44wilky44 Posted May 10, 2009 Report Share Posted May 10, 2009 just removed piston from montesa 250 1999 and the back side of it, half of its has come off in bits, is this a common problem,or dose anyone know why it happens will i have to rebuild the bottom end, thanks lee Quote Link to comment Share on other sites More sharing options...
fsb Posted May 10, 2009 Report Share Posted May 10, 2009 It's a common problem when pistons are used too long. The clearence between piston and cylinder increases and the piston starts to rattle. And after a while the pistons breaks. Just keep an eye on the operating hours of your engine in future and dismount your top end after 100 hours to do some measurements. There should be some information for cylinder/piston wear in your manual. Do you have something like Aluminium-powder in your crankhouse or are there just some broken bits from your piston? Does the crank still rotate smooth or does it stick/clamp in some positions? Safest thing would be to install a new conrod kit with new bearings but maybe you just have to clean it. It's difficulty to say without seeing it by myself. Don't you have a good dealer who could take a look to your motor? bye, Felix Quote Link to comment Share on other sites More sharing options...
fastducs Posted May 11, 2009 Report Share Posted May 11, 2009 Did the piston fall apart as you were removing it? Or was the piston skirt already broken with bits of it flying around the bottom end? If all you see is loose pieces of piston skirt with no damage to the crankshaft, connecting rod, or the engine cases, then count your blessings. On a 2-stroke engine the transmission is sealed from the crankshaft, so it is safe from piston skirt debris. I would remove engine from frame, turn it upside-down and flush the crankshaft area really well, and inspect crankshaft main bearings and connecting rod big-end bearing. If all looks well, then put in a new piston, new little end bearing, new wrist pin, and new clips. Quote Link to comment Share on other sites More sharing options...
44wilky44 Posted May 11, 2009 Author Report Share Posted May 11, 2009 it looks like its gradualy worn away and when its finaly gone a small piece has scaped up the head and past the rings which i removed from the top of the head, the con rod is very smooth it has a small rock from left to right dont know whether this is normal, (what would i use to flush out the engine) thanks for rep all lee Quote Link to comment Share on other sites More sharing options...
fastducs Posted May 12, 2009 Report Share Posted May 12, 2009 Do you have photos? A picture is worth a thousand words. If a piece of piston skirt went past the rings and into the head, then there is likely damage to the cylinder wall. Do you see any damage / scoring? Again, without good photos it's hard to give any advice. Quote Link to comment Share on other sites More sharing options...
44wilky44 Posted May 12, 2009 Author Report Share Posted May 12, 2009 the head is a gonna it needs a re-skim, the in side is rough, i was just thinking about the bits of piston (profile pic) there is load missing, just wanted to know what i could use to flush out the con rod part with. thank lee Quote Link to comment Share on other sites More sharing options...
fastducs Posted May 13, 2009 Report Share Posted May 13, 2009 You could try flushing out the bottom end with brake parts cleaner. Quote Link to comment Share on other sites More sharing options...
dave dix Posted May 14, 2009 Report Share Posted May 14, 2009 Try diesel or kerosene (paraffin) to flush out with then a drop of petrol / oil mix Dave Quote Link to comment Share on other sites More sharing options...
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