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Discussion Starter · #1 ·
I’ve rebuilt several KYB mono tube shocks in the past but I attempted my first piggy back rebuild today. I have a few questions and am wondering if someone would mind if I gave them a call to discuss some doubt that I have about my workmanship. Figure it may be easier on the phone.
Please PM if you are able to help.
Thanks
 

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I followed this. Only concern i have is they want the floating piston bottomed out in bore but to me it seems odd but i beleive the skidoo service says same thing.

Im no expert but have followed above on mine to freshen up. Leave revalves for experts
 

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Discussion Starter · #5 ·
Yes, I have seen that video. Seems to cover the basics and like you said leave the valving to the experts.
With the floating piston bottomed out the shock will be softer as there will be less oil and more Nitrogen. I believe there is about 15mm difference between factory spec. and the piston bottomed out. FWIW: I put mine to the bottom.
 

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What do you guys mean by having the IFP "bottomed out?

The Nitrogen is there to pressurize the oil so it wont cavitate when passing through the shim stack/piston AND provide a space for increased oil volume when the shock is compressed.

If the IFP height is set too deep and the IFP hits the bottom in a piggyback even one millimeter before the shock reaches fully extended length the oil will become unpressurized, or more precisely subjected to "vacuum", which will more or less guarantee that gas will bypass the IFP into the oil. Putting the IFP too shallow could wreck the shock if the IFP hits the piggyback "lid" before the shock is fully compressed

For a non piggyback shock the oil volume displaced by the piston rod when the shock is compressed needs somewhere to go, so the volume between the IFP and the top of the shock will need to be at least as large as the volume of the piston rod.

There is a, (sometimes quite wide), range for IFP depth but getting outside it in any direction could have a price....
 

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Discussion Starter · #7 · (Edited)
Good catch Simnil: I should have explained my approach further when I mentioned bottoming out the IFP.
I bottom it out only while inserting the shaft/bleeding the shock. Once the cap of the shaft is screwed in the IFP rises approximately 0.5cm so it’s not directly on the bottom when the shaft of the shock is fully extended.
 

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On a pro 40 for the rear arm if you follow the published IFP depth the shock rod will never extend. Bottom IFP out always and it will be good.

Sent from my SM-G781W using Tapatalk
 

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What do you guys mean by having the IFP "bottomed out?

The Nitrogen is there to pressurize the oil so it wont cavitate when passing through the shim stack/piston AND provide a space for increased oil volume when the shock is compressed.

If the IFP height is set too deep and the IFP hits the bottom in a piggyback even one millimeter before the shock reaches fully extended length the oil will become unpressurized, or more precisely subjected to "vacuum", which will more or less guarantee that gas will bypass the IFP into the oil. Putting the IFP too shallow could wreck the shock if the IFP hits the piggyback "lid" before the shock is fully compressed

For a non piggyback shock the oil volume displaced by the piston rod when the shock is compressed needs somewhere to go, so the volume between the IFP and the top of the shock will need to be at least as large as the volume of the piston rod.

There is a, (sometimes quite wide), range for IFP depth but getting outside it in any direction could have a price....

It is impossible for the IFP to be bottomed out after assembly when done correctly. During assembly you can set the IFP all the way to the bottom of the reservoir bore. When you install the seal head on the shaft side, the volume of the seal head will displace the oil and push the IFP away from the reservoir bottom. It is important to make sure the shaft is fully extended during assembly though.
 

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Good catch Simnil: I should have explained my approach further when I mentioned bottoming out the IFP.
I bottom it out only while inserting the shaft/bleeding the shock. Once the cap of the shaft is screwed in the IFP rises approximately 0.5cm so it’s not directly on the bottom when the shaft of the shock is fully extended.
Now I'm with you! :)
 

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When you install the seal head on the shaft side, the volume of the seal head will displace the oil and push the IFP away from the reservoir bottom.
That is part of the design idea behind the quickest ever shock absorber to service - Ohlins regular 46 mm shock absorber. The displacement of the seal head matches the displacement of the free piston. Release the nitrogen pressure, remove the silly simple circlip that holds the seal head in the cylinder tube, re-pressurize and the seal head lifts exactly the amount it needs to lift to come out of the cylinder tube...

The brave guys even have tools that requires no de- and re-pressurization...Shock service and damping spec changes without the need of a nitrogen bottle...
 
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