I finally got around to doing the crank seals and waterpump seals on my 96 Xplorer 400. I ordered the complete seal kit with the water pump bearing and the complete engine gasket set. I figured while I was this far into it, I might as well pull the head and put a new head gasket and base gasket in since I will have everything taken off and out of the way. Question is, what should I do if I pull the cylinder and find out it has a stock polaris piston in it? Didnt they have problems with the skirts shattering? The guy I bought it from told me he had just put rings in it before I bought it, but never said anything about the piston. The machine has just turned 5000 miles on it. I would think that would be on the outer limit of the stock piston if it was prone to break its skirts. I know 96 had two different types of pistons, and mine has the round ports on the intake side, so it would be a late 96. So, how do I tell if I have a stock piston, and if I do, should I just bite the bullet and get a new wiseco piston and rings assembly? The thing is, I want to only take this thing apart once and if I got it tore down now and I just have to spend a little more money for peace of mind instead of having issues later, I'm all for it.

Thanks,
Mike

 

Wiseco will have a etched part number along with the over size marked on the top. Oem cast piston will just have a number 5 or 50. Always have the cylinder measure against the standard bore size of 83mm. Unless it was parked in a garage for most of it's life it will need an oversize piston and bore job.Don't go cheap since you've gone this far. This is just an example:http://www.ebay.com/itm/Wiseco-Top-End-Kit-83-5MM-For-Polaris-400-ATV-Models-94-03-PK1108-166582-/391073583082?pt=Motorcycles_Parts_Accessories&hash=item5b0dcd4fea&vxp=mtr Or you can just buy the piston and get the gaskets separately.We used a lot of Cometic gaskets also as they're good quality.

 

Thanks OPT,

I did my seals today along with the waterpump bearing. Crank seals were definitely needing to be replaced but the waterpump looked like brand new, no pitting or rust on the bearing. I replaced the bearing and seals anyways since I had it apart. My crank bearings and rod bearings were in great shape so I should be good there. I was worried about this job because everyone said the clutch and flywheel removal can be a pain. My clutch was a little stubborn but popped off with the clutch tool but my flywheel came off like butter. Maybe it was because I have a really nice puller for doing harmonic balancers and flywheels.

The piston in my machine had 38B50 stamped in the top so it is stock. I pulled the jug and ran my micrometer around the cylinder in a bunch of spots and had readings from 83.60 to 84.00 millemeters. I ordered up a Wiseco piston kit that is 84.50 mm. The cylinder looked good and there were no bad spots or pieces missing from the piston so I think the guy actually did throw some rings in it, but it definitely needs a bore job and ditch the factory piston before it decides to grenade.

I am sending the jug out for a bore and hone and I am sending the head along with it to check for warpage and to have that straightend up if it needs it. Hopefully I will have a 400 running by next weekend, lol.

Thanks,
Mike

 

If it actually ran with these measurements,it was an accident ready to happen at any time. Keep us updated.

 

Do you think it may have been possible that at one time, the piston was replaced with another Polaris piston? I know the readings were a good millimeter over what is stock at 83mm. The highest readings were at the top in the area of highest ring travel at exactly 84mm. I have rebuilt a lot of engines in my life, but most were V8's or smaller four strokes like lawn tractors. I know the service limit is usually less than 5 thousands and a full millimeter is a good chunk of range on a smaller two stroke piston. I am shocked it did run and even had good compression but I have found around here, when someone says they rebuilt something, it is done as cheap as possible. Maybe the previous owner knew it was getting weak and just threw a set of rings in it to make it run and sold it. The rest of the machine doesnt show excess wear so it was taken car of.

 

Just a little note:
Be sure to pay attention to the mfg's spec for piston ring end gap. On my Wiseco 2-cycle piston the end gap for the top ring and second ring are different. If you make them the same the mfg says it will cause top ring flutter. (I had never heard of this before)

 

Piston to cylinder clearance service limit is .006 in the manual,but basically around .004-.005 is when you start having problems. Hard starting,dies when warm,etc. Beyond .006 you're looking at breaking piston skirts. Had plenty of them over the years that people kept driving until they dropped.No telling what the previous owner may have done. I've seen plenty of stock pistons installed in worn out cylinders. Wiseco I think still recommends .003 clearance between the piston and cylinder,but I always set them up around .002 and broke them in slowly. Yes when someone says they "rebuilt" the top end,don't always assume that they had it bored and a new piston installed. Some even consider it "rebuilt" if they just drop a new set of std rings in only to find out it won't run or won't last long.