it all started a few months ago, when the coolant line fell off the cylinder head. Ever since, ive melted 3 pistons and warped a head. I cant for the life of me figure out why. I used brand new antifreeze, removed oil injection and pre-mixed the gas, i put on an hot seat billet head with o-ring seals, brand new wiseco piston. I thought everything was up to snuff, but every time after running for a couple of minutes the piston melts. anyone have any ideas?

1996 Polaris scrambler 400 4x4 2 stroke.

 

Where does it melt?

what does it look like when it melts?

There could be many reasons why, but you need to discribe (photos if possable) what the piston looks like.

 

I am NOT a mechanic. Could it be a bent piston rod?

 

always seems to melt on the exhaust side, as your sitting on it, ranging from about 9-1 o'clock, the last one a big chunk was melted, the one before that wasnt as bad, but enough to lock the rings in place

 

the only thing that comes to mind is excess heat. from wrong jetting, sucking air, lack or coolant running thru the cylinder form blockage? not sure. the plug is the right heat range right?

 

Well lets see:

When you say exhaust side do you mean exhaust port or "where the pipe routes around the motor"

When did you change to the new head?(if any nicks in the combustion chamber of the head protruding then you will have a hot spot there)

Is there an exhaust leak?

Like Sten said heat range on plug is very important but "most" of the time you will burn a hole right through the top at the spark plug, not the edges.

Jetting is also very important, "most" of the time it will erode the edge of the piston at the exhaust port from excess combustion temps.(air leaks are not good for jetting)

Of course cooling issues with the radiator and water pump can make the piston more susceptible to failing.

Could be a jet plugged in the carb making it run lean.

Compression to high.(to low octane of a fuel)

Or any combination of these and more could cause a problem like this.

On a side note:

Check to see if you have lines scared down the sides of the piston normally at (12 being the exhaust port) 1:30-4:30-7:30-10:30 when you are setting on the ATV.

If you have these "lines" then your bore is to tight or you "cold seized" the piston.
It's caused by not letting the motor warm up before riding.

 

i like this description. ["Check to see if you have lines scared down the sides of the piston normally at (12 being the exhaust port) 1:30-4:30-7:30-10:30 when you are setting on the ATV.

If you have these "lines" then your bore is to tight or you "cold seized" the piston.
It's caused by not letting the motor warm up before riding"] this something people dont even realize can happen on a liquid cool engines when its cold out side. i've seen alot of damage done when you try to take off and nto let the engine warm up. your right about the hole being mostly on the top of the piston from too much heat from the plug. most of the time if you check the plug it will be white in color and the tip of the electrode will be as well if its running that lean to burn.

 

Snowman,
Did you open the bleeder screw on the head when re-filling the coolant system?
An airlock at the head/cylinder might be able to keep the coolant from flowing.

 

have had crank seal problems on both my popo 2 strokers...my problems were the rings seizeing and the breaking causeing nasty score marks...

 

Did you keep the compression stock?