Would a leaking base gasket cause a piston to lose its rings? I am finishing up the 93 sportsman 350L that I bought awhile ago and I am looking at the old piston. When I took this motor apart, the base gasket was crapped out in the front of the motor but wasn't leaking coolant. I pulled the jug and the piston wasnt scored but the bottom ring had melted about an inch out of it by the exhaust port, causing damage to the piston. I got a new piston, bored the cylinder and went through the motor. The bottom end and rod are very tight so I put new crank seals in while I had it apart. Nothing else was out of the ordinary, the head gasket wasn't blown and the carb boot was good also. Would an air leak at the base cause this?

Thanks,
Mike

 

Absolutely a leaking base gasket could cause this.Would allow air to enter and lean the fuel mixture plus can cause excessive heat. Almost all the sites that show damaged pistons show this: Scuffing on the exhaust side only: The bike is running too lean, whether it's caused by the wrong jetting, clogged jets, or an air leak that can cause heat build up. It can also be caused by using low-octane fuel when you have your bike jetted for the good stuff. Lean jetting is usually accompanied by a loud, ringy, "pinging" noise coming out of the cylinder when the bike is under a load. If it pings, shut it off and find out why it's running lean. If the bike idles at a high speed and refuses to idle down, you have an air leak somewhere. Fix it.

 

Thanks OPT,

When I saw the gasket I assumed I would find a bottom end full of coolant but I was lucky and it was bone dry except for a little spot of oil/gas mixture right at the bottom. I am used to seeing base gaskets failing on snowmobiles and coolant taking out the bearings. This one has a solid bottom end and rod so I just put new seals in and buttoned up the jug after I made sure I didn't have any pieces of damaged ring laying around. I guess I keep forgetting that the bottom end is part of the intake system on a two stroke and it has to be sealed up as well.

Thanks,
Mike

 

This is about as simple a video that you can find on how a two stroke works. Yes the bottom end must be sealed(gaskets and crank seals) for the air,gas,oil mixture to compress properly and be pushed up the transfer ports for ignition.