Hey! Has anyone heard of/or experienced problems with piston skirt cracking or anything similar to this.

1997 Sportsman 400 4X4

 

Zoom Zoom
On a recent engine mod / rebuild I did some crack testing on most parts and found the stock piston did have some hair line cracks in the skirt area.
I already had a wiseco +40 ready to drop in so it didn't bother me. I also found some cracks in the jug which were due to casting defects ( not a common problem ) and I had them repaired before porting , ect.
If you have any doubts - [red] WISECO IT ! [/RED]
Andrew
97 Sport 400
Australia

 

Have a 96 400L that I rebuilt this last spring. Major reason for rebuild was a broken piston. Took piston and jug to a small engine shop and they said it was only .004 out of spec. I have heard in this forum that Polaris pistons are not of the highest quality. I put a .010 oversized Polaris piston back in, it was all I could get without waiting 2 weeks, Gotta ride ya know. Its been about 9 months and no problems. Course with 2 wd its not goin very far up here this time of year. Hope this helps.

 

Polaris stock pistons are cast and not forged like a wiseco. The stock cast piston does not allow for near as much expanding and contracting as the forged wiseco. ONce the cylinder bore is worn to a certian extent, the piston has some "play" b/t itself and the sidewalls(skirt). This "play" allows for the piston to wobble, bounce, and smack around on the skirt, eventually causing a crack.

Drop in a wiseco, warm your bike throughly b/f riding and you will be just fine.

EC

 

It's a very common problem with stock cast pistons, particularly if they are hammered hard.

 

GM, Ford and Chrysler all install aluminum cast pistons in their power trains with no ill effects. They all manufacture thin wall pistons (skirts). Same principal here, what do you think is the difference? Why would Polaris have a problem with cracks in the skirt when other manufacturers use the same process and have little if any? Dumb question? Maybe but I'm trying to get smart. Help me out...
Chuck, Suzuki KQ 300, GPS & lunch...

 

Because 4-wheeler's are rode a hell of alot harder than a car!

 

You my friend, obviously don't know anything about testing automobile power plants...
Chuck, Suzuki KQ 300, GPS & lunch...

 

I am no expert, but I am going to say that the skirts in quads in general use a lighter, softer material than what is used in car engine production. ATV's also stay at Higher RPM's for longer than most "sain" people would run their cars at. JMO.

EC

 

A lot is to do with the piston design and materials. In addition, the rpms turned are much higher than in a factory automobile. That is probably the single most important factor. This is very noticeable after modificatons have been done to raise the engine rpm's upward, like after a pie, or other mods. The first thing to go is usually the cast piston. The continued full throttle hammering at rpms that are 500-1,000 rpm more than stock is what causes the problem.