .0006" more than what it was when it was assembled. There is very little wear on any of the parts - that is, the kind of wear you can physically measure. The engine these parts were in was run during a combination of hill racing and pleasure riding for a period totaling 30 hours riding time. The cost of replacing the piston, rings, clips, wrist pin, bearing, the cost of a base gasket and a hone is about $80-$90 (give or take). A few dollars an hour. The parts are changed out (replaced) in 40 minutes using hand tools. Running in the new replacement parts will be quick and painless.

If the bore was warn out it would cost about $50 more and I'd have to give more attention to running in the new surfaces. It also takes more time to perform and wastes a bore size. If the top end wasn't checked until the piston broke it could take the crank with it (about $300 new or $150 rebuilt). It could also take out the cylinder itself (about $400 plus the cost of porting - $$$) and maybe damage the head (more $$$). Though the parts that come out often "look" fine - it is difficult to establish how much longer they could have lasted at a steady state of continued operation. It's not worth taking unnecessary risks with internal engine parts - they were designed to be replaced often. They're relatively cheap and easy to exchange so there's no reason not to replace them. People who change parts out on a regular schedule spend less money on parts in the long run than people who wait until replacement is indicated by other means.

Rick

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