Dill,
Save your breath, it's hopeless. Even though I'm not a big fan of forged pistons, I'm with YOU on this one!
ERIC

 

you can side with who you want the fact STILL remains that a forged piston expands and contracts slower than a cast piston, thus lowering ring life.

im not the only one who thinks this way, most pros use cast pistons.

for two strokes, cast pistons are better.

 

most pro's use cast pistons because they don't require nearly the breakin procedure that a forged piston does. They can drop a piston in between motos and go.

For someone who changes piston every race, go cast. For those with really high output engines that run high compression, alky, or NOS, forged is the way to go. 4 strokes are every bit as capable as turning the same RPM as a 2 stroke, at least in an ATV application. In many engines in fact, a 4 stroke turns more RPMS.

The fact of the matter is that a forged piston is probably much more closer to the same density and heat retention capability of the sleeve compared to that of the cast piston. So I don't understand your argument here. You're comparing apples to oranges. I run my forged pistons with .0025 of an inch clearance..I don't think my 4 strokes are much, if any tighter.

I ride alot. I monitor my compression after every ride, and when it falls more than 10 lbs, I rebuild my engines. This rarely happens until the end of a riding year, and this riding every weekend. I guess proper maintenance does work.


Topends on 2 strokes need to be rebuilt more often because the piston and rings are moving over openings in the cylinder walls, where a 4 stroke doesn't. It's not the heat that wears pistons, its running at over 8000 RPM over something with holes cut in it that does wear them.

 

.0025" right? Pretty tight for a forged piston!
ERIC

 

yes my bad..its cold in the office here and I hit the button 3 times but only 2 came out. edited my post to reflect [img]i/expressions/face-icon-small-smile.gif[/img]

and you're right, it is a little tight, but when you take your time and break the bike in correctly, its fine. Get anxious and rip on it too fast...and well you know what could happen then.

 

Beerock,
Why do you think a piston expands and contracts so much. I would think it would expand as is gets warm and would stay the same shape untill the engine cools. I could see it change after not running for 10-15 minuites. I doubt it changes very much after the engine has reached normal operating temperatures.

2 stoke/4 stroke, whats the difference? They both get hot!

ERIC

 

<< most pro's use cast pistons because they don't require nearly the breakin procedure that a forged piston does. They can drop a piston in between motos and go. >>



No pro drops in a piston in between motos unless something is wrong, they dont use forged because it is harder than cast.






<< For someone who changes piston every race, go cast. For those with really high output engines that run high compression, alky, or NOS, forged is the way to go. 4 strokes are every bit as capable as turning the same RPM as a 2 stroke, at least in an ATV application. In many engines in fact, a 4 stroke turns more RPMS. >>



Do you not think the pros engines are high compression? You bet your a$$ they are.
I would definately use a forged piston with alky or nos. no need for hi comp



<< The fact of the matter is that a forged piston is probably much more closer to the same density and heat retention capability of the sleeve compared to that of the cast piston. So I don't understand your argument here. You're comparing apples to oranges. >>



Exactly! Because of the density of the piston being harder than the cast this means it wears the bore and rings out FASTER!



<< It's not the heat that wears pistons, its running at over 8000 RPM over something with holes cut in it that does wear them >>



EXACTLY! so because of the forged piston contracting slower than a cast the rings and bore wear out quicker!

Do you see my point now?The high rpms build up more heat allowing more expansion- the expansion isnt the problem, its when everything cools down a bit because the high rpms arent being turned so the engine cools down AND CONTRACTS. It wears the rings and bore out on the forged piston faster, since the forged piston contracts slower.Aside form the fact that the forged piston is harder than the cast piston.

Eric, after running a two stroke like a 250r, which coincidentally revs alot higher than any four stroke around. the heat build up very high, when you slow down and are just done from a couple of long extended wide open runs the engine is very hot,A LOT HOTTER THAN &quot;normal conditions&quot;, then when you cruise around for a while to get your energy back the engine contracts and cools down this is where the quicker contraction of a cast piston extends the ring and bore life.Aside from the fact that the cast piston is softer thus wearing the bore and rings out at a slower rate..................

 

Hmmm, I thought the radiator was to cool the cylinder. More like it will get hot from going slow!
ERIC

 

nope, I do not see your point. Pro's often change out their pistons, maybe not between motos, but surely between races. They DO in fact run cast pistons for the reason I listed. You can drop them in and go. For a piston run a race or two, who cares? For someone who wants their engine to LAST, its a big deal.

Having a piston that is denser does not wear the bore faster. I still fail to see your point here. Do you think a denser piston scrapes the bore or something? Being that something is dense does not make it 'hard'. Lead is denser than alum or steel, yet run it at 8000 rpm against some steel, and it will smear the lead all over the bore. In a correct running engine, the piston will never touch the cylinder walls much anyways. This argument is silly.

I still fail to see how with a cast bore liner, the forged piston causes wear more than a cast piston. The bore is not contracting faster than either a cast or forged piston, so your theory about the bore 'collapsing' around a piston is rediculous. Any differences of this kind would be taken care of by the ring end gap. Frankly, that is the stupidest thing I've ever heard.

 

<< Hmmm, I thought the radiator was to cool the cylinder. More like it will get hot from going slow! >>



your right it will get even hotter after a wide open throttle run returning to a slower speed, but after a certain amount of time the tempurature will go below that of the motor revving high.

dill,
pro change there pistons from race to race


<< Having a piston that is denser does not wear the bore faster. I still fail to see your point here. Do you think a denser piston scrapes the bore or something? Being that something is dense does not make it 'hard'. Lead is denser than alum or steel, yet run it at 8000 rpm against some steel, and it will smear the lead all over the bore. In a correct running engine, the piston will never touch the cylinder walls much anyways. This argument is silly. >>



Oh boy, a forged piston is harder than a cast piston plain and simple. it wears the bore faster because of this and because of the rate in which the piston expands and contracts.

this is the stupidest thing you ever heard because you are set in your beliefs.

alot of people belive the forged pitson is better, but in actuality it is not.(not for high powered two strokes anyhow)