Following on from my post yesterday, I have now stripped my 1994 400L down and have found that the exhaust-side piston skirt has broken off.

It was only just being run-in, but the piston was a cheapo Namura piston... so much for saving money! (The quad is only for light use around a pretty small estate, so I wasn't worried about a super-dooper Wiseco before)

Any ideas why this happened so soon after installing the piston?

I was running pre-mix and breaking the engine in very gently when it went.

The reason it had a new piston is because the old skirt broke on the same side... I had assumed it was because the oil pump broke down and it seized... now I'm thinking it may have damaged the cylinder. Obviously I won't be replacing it with a cheap, crappy Namura piston!

 

too much piston to wall clearance????

 

Did you have the cylinder bored to the next size? If not then too much clearance sounds like the only reasonable explanation. As the motor turns over the rod tries to push the exhaust side of the piston into the wall. It's kind of funny though, the intake skirts are notorious for breaking usually.

 

More than likely it was the thrust side of the piston. I would have a lot of questions like piston fit and rod clearances etc. etc. I never heard of the brand of piston you used but how many hours were on it? If it were me I would bore and hone cylinder maybe 0.020 over and fit a new piston and check that rod. That's just to start. I am not sure about Polaris engines but there is a front and rear to most pistons. Was the piston in correctly?

 

Sounds like incorrect piston to cylinder wall clearances.

 

Taking the cylinder to my local head-shop tomorrow to have it bored to the next size... the piston to cyl clearance was about 0.005 inch. Within the tolerances in my manual, but not ideal. I'm convinced the crappy piston is to blame!

I'm so pi55ed off at myself for trying to economise (the cost of my lost personal/riding time is ten times that of the piston!) the only good news is that once the engine is back together, I can get the bike up and running in half the time it took me before! All this practice is good and character building!

 

Make sure the cylinder is honed after boring.

 

The piston was in fine and was brand new... this is my first ATV but I spent years taking my motocross bikes apart and putting them back together... I know where the arrow goes! Thanks though.

 

Will do Florida... will do.

On another matter, are there any good Polaris spares dealers in Florida? I regularly visit Florida (business interests out there!) but can never find anywhere that sells parts.

 

Polaris of Daytona Beach......Seminole PowerSports in Sanford.....St Augustine Powersports.....Sorry I can not vouch for any of them as I have had no dealings with them. The one in Daytona Beach deals strickly with Polaris. There must be a good size distribution center in Florida because I have heard they can have parts in a few days.