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Baffled! Need help - KYB Free Piston problem

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This is a continuation from the "Sanity Check" thread below. 2005 YZ250F

Now we have a problem. (Caveat - I have used Dave J's cartridge bleeding procedure with good success for about ten jobs now).

As discussed in previous thread, I used a 7mm spacer on top of the ICS spring. It did not work. With that spacer in there, the damping rod will only go to about 4 inches from the end of the cartridge and stop. Dead stop, hydraulic lock feel to it. I'm thinking the free piston would not raise high enough. I never could get any fluid to purge from the transfer holes. Perhaps the ICS spring reached it's mechanical limit.

I took the 7mm spacers out and now the rod will go to within about 2 inches of the cartridge end and stop with all the same symptoms as above. On my 5th try now and no change. I have never had this problem. On my last try, I backed the base assy cap out two full turns (Raising the entire assy slightly) and the rod moves all the way in normally. This system is 100% stock and I am sure no one has ever opened it up. I took the base assy apart to make sure the ICS spring retainers are positioned right and they are. Everything in this system is stock now except for removing some HS shims in the compression stack. Bazaar!!!! Anyone have any ideas what could keep the piston from traveling it's designed distance upwards?? This entire system looks brand new, not a scratch or burr anywhere.

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It seems to me that it is more than likely one of two things:

  • The piston is topping out (because the wrong piston or shaft is used. The '05's had a different piston, although other than the bottom of it, I don't know what the difference was)
  • The spring is coil binding.

Assemble the unit without the ICS spring. The rod won't extend by itself like it's supposed to, so you'll have to pull it down, but if you can move the rod up far enough for the piston to clear the bleed holes, it's the springs. If not, it's the piston. You could even mock it up without oil just using the cap and the piston. The cap shouldn't push it down beyond the bleeds.

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Thanks for the response. Just came back in from the shop and here is what I did (before seeing your reply). I bled the cartridge just like normal and when the damping rod stopped a few inches below the cartridge, I backed off of the base assy cap one full turn (equals a little more than 1.25mm of lift) and tried inserting the rod again. It then purged out the transfer holes normally. Once it was purged and the rod would insert and extend nornally the way it supposed to, I tightened the base assy cap fully. Apparently the 1.25 mm difference was a big deal. I am hoping there is no need for the purge to happen while the system is in operation. The damper works beautifully right now it's entire travel with the cap tight. What a freeeeking pain!!

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The trouble is that if it won't move the piston past the bleed holes when it's assembled, it will break something later, even if it works now. For one thing, the oil can expand thermally and need to escape the cartridge when fully compressed. For another, when the fork is bottomed, the oil locks expose the rod seals to a high level of pressure that can force oil into the cartridge from the outer chamber, overfilling it. If that oil can't move the piston far enough to blow off the excess through the bleeds, it will lock, and something will give.

You need to find out what's up and fix it.

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Ok, I understand. The solution for now since the system is all back togetheris to simply rotate the base assy cap one turn from tight. It essentially equals one thread but makes me a little concerned. But the O-Rings are still in place and everything seals properly and the caps shouldn't move since they are so hard to turn anyway with the O-Ring pressure. All I can figure is that someone put different springs in it at some point. With the springs removed, the piston moves smoothly all the way up to the cap. So, maybe it has different springs. The system really looked as though it had never been touched however. I guess it would have been better to grind 1.25 mm off the ends of the springs? I'll do that next time around or order replacement springs. This was a simple job that went south.

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Maybe I'm confused a little but you can easily hydro lock these forks by overfilling them. All it takes is using Dave's procedure and pushing down rather hard to start threading the cap. You'll get the cap all the way on and the rod will stop 4-5" from going all the way in.

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Have seen this before and found that the springs were not the right length Make sure bfore riding the damage caused by not making sure is costly

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Maybe I'm confused a little but you can easily hydro lock these forks by overfilling them. All it takes is using Dave's procedure and pushing down rather hard to start threading the cap. You'll get the cap all the way on and the rod will stop 4-5" from going all the way in.

When things work right, you can't overfill the cartridge. They are designed to be overfilled, in fact, and then the air and any excess oil exhausted on the first full compression as it forces the free piston past a set of holes drilled in the cartridge for that purpose. If the piston won't move that far, yes it will lock and break something, but not because the cartridge is too full.

The problem the SMART free piston mod addresses is what happens when the seals, usually the one on in the piston that seals to the shaft, allow oil to pool in the upper chamber of the piston where the ICS sits. Too much oil and the air pressure within the piston rises to the point where the piston fractures. Drilling the holes lets the oil escape and balances air pressure on both sides of the piston. The same thing can happen because the ICS coils binds because it was shimmed too much, but in that case, drilling the holes won't help.

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Guess I'm still confused then. I can get this to happen with stock 05 carts, 05 carts with modded pistons and carts with modded pistons and epnp springs. If I'm reading you right then I shouldn't be able to do this on 05 forks without shims on the ICS and modded spacers but I can?

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When things work right, you can't overfill the cartridge.

yes you can - never achieved it on Showa's but KYB can be overfilled resulting in an oil lock.

I can reproduce this whenever required.

But - and that's where you are right - using the right procedure when filling and bleeding the cartridge, it won't lock.

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yes you can - never achieved it on Showa's but KYB can be overfilled resulting in an oil lock.

