There is a big difference between running a car engine 4or 6 cylinder at 6000-7000 rpm with cast pistons
and a two stroke engine running at 9,000-10,000 rpm with a cast piston.
in a car 4 pistons travel from TDC to BDC 100 times per second.and are most often cooled by oil jets,that squirt oil onto the bottom of the pistons, these are liguid cooled 4 strokes
In a two strke engine 1 piston has to travel from TDC to BDC over 170 times per second,the only lubrication is the fraction of oil that is mixed in with the incoming fuel!some of wich are air cooled engines!
and you think they are under the same amount of stress! then why is it that cars can go 500,000 miles before a rebuild and a quad cant!
Cast pistons are a cheap way to make and save money.Cast aluminum is very brittle and wears much faster the a forged aluminum piston will.
and are pone to burning and melting much faster than an aftermarket forged piston.

 

Cast pistons are the least expensive, that's why the OEM's use them. Automotive pistons have an expansion limiting piece of steel cast into them. Cast pistons allow closer clearances, which translates into less noise, less oil consumption, and more friction. They are, as stated above, more brittle, less tolerant of high heat, detonation and more prone to fatigue cracking.
Forged pistons are used exclusively in automotive type racing engine, unless the rule book prohibits their use.
Forged pistons are stronger, due to a more refined metal grain structure, and are made of better material than a casting. They are more tolerant of high heat, detonation, and are not prone to cracking.
They do have greater thermal expansion, and must have larger skirt to cylinder wall clearance. They are noisier, use more oil in a four stroke engine, and normally lighter, as the higher strength of the material allows the use of thinner sectional area. The oil consumption issue is a moot point in an oil injected or pre-mix fueled two stroke engine.

 

JACK, QUADBUSTER, WRENCH101 AND Specialist CAPTNEMO:
You are all in agreement in some form. And I can not dispute any of what you say, nor do I intend to try. However, the question I posed was not the difference between car and quad pistons but rather that we have the technology to build a cast aluminum piston that can withstand the forces inside a larger more powerful engine and I'm sure the bike manufacturers have the same technology available to them, so if Polaris, is having trouble with their piston skirts cracking, what are they doing different from other quad manufacturers?
My guess is Polaris, builds a very good piston and there's something else we're missing here. I just asked a question.
Chuck, Suzuki KQ 300, GPS & lunch...

 

Yahoo, great comments guys! I'm not having problems with my machine, (knock on wood) but this started with my buddy & I talking about the subject as he knows people who have had breakdowns caused by pistons defects, ( that's how I look at it)...but I'm sure learning a lot from your response on the subject. It's kinda disapointing that polaris would think of putting something as flawed as a piston in an otherwise fine machine, just doesn't make sense.

 

 

Part of the reason that the pistons crack much sooner on the Xplorer than a car is the engine design, 2 cycle vs. 4 cycle. When a 2 cycle gets a certain amount of wear, the piston starts to wobble from side to side on the wrist pin. This gets worse and worse as the piston/cylinder wears until it actually starts to 'hammer' against the cylinder wall. This is the so-called piston 'slap'. This hammering will eventually crack the skirt. Since the only oil in the engine is the fine mist carried along with the fuel, there is little cusion when this happens.

Compare this to a 4 cycle, which has its own oil supply under the piston. Some engines even spray oil on the bottom of/under the piston. When the piston wears, the oil cusions the 'slap' so there isn't near the stress on the skirt, so they are much less likely to crack. That is why a 4 cycle engine just starts using oil. The piston gets 'loose' in the bore, allowing more oil to be 'pumped' past the rings. Ring wear doesn't help, either.

The trick to long 2 cycle life is keep the air running through it as clean as humanly possible, and use good oil to lube it. As soon as you hear a funny 'knick, knick, knick' at idle, and low RPM acceleration, it is time for a new piston (saves the bottom end, if nothing else).

 

Thank you Farmr123. You're a man's, mechanic library. I think I understand what you're saying. The air/fuel mixture must past thru the carb and enters the crankcase then on the power stroke the mixture is forced into the combustion chamber where it explods on ignition. The oil that's in the air/fuel mixture is used to lubricate the crankshaft, piston pin and cylinder walls. So that's the only lubrication a two cycle engine will see. I've been reading...
Owners of two cycle engines tend to put more oil in the fuel/oil ratio than the manufacturers suggest. The manufacturers know this too so they are not too concerned about it. But if you put too much oil in the mixture, the engine starts to bog, miss or don't start at all. To avoid cracks, it would probably be a good idea to warm your two cycle engine before hard riding, would you agree Farmr123? Early wear and cracked piston skirts is not a Polaris problem. It's a two cycle engine probem.
This was fun... Thanks Zoom Zoom...
Chuck, Suzuki KQ 300, GPS & lunch...

 

One other thought, a two stroke engine uses thin viscosity oil mixed with fuel. A four stroke engine typically uses a heavier lubricating oil that is sprayed onto the cylinder walls by the rod bearings. The heavier oil will cushion and quiet the movement of the piston in the four stroke engine, where as the two stroke's thin fuel / oil mix doesn't add much cushioning effect.
I found out the hard way that when you start making more horsepower with the 400 engine, you need to begin pre-mixing some oil in addition to what the oil injection system provides.
Since more HP requires more fuel to be burned, the oil injection doesn't compensate.

 

Warm up your 2 cycle before hard ridindg? Yes, yes, yes! Keep this in mind. The cylinder has a water jacket, the piston doesn't. Obviously, the piston is going to run 'warmer' than the cylinder. Because of this, the piston has to start out a little 'loose' when it is cold. Hard riding when cold will just make piston slap worse (until operating temp is reached). In a worst-case scenario, you can have BIG problems, especially in a new or rebuilt engine as things are still tighter than when fully broken in. If you run hard too quickly, the piston will warm up before the cylinder (because of the water jacket). In some cases, it can swell so much so quickly, the piston will stick in the cylinder. This is called a 'cold siezeure' by mechanics. It really sucks as it happens soon after you get going and is most prone to happen on a new or recently rebuilt engine.

 

In addition to the excellent points raised by Farmr123 who understands "cold seizure, piston slap and low ash lubricants".

Let me raise one more point of conversation for Captain Nemo.

TC-W3 oils do not provide the amount of protection in a 2 cyle application (piston scuffing) as an oil with a low ash content labeled ISO-L-EGD

With each modification which leads to an increase in performance of your atv you have proportionally decreased the manufacturer's designed in reliabilty/safety margin. These modifications require a 2 stroke lubricant of a higher quality.

Currently the highest available quality 2 stroke engine lubricants are labeled ISO-L-EGD.

Remember that classification. Everything else is snake oil.

I realize that once again, I have crossed the line and made a comment on "oil."

Oil is a personal thing, it is much like religion,
your preference in women, and the choosing of a political party.

But the facts remain: If the bottle of 2-stroke oil in your hand doesn't say ISO-L-EGD, you do not have the best two cycle oil available.

In a modified ATV a person couldn't do better than looking at and researching the best snowmobile oils available.