- 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.
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- 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.
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- 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.
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- Rick
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