broken polaris piston

m38willys · October 14, 2006, 09:41:15 PM

Hey gang. been gone a while. uhhhhh what makes a piston come apart? I have a polaris 400L. good oil mixture, and not ran hot. I don't have it all the way apart yet, but the skirt on the intake side is broken off and I don't know what else is damaged yet. Any ideas?

310R_Dude · October 14, 2006, 10:45:15 PM

How much time on this piston/rings do you have?

Even if the follow everything correct... time will play apart on a
2 stroke motors lifespan.

Ex: Excessive wear(too much time between re-ringing/bore jobs) will
create "slop" in the the cylinder and allow the piston to rock back and
forth. Which usually breaks the skirt.

"Usually" not much damage to the rest of the motor is done due to the
piston being made of softer aluminum. Good Luck!

I am not an eXpert, jus' trying to help.

Dan

P.S.
I had this happen to me on my very first 250R. At 10 months I re-rung it
but they did not tell me how loose my clearances were... at 12 months
it ate the piston just as you described. THEN they told me it needed bored
before!!!

m38willys · October 14, 2006, 11:58:42 PM

yeah, thanks. The cylinder looks great. No deep scoring like I expected to see after a piston came apart. I'll take the bottom end apart and make usre everything is alright in there before I put it back together.

What does a cylinder bore cost? Any reason a regular machine shop can't do it?

thanks again for the info.

m38willys · October 15, 2006, 01:19:42 AM

OK, did some measuring in the bore. The bottom of the cylinder bore is almost 11 thousands of an inch larger than the top of the bore. no wonder. guess I'll take the case apart and make sure nothing is wrong down below and bore and piston it.

TOY_DOCTOR · October 16, 2006, 12:50:12 AM

Be sure to check out the water pump bearings, The 400's are known for the water pump bearings going bad which is actually the counter balancer that drives the oil injection pump, when the bearings go bad it stops pumping oil and you can get the same thing that happened to you.. just make sure you check this before putting in a new piston....

m38willys · October 16, 2006, 11:46:58 PM

Thanks for the tip. I replaced the waterpump/counterbalancer bearings 2 yrs ago when the waterpump seal went out. But I will check those too while I'm in there. Anything else I should look for? thanks again

310R_Dude · October 17, 2006, 07:51:53 PM

Inspect the cases for any hairline cracks or otherwise!!!

The "second" time I blew my 250R up was due to my mechanic
NOT seeing a crack in the case so when it went back together...
oil from bottom end was leaking in and then during break in it
blew up again. ARRRRRRGH.

Yeah... it was a curse of about 6 mons. me and that "R". LOL

NOW... Git' it done.