I can reproduce this whenever required.

But - and that's where you are right - using the right procedure when filling and bleeding the cartridge, it won't lock.

I had this happen on some 05 crf forks recently. I was showing my friend how to bleed the inner chamber, having him do it while I talked him through it. He's a bigger guy and he pressed down pretty hard and got the cap to thread in and it locked up about 3-4" from full stroke. I knew that wouldn't work but it helped to show him what would happen if he did it that way.

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yes you can - ...KYB can be overfilled resulting in an oil lock.

I can reproduce this whenever required.

I'd like you to explain how, please. With no oil on top of the piston, it should not be possible.

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I'd like you to explain how, please. With no oil on top of the piston, it should not be possible.

Try it for yourself. Overfill the cartridge and only remove enough oil to get the cap on with a good amount of force.

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I'd like you to explain how, please. With no oil on top of the piston, it should not be possible.

if you are able to insert the base valve assembly, then it will bleed the excessive oil.

but the more oil is in the cart, the more you have to compress the ICS to insert the BV assembly, and that could be a real big amount of force - not possible without special tools...

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If you fully compress the damper rod and fill the cartridge until it runs out the bleeds, you can't do what you describe. The piston doesn't seal in the cartridge barrel until it passes the bleed holes. At that point, oil is trapped, and to advance the piston farther down the bore, the rod must be allowed to extend as the piston displaces the oil. If you insert the compression assembly with the rod extended more than about a half inch and the cartridge filled to the same level, you will run into a situation where a lot of pressure is required to insert the valve far enough to thread it because the ICS has to be compressed to do so. But the point is that if you could muster the strength to assemble it, the cartridge itself would still not be hydro-locked because when the rod is compressed, the free piston will normally rise until the bleed holes clear without any mechanical interference, and the excess oil will be blown off.

The problem in the OP's case is that something is preventing that from happening; the piston is incorrect and too long for the space between the bleeds and the bottom of the fully threaded cap, or the ICS is coil binding.

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If you fully compress the damper rod and fill the cartridge until it runs out the bleeds, you can't do what you describe. The piston doesn't seal in the cartridge barrel until it passes the bleed holes. At that point, oil is trapped, and to advance the piston farther down the bore, the rod must be allowed to extend as the piston displaces the oil. If you insert the compression assembly with the rod extended more than about a half inch and the cartridge filled to the same level, you will run into a situation where a lot of pressure is required to insert the valve far enough to thread it because the ICS has to be compressed to do so. But the point is that if you could muster the strength to assemble it, the cartridge itself would still not be hydro-locked because when the rod is compressed, the free piston will normally rise until the bleed holes clear without any mechanical interference, and the excess oil will be blown off.

The problem in the OP's case is that something is preventing that from happening; the piston is incorrect and too long for the space between the bleeds and the bottom of the fully threaded cap, or the ICS is coil binding.

I agree. In 08, if you used the FC ics 1.6kg springs from 07 and earlier, some of them would coil bind and prevent the piston from moving high enough to bleed off the excess oil. That is, if you filled the cartridge with enough oil to get a good purging! Not long after they came out with a spring that had more compression distance before coils touched.

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If you fully compress the damper rod and fill the cartridge until it runs out the bleeds, you can't do what you describe.

I'm sure you're right :smirk:

but I was talking about a fully extended rod...

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And in that case, what happens is that it become impossible to assemble from a practical viewpoint. But if you could push hard enough, the free piston would remain where it is at that point, and you would be compressing the ICS in order to assemble the cap into the cartridge. When you then first stroke the rod, oil will begin exhausting from the bleed ports almost immediately, instead of at the very last, and keep on doing so until the rod tops out.

As long as the piston can move far enough to clear the bleed ports, it can't lock the rod as the rod is compressed, and will adjust its own fluid level. If it can't, there's trouble ahead.

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Wow this is an informative discussion. I keep thinking about what I did over and over again and I keep coming up with the same results. Both cartridges did the exact same thing.

Final assessment? For whatever reason, the ICS springs are preventing the free pistons from rising high enough to clear the bleed holes. If I back the base valve assy caps out one full turn, the pistons clear the bleed holes normally. One turn equals about 1.25mm.

So it makes me question the recommendations to add a 7mm spacer on top of the ICS spring to lower the free piston.

We were pretty darn sure that these forks had never been off the bike. Now we are wondering if someone tried to use different springs along the way. At least we know that the shims had not been touched since the peening was original. There were ZERO tool marks anywhere on this system. That's hard to do even with the correct tools. I wish I would have measured spring length now so that I could research the original length to help cure my curiosity.

The current status is that they are on the bike with the base assy caps backed out one turn which is barely noticeable. The racer is going to check periodically to make sure that they are not moving at all.

Not my preferred way of operating but for now it will have to do.

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The '05 did have a fault in their construction in that the IC springs are too short to fully extend the free piston on its shaft; negative preload, if you will. If you assemble them, then bleed them with a full stroke, and then fully extend the rod, the oil pulls the piston down to where the spring has clearance, just "floating" on the oil without any preload in the last bit of extension. But sometimes, 7mm is too much too shim them because of exactly what's happened here. What's really needed is a proper set of springs for the '05 setup, or a conversion to later parts. Not sure how much is involved in that last, though.

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