Damaged free piston - not the usual - pics


9 replies to this topic
  • crf450319

Posted July 06, 2011 - 06:48 AM

#1

Hey,

I've got a buddy's yz450f forks apart, one of his seals was leaking and he asked if I'd fix 'em up. I took the cartridge out of one of the forks and the dampning rod would compress without any effort, and would stay compressed. My first thoughts were either a cracked free piston, or leaking cartridge/ICS seal. Knowing that the free piston hadn't been modded (holes drilled) I figured that was the most likely culprit.

I pulled the base valve assembly out and below is what I saw. My question is this, is the damage pictured typical of the standard free piston damage ? I've seen pictured of KYB free pistons that look like they've imploded and my guess is that the one that I'm working on was on it's way to that same fate, but sustained the damage shown before it could implode ?

Thoughts from suspension tech's ?
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  • grayracer513

Posted July 06, 2011 - 07:13 AM

#2

It's not typical of the ones I've seen, although it could be a result of the same causes. OTOH, it also looks as if it could have resulted from the lower bushing catching on something and breaking the land away from the seal groove. Is there any kind of a sharp edge on either of the bleed holes in the cartridge barrel?

BTW, Dave Johnson at SMART Performance can get all of those individual seals and parts separate from the complete assembly.

  • crf450319

Posted July 06, 2011 - 08:17 AM

#3

Thanks for the reply Gray.

I noticed that the bottom bushing had been stressed at some point, as the two ends were a couple mm off from being perfectly lined up with each other. But, they hadn't pulled away from the free piston.

I checked the inner walls & bleed holes of the cartridge but they're fine, not a mark/gouge to be seen.

I guess, in the end it doesn't really matter what caused it. I was just curious as I haven't seen this sort of damage before. None the less, I'll have Dave send out some new parts.

  • grayracer513

Posted July 06, 2011 - 08:31 AM

#4

I guess, in the end it doesn't really matter what caused it.

As long as the causal condition no longer exists, that's true.

It is odd, though, isn't it? My first thought was that it was excessive cartridge pressure pushing the bushing up into the lands of the seal groove, but it would have to be quite a pressure spike to do that, I would think. Something along the lines of what might happen if too thick a shim had been placed under the ICS spring, causing it to coil bind before the piston cleared the bleed holes.

Was there a lot of oil on top of the piston?

  • crf450319

Posted July 06, 2011 - 09:03 AM

#5

As long as the causal condition no longer exists, that's true.

It is odd, though, isn't it? My first thought was that it was excessive cartridge pressure pushing the bushing up into the lands of the seal groove, but it would have to be quite a pressure spike to do that, I would think. Something along the lines of what might happen if too thick a shim had been placed under the ICS spring, causing it to coil bind before the piston cleared the bleed holes.

Was there a lot of oil on top of the piston?


Ya, true enough. I made the assumption that it was caused by the same mechanism as the standard damage, and also assumed that performing Dave's mod would solve the problem. A lot of assumptions, when as you've said the cause could be something else.

I didn't notice an excess amount of oil on top of the piston, in fact there was very little oil left in the cartridge itself. I thought it had all blown by the piston, made it's way through the bleed holes and into the fork tube. Of course, after seeing the damage I could understand why there was no resistance while compressing/extending the dampning rod into/out of the cartridge. The rod itself was static, regadless of where it was in it's stroke.

My buddy just bought the bike four or five months ago used, and to his knowledge the forks have never been apart or modded.

The other base valve assembly is fine, when the standard damage to the free piston occurs do both pistons usually come out damaged ? Obviously, if one fork experienced a sharp hit the other one got it as well. I don't know, just thinking out loud..

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  • grayracer513

Posted July 06, 2011 - 10:54 AM

#6

The other base valve assembly is fine, when the standard damage to the free piston occurs do both pistons usually come out damaged ?

No, not usually. The main stream thought on the matter is that with an unmodified free piston, oil can leak up into the closed air space on top of the piston. When enough has leaked into the area to take up enough air space, the resulting pressure created under full compression becomes great enough to burst the sides of the piston out. One piston can leak and the other not, so they don't necessarily both fail together. By drilling the free piston per the SMART mod, the oil has somewhere to escape to, and the air pressure inside and outside of the upper chamber is always in balance.

There are other causes, like the coil bound springs I mentioned, but only if the fork was modded somehow.

It would be prudent to change the rod seals out in this case (requires a special tool to unscrew the seal head), but the fluid loss was most likely all due to it being pumped out past the free piston.

  • Wiz636

Posted July 06, 2011 - 04:47 PM

#7

I have experienced the same problem in my '08 though not to that extent. Both sides had breaks in the plastic that chewed up the o-rings. It has happened twice....not sure what causes it.

  • todds924

Posted July 06, 2011 - 06:28 PM

#8

When I took my forks apart for service after Enzo had them........
I found that they had drilled 2 holes to prevent this

  • Wiz636

Posted July 07, 2011 - 11:02 AM

#9

I had drilled the holes also...still happened. :thumbsup:

  • primerib108

Posted July 07, 2011 - 11:29 AM

#10

I had cracked free pistons that look similar to OP's.......called up the local suspension shop who had the metal replacements. No problems now.





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