matt_k Posted June 21, 2011 Share Posted June 21, 2011 My mate runs and motor cross shop and was questioning me about how often i change my piston and rings on our paramotors. I told him the manual for a top80 says piston and ring every 100hrs. The piston and rings on there bikes are changed every 20-25hrs, they are two stroke machines, run at around 9000 revs about the same as ours and use the same oils they even run vertex pistons the same as Pap. Are top80 right in saying 100hrs or should i be looking at changing components more often? The air filter on my top80 does it have a sponge filter inside like a bike or is it just an empty box? Yep you guessed it i am a dumb arse carpenter Quote Link to comment Share on other sites More sharing options...
norbertflyer Posted June 21, 2011 Share Posted June 21, 2011 I would bet piston life is inversely proportional to the max power/litre cubed. Or something like that. Richard 430cc, 21hp, 819 hours, no rebuild (yet)..... Quote Link to comment Share on other sites More sharing options...
t_andrews Posted June 21, 2011 Share Posted June 21, 2011 Short answer is: When it needs to be replaced. If your cylinder looks ok, piston skirt looks ok, piston top looks ok, consumption is ok, then it's ok... Problem is, many don't check it and end up with failures as time to replace (piston, rings, cylinder). Compression tests, leakdown tests, plug hole inspection, intake/exhaust port inspections all add up to preventative maintenance. These days a mate is likely to have a scope to check inside the cylinder without teardown, crankcase from exhaust side even. So if it ain't broke, check that it ain't about to break and fly on! For Moto-X folks that redline all the time, and need never break it in a race, sure, 20-25hrs *is* preventative. Expensive, but preventative. Manufacturer specs are a guideline, and one for which there is no recourse even if they're followed and a failure results (YMMV). Par example: Mini2 (plus) manual says never exceed 1076EGT on takeoff as max temp. LOL Well, if you adhere to that one you'll be running an awful long way and most would see 1300+ where the power is. http://www.magictrikes.us/files/Manual_Mini_2_Plus.pdf (leeched link, thanks magictrikes) Quote Link to comment Share on other sites More sharing options...
notch Posted June 22, 2011 Share Posted June 22, 2011 Motocross engines run at higher rpm - 14,000 is not uncommon. The wear on an engine of this type is exponential with RPM. A 125cc doing 9500 rpm will probably last 4x longer than one doing 14000 RPM. Also, PM engines have no power valves which increase wear on the piston and cylinder. Quote Link to comment Share on other sites More sharing options...
enzo200500 Posted June 22, 2011 Share Posted June 22, 2011 Motocross engines run at higher rpm - 14,000 is not uncommon. The wear on an engine of this type is exponential with RPM. A 125cc doing 9500 rpm will probably last 4x longer than one doing 14000 RPM.Also, PM engines have no power valves which increase wear on the piston and cylinder. Could you please explain how the addition of a power valve increases piston/cylinder wear? If the power valve is set correctly and is not in contact with the piston, where does the wear come from? Quote Link to comment Share on other sites More sharing options...
notch Posted June 22, 2011 Share Posted June 22, 2011 Usually, a much larger hole in the cylinder is required to facilitate the powervalve. The piston and bore end up becoming oval quicker with less contact area. Quote Link to comment Share on other sites More sharing options...
enzo200500 Posted June 22, 2011 Share Posted June 22, 2011 If the bores are Nicasil (or similar coating), then for the bore to become 'out of tolerance', surely the plating would have been stripped? So, an out of tolerance piston/bore would be the least of my worries! If the piston is wearing to that extent, it can only be down to 2 things as I understand. 1. Incorrect warmup procedures - usually leading to a 4 point seize on the piston, due to it expanding faster than the cylider walls. 2. Failure of the pre-mix oil to do its job correctly. A lean mixture will cause the piston to wear faster, but the engine will usually seize before bore wear is noticed. In both cases the piston will exhibit signs of scuffing/picking up. Usually evident by inspection through the exhaust port. Any piston material smeared or left on the cylinder walls can usually be cleaned with a cotton bud dipped in acid. Usually spirit of Salts/Hydrochloric Acid, don't get on bare ally though! On the RGV's I work on, you can normally be expected to get at least 3 sets of piston/rings to one coating of Nikasil (if undamaged). Normal service intervals are about 10-12k miles for piston kits. And these engines do spend time in the higher rev range. Quote Link to comment Share on other sites More sharing options...
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