my friend brought me a 400 scrambler 2 stroke that he bought off another one of my friends... knowing that the motor was hammered before he bought it he brought it to me to take apart and fix... i just got done tearing it down to find that the piston was broken... 2 yrs ago the orginal owner melted the piston in it bc of lack of coolant and we tore it down and had it bored and rebuilt it... well this time when i removed the head i found chunks of metal belonging to the piston... the piston broke on the bottom side... it looks like the rod hit it and broke it... well actually thats what did happen... there doesnt seem to be anything wrong with the wrist pin or bearing and there doesnt seem to be any play in the crank bearing... i was just wondering what would allow the piston to hit the rod... if water got into the motor could that have done it? when i remove the carb it was half full of water rather then gas... the cylinder looks ok another then one small scratch found the chunks of piston flying around in there... well if anyone has any ideas or has something similar happer please let me know... thanks...

p.s. sorry for the lenght of my post

 

i would say the wrong piston got installed when it was bored or they did not match the bore with the new piston. if the tolerances are too far off then the skirts will slap up against the cylinder walls causing them to break off. if i were you i would go ahead and pull the crankcase out of the frame and replace the bearings and seals just incase there are some small bits of piston still in there.

 

The 400 polaris 2-stroke engines have a history of breaking stock skirts. The pistons are cast..........wear a long time and all, but if it does develop rattle they are prone to break the skirt off. What happened in your case was a piece of the skirt broke off, fell, and got jambed between the rod and crankcase.........the rod did not somehow come in contact with the skirt before it broke off.

If you want it to be more durable buy an oversize wiseco, get the piston bored to fit with the proper clearance, and always warm it up good before getting on it. Take a compression reading after the first tank of fuel so you can take periodic compression readings and know when it gets low enough to re-ring or rebuild. Rick Ritter with Ritter Cycle Racing is an excellent source for advice on these engines.

Waylan

 

thanks guys for the info... do you happen to know the clearance for the piston and the cylinder for that motor? i can see now that i have cleaned it up that the skirt was rubbing in there... the cylinder still looks good inside... theres no marks and no scratches... i think all that needs to be honed... i cant remember for sure but i think my friend had the motor bored and then bought the piston but i cant remember... so maybe that was the case... i would have to go check for sure but i would have to say that theres no more the .005 of clearance per side... and that leaning on the high side... but thank for the info... it makes more sense now

 

.0015-.002 clearance on piston to cylinder if boring. .005 clearance too much