OEM VS Forged Piston??


9 replies to this topic
  • Dan_187

Posted July 29, 2008 - 10:10 PM

#1

Since I have my bike apart I'm going to throw a new piston in it to be safe for another 100,000 miles.

I read an article in MXA where they said a forged piston is better than the cast oem because it stays stronger in high heat situations.

I want to stay with stock compression, the only reason I'm thinking of a forged is for durability and it is somewhat cheaper than oem. What do you guys think?

  • foxz

Posted July 29, 2008 - 11:24 PM

#2

yea..if its the same as the car pistons, the forged stays true during the heat and cast expands...thast my understandment at least...

  • grayracer513

Posted July 30, 2008 - 06:14 AM

#3

Just the opposite, actually, although modern metallurgy has improved things. Aluminum naturally tends to expand and contract a lot with temp changes. This was particularly problematic when you had an aluminum piston in an iron bore, as the iron, even an iron sleeve in an alloy barrel, would expend much less. This meant that pistons had to be sized smaller than they should be so that when they arrived at their operating temperature, they would have expanded to the correct dimensions. One then hopes it won't get too much hotter than that.

To control this, other elements are alloyed with the aluminum, usually silicon. Silicon is very effective in reducing expansion in aluminum, and together with the use of plated aluminum cylinders, piston clearance can be held to a very small value with less risk of seizing when overheated.

Silicon, however, makes aluminum less malleable (the quality that allows a metal to be beaten into another shape without loosing strength), and so makes it less suitable for forging. Very little can be used in cold forging processes. The best modern forged pistons are processed by a means that might more accurately be called pressure casting, and with this, the material can contain nearly as much silicon as a cast piston without interfering with the process. Unfortunately, most aftermarket pistons are not made by the best of these processes because they don't lend themselves to small volume production. This can be seen by the fact that manufacturers like Wiseco still require a looser than OEM fit to the cylinder.

Forged pistons were preferred years ago because they were stronger. They still are, when well made, but modern casting processes have improved too, and the gap is nowhere near as large anymore between the strength of a top quality cast piston and forgings.

If it were my bike, which is used for desert racing, recreation, and a little MX, I would use an OEM piston. If it was an engine for pro level racing, or for dirt track, SM, or road racing, then I would choose a CP piston, personally.

  • Wiz636

Posted July 30, 2008 - 12:20 PM

#4

Just the opposite, actually, although modern metallurgy has improved things. Aluminum naturally tends to expand and contract a lot with temp changes. This was particularly problematic when you had an aluminum piston in an iron bore, as the iron, even an iron sleeve in an alloy barrel, would expend much less. This meant that pistons had to be sized smaller than they should be so that when they arrived at their operating temperature, they would have expanded to the correct dimensions. One then hopes it won't get too much hotter than that.

To control this, other elements are alloyed with the aluminum, usually silicon. Silicon is very effective in reducing expansion in aluminum, and together with the use of plated aluminum cylinders, piston clearance can be held to a very small value with less risk of seizing when overheated.

Silicon, however, makes aluminum less malleable (the quality that allows a metal to be beaten into another shape without loosing strength), and so makes it less suitable for forging. Very little can be used in cold forging processes. The best modern forged pistons are processed by a means that might more accurately be called pressure casting, and with this, the material can contain nearly as much silicon as a cast piston without interfering with the process. Unfortunately, most aftermarket pistons are not made by the best of these processes because they don't lend themselves to small volume production. This can be seen by the fact that manufacturers like Wiseco still require a looser than OEM fit to the cylinder.

Forged pistons were preferred years ago because they were stronger. They still are, when well made, but modern casting processes have improved too, and the gap is nowhere near as large anymore between the strength of a top quality cast piston and forgings.

If it were my bike, which is used for desert racing, recreation, and a little MX, I would use an OEM piston. If it was an engine for pro level racing, or for dirt track, SM, or road racing, then I would choose a CP piston, personally.


Good stuff there Gray.

Here's a question: Bought my son's YZ250 smoker used and it has an LA Sleeve'd cylinder. Would you recommend cast or forged pistons for use with that cylinder?

  • grayracer513

Posted July 30, 2008 - 01:00 PM

#5

I would lean toward an OEM cast piston, but it's a good question, and the best advice might well be to ask LA Sleeve which one they intended it for.

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  • Wiz636

Posted July 30, 2008 - 01:07 PM

#6

I would lean toward an OEM cast piston, but it's a good question, and the best advice might well be to ask LA Sleeve which one they intended it for.


A rather obvious piece of advice (calling LA Sleeve)...duh Wiz.

So I called them and in a rather bored tone was told that 'it doesn't matter' but they use OEM.

  • grayracer513

Posted July 30, 2008 - 01:33 PM

#7

A rather obvious piece of advice (calling LA Sleeve)...duh Wiz.

So I called them and in a rather bored tone was told that 'it doesn't matter' but they use OEM.

If they're tired of giving that advice out on the phone, maybe they should print it in some documentation to send out with their product. Ya think?

  • Wiz636

Posted July 30, 2008 - 01:39 PM

#8

If they're tired of giving that advice out on the phone, maybe they should print it in some documentation to send out with their product. Ya think?


That's the way I see it.

I also looked all over their website (faq's, technical, etc.) thinking it would make some mention of pistons but with no luck. So I called...

  • Dan_187

Posted July 30, 2008 - 07:17 PM

#9

Thanks for the input guys, I went ahead and ordered me an oem one. hopefully ill be out there again in no time :ride:

  • Justin89

Posted July 30, 2008 - 08:45 PM

#10

I just put in an OEM piston in my bike..no complaints here:thumbsup:





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