08 yz450f broken free piston


7 replies to this topic
  • Yamaryder29

Posted January 07, 2012 - 04:32 PM

#1

I was doing a service on my forks today when I found a broken free piston on the left fork. It split from top to bottom and kinda spider webbed. I've searched around and found other threads that talk about the modification that smart performance has, but am unsure of the exact details. How does this effect the handling?

What are the pro's about this modification and are there any con's?
I've also read about aluminum replacements. Does any one know where I can find them online?

  • grayracer513

Posted January 07, 2012 - 09:33 PM

#2

Two threads that refer to this issue are these, for those not familiar with the problem:

http://www.thumperta...ad.php?t=650136
http://www.smartperf.../YZMODASSEM.htm

The problem is that, as built, the free piston traps a volume of air above it. If the seal between the base valve stem and piston leaks a little, oil accumulates on top of the piston, reducing the air volume. At some point then, the oil accumulation causes the full stroke air pressure in the trapped air above the piston to exceed the strength of the piston, and the piston bursts out into the outer chamber.

Drilling the holes allows any such oil to escape to the outer chamber via the cartridge bleed/overflow holes that are already in place. Doing this won't prevent every possible failure of the piston, but it eliminates 90% of them.

As to how it affects fork operation, there's a little disagreement about whether it does, and what it does. The debate centers around the question of whether there's a difference in the air pressures within the inner and outer chambers at full stroke. Most believe that the air pressure over the free piston is lower when it's drilled, and that will reduce the rate at which trapped air adds to the spring rate as the fork is compressed, and that that will make the fork plusher. Maybe, but the fork ceratinly gets harsher as a free piston fills with oil, so that much can be prevented.

SMART Performance sells an excellent aluminum piston. They're expensive though.

  • Stu2

Posted January 07, 2012 - 11:18 PM

#3

As gray racer says, drill em out, I have had one crack after drilling but it was more from being old and brittle than anything else,

Now I run the metal kyb ones with the open sides, a bit pricey but I can take them out when I sell the bike and re use them

  • Yamaryder29

Posted January 08, 2012 - 10:01 AM

#4

As gray racer says, drill em out, I have had one crack after drilling but it was more from being old and brittle than anything else,

Now I run the metal kyb ones with the open sides, a bit pricey but I can take them out when I sell the bike and re use them


Did you see any differance in the handling with the open kyb pistons? and do you have to use a differant amount of fluid on the inner chamber?

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  • grayracer513

Posted January 08, 2012 - 11:11 AM

#5

The pistons don't alter the inner chamber oil volume, and the oil in that chamber is simply filled completely full, and bled free of air, so that doesn't change. The vented pistons by themselves are not going to make a very perceptible difference in the fork action.

  • Yamaryder29

Posted January 08, 2012 - 11:53 AM

#6

So the inner chamber will still hold 200 cc and the outter holds 350cc? Sorry if this seems stupid, but this is my first time changing the seals on this style fork.

  • Stu2

Posted January 09, 2012 - 01:16 AM

#7

As stated the oil amount is still the same, I revalved my suspension on installation of the metal free pistons so I would not feel a difference

I do it to save broken free pistons, some say it does make the susp a little softer

  • Yamaryder29

Posted January 09, 2012 - 04:17 AM

#8

As stated the oil amount is still the same, I revalved my suspension on installation of the metal free pistons so I would not feel a difference

I do it to save broken free pistons, some say it does make the susp a little softer


I'm gonna go with aluminum, so I don't have to worry about it. I'll wait to revalve it until I can get a feel for the new suspension and see how it feels





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