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Shattered piston on 2011 sportsman 850

5K views 35 replies 4 participants last post by  LBeckett 
#1 ·
I have an 850 sportsman with a locked up motor. I open it up and the rod is bent, and the piston is shattered. But in the other cylinder everything is fine. What caused this? Could the spark plugs be firing at the wrong time? If so how do I fix it from happening again?
 
#17 ·
Unlikely, but it depends on how and why the rod bent - if the rod cap came off (it would have been knocking long before the cap came off) and the crank hit the rod just right to bend it between the steel crank and the aluminum case without damaging the case, then a loose cap may have caused it. The mechanic who tears it down to repair it will have to issue his opinion based on the evidence and his opinion will be just that unless he discovers evidence beyond a doubt of the cause of the failure.
 
#22 ·
From the pics, it looks to me like the piston may have contacted a valve which broke the piston, but it should have been hammering long before that happened. Is the cap just loose, of it the rod cap bolt broken? Also, from the pics I don't see that the rod is bent?
 
#27 ·
OH - so now we know the engine had been screwed with prior to the failure - any mechanic is capable of forgetfulness - it happens to the best - the mechanic just swallows his pride and fixes it assuring that the next time it is right.
 
#31 ·
Any failure like rod, piston, cam chain, crankshaft or ingestion of a foreign object (Honda had a series of small 125/200 singles that somehow had a 6 mm washer get from the air box into the combustion chamber - they had a recall to replace the air box, cylinder, head, valves and/or piston depending on the damage - those with undamaged engines just got a new air box).

I did not see any indication on the valves of contact with the piston, but I only had one pic to view and pictures do not reveal things well unless the photographer knows how to highlight the details.

Give the valves a liquid test - use lacquer thinner, mineral spirits, kerosene or diesel fuel - orient the head with the ports up, fill the port with liquid and observe the leakage past the valves - seeping is OK, flow is bad - check both intake and exhaust the same way - if you fill the port and see fast seepage, that does not necessarily indicate a bent valve - a chip of carbon could be holding the valve open .0005 (half of a thousandths) and that will let a low viscosity liquid out quickly without the valve being bad. If you get this type of indication, simply remove the valves, clean them, clean the seats and lap the valves - during the lapping process you will discover if the valve is actually bent - if no bent valves, after lapping the head will be ready for assembly.
 
#33 ·
I can see where the piston hit the head and the piston broke in line where it contacted the head, but I see no evidence of valve contact with the piston either on the valve or piston - the head may have prevented the piston from contacting the valves
 
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