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KYB Free Piston


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This plastic ring adds to the longevity of the lower o-ring. I would suggest removing the upper o-ring. It adds drag as well as threatens the life of the free piston. I would suggest you also drill a 3mm hole in the middle of the free piston halfway between the two o-ring lands.

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Before I drilled holes, between 1 and 2 o-rings was closed air chamber, right? ( i find spring with black corrosion). It creat additional damping. Now piston drilled and air and oil from out fork chamber move freely. Can I remove 1-2 rings like showa style?

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I broke that plastic free piston. Dave J sells aluminum replacements that are totally worth the piece of mind IMHO. They also have holes already designed into them. I suppose the plastics ones would be less prone to breaking with the holes drilled though...

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I broke that plastic free piston. Dave J sells aluminum replacements that are totally worth the piece of mind IMHO. They also have holes already designed into them. I suppose the plastics ones would be less prone to breaking with the holes drilled though...

You broke stock piston ore mod. whith holes ?

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e82b14ea0b75.jpg

The ring provides a seat for the O-ring. That ring coupled with the slots in the piston gland allow the O-ring to act as a one way check.

After a cavitation event blows fluid out of the shock the slot in the piston gland allows the fluid to seep back into the shock and past the O-ring which does not seal well against vacuum.

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Hello !

Question is, what for plastic ring and how it works?

Thanks.

That Teflon ring is called a "backup ring". It is there to prevent the o-ring from extruding through the clearance gap between the free piston and its bore. They are commonly used when sealing between a cylindrical interface, especially when the two parts must move relative to each other (because there must be clearance). The more rigid Teflon ring provides the extrusion resistance without adding too much friction, and the softer rubber o-ring performs the sealing function.

There might be some more/better explanation on the web if you search. I have an internal design standard at work that we use for designing sealed interfaces, but I can't post that up, obviously.

Edited by adynes
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That Teflon ring is called a "backup ring". It is there to prevent the o-ring from extruding through the clearance gap between the free piston and its bore. They are commonly used when sealing between a cylindrical interface, especially when the two parts must move relative to each other (because there must be clearance). The more rigid Teflon ring provides the extrusion resistance without adding too much friction, and the softer rubber o-ring performs the sealing function.

There might be some more/better explanation on the web if you search. I have an internal design standard at work that we use for designing sealed interfaces, but I can't post that up, obviously.

­čśĆ

http://www.ahpseals.com/products/backup.php

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e82b14ea0b75.jpg[/url]

The ring provides a seat for the O-ring. That ring coupled with the slots in the piston gland allow the O-ring to act as a one way check.

After a cavitation event blows fluid out of the shock the slot in the piston gland allows the fluid to seep back into the shock and past the O-ring which does not seal well against vacuum.

Interesting. ­čśĆ

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Ah, something that makes sense.

Having spent something like two decades working with automatic transmissions, I'm very familiar with scarf cut nylon/Teflon seals like these, but I never understood why there would be one here. This explains it.

heavy equipment manufacturers use these all the time in the seal assemblies on hydraulic cylinders. they see excess of 5000 psi sometimes.

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The thing with such seals in hydraulic equipment, like the automatic transmissions I spent two decades working on, is that they have to leak in order to seal. By that I mean that they depend on oil flowing underneath them to drive them outward against the cylinder in which they work, so they always let a certain amount of fluid get away as the circuit starts up. Such leaks are of no consequence in that situation. But this means they don't seal at low pressure differentials well, and wouldn't work as a free piston seal, since there is little or no pressure delta near full extension.

Used here to back up an O-ring, they perform a different function.

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Grayracer

Ther upper o-ring creates the seal behind the free piston. It is this seal that eventually causes its downfall due to pressure increase, followed by splitting.

Then once Dave's recommended piston drilling is done, the seal is no longer either a threat, nor of any further use, unless I'm missing something.
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Grayracer

I think you'll find Dave is using my recommended drilling from back in 05 when I first posted a solution for the much plagued KYB TC fork. (their first example) Never the less, the drilling will indeed eliminate the need to remove the o-ring and removing the o-ring will eliminate the need for the drilling. But the upper o-ring will always be a source of drag. I can't see the need to keep it there.....particularly if you have already drilled a hole.

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Grayracer

I think you'll find Dave is using my recommended drilling from back in 05 when I first posted a solution for the much plagued KYB TC fork. (their first example) Never the less, the drilling will indeed eliminate the need to remove the o-ring and removing the o-ring will eliminate the need for the drilling. But the upper o-ring will always be a source of drag. I can't see the need to keep it there.....particularly if you have already drilled a hole.

I don't think I'm cool with running it without the holes.

When removing just the o-ring, you risk the free piston acting like a large cup, and filling up with fluid. This is why we run two holes at the bottom and then two holes at the top.

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