I bought a 1998 polaris Xpress 300 from an owner who ran it threw water at high speed and then the atv struggled to run for a few minutes then died and never ran after that.

After they spent a bunch of money on it changing parts they sold it to me for cheap. I took a compression and it had little. So, I tore the motor down and found the piston on the left and right primarily was scraped up and a little melted. So the piston was damaged but the cylinder didn't look too bad. It was a little ruff in a few spots, but it seemed to be from some of the melted piston head stuck to the cylinder.

What do you guys think about honing the cylinder? What do you think the chances are thats all it needs rather than being bored out ? I didn't see any pitting, like I said it seemed more like a glaze from the piston. Aslo, since I didn't own the atv when it happened it is harder for me to tell excactly why it happened. But, I did find the ot be a lot of corbization on the underside of the piston head and it filled in a few of the holes on the top sides of the piston by the rings.

ANy ideas as to why there was piston failure? I have ideas, but would like to hear more ideas from those who have had piston failure experiences or know their stuff on this topic.

The motor has one cylinder, gas is gravity fed, seperate oil tank w oil pump.

Wish I could get pictures on heres of it? Maybe I could.

Would love as much feedback as I can get.

Thank You!
KELLY

 

maybe real hot piston then cold water put on it caused it

 

Thanks- that was a possibility I was thinkin as well.

 

i was thinking the same as The Kid. it will cause shock to the piston.

 

Some more comments would be great!
Thanks

 

Anyone?

 

Was the piston melted above the rings by the exhaust port... that can be caused by a leanout or timing issue or too hot of plugs.
If you have to buy a new piston.... without seeing your cyl I would recommend getting a the first oversize above what you have allready and rebore it...if it is a "nikasil cyl then it isn't reborable... don't know anything about the model of your bike so I can't give you advice there. If it is scuffed below the rings on both sides then it could be from the water bath.... got pictures?? Most of my experience is with sleds.........hope this helps.

 

Thanks for the reply Jomax

Not above the rings, but to the rings.
I am guessing water also. Vertical scratching/scoring in about 3 diff spots on the piston. I will try to take pic.

My piston size is 74.50mm (1mm=.039'')

Was thinking of going with an over bore piston size of .010'' or .020''

Does that sound about right? Less than 1mm

Thanks for the input!!!

Kelly

 

Yes that sounds right .010 is the same as .25mm... they may only come in mm., also take a look at the underside of the piston (beneath the top of the piston)... if it is a dark chocolate color it sometimes points to lean/pre-ignition condition,. you definitely want to make sure that everything is right before you fire it up again. More than likely they messed it up when it went thru the water somehow though. One other thing.... take your cyl to someone that is going to do the work and let him give you a idea if the first oversize will "clean" the cyl, you don't want to buy any extra pistons. Most shops want the piston first so they can give it the correct clearance when they bore it... at least I do . One other thing, clean the top of the original piston and make sure there is not a oversize stamp on the top of it already.

 

That was one other thing I did notice, the underside was kinda caked with that dark chocholate grit. I will make sure I get the mixure right when things are all put back together.

Also, thanks, I did today after work take the cylinder and piston in for that reason so I had an idea of what o/s piston to get. The piston does look to be factory.