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  1. #1
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    Breaking and entering the damper circuit on a Fox air shock

    I searched a bit on self tinkering the oil section of a Fox rear air shock, but found little. Obviously, Fox does not want us to go anywhere near this. But as I trusted a guy and bought a used Stumpy FSR with a fully functioning RP 23, but ended up with a broken Specialized branded Fox Triad shock, I thought I might as well give it a go. Obviously, this will void any warranty and one might quite possibly damage something in the process. So beware.

    (EDITED aug15 with some new thoughts)

    Anyway, I thought I might as well document things with some pics:

    Here is the shock in "air sleeve service mode". Air sleve gone and air piston seal removed (it's easy to find info on hpw to get this far). I've also taken out the plastic goo from the nitro valve pellet retainer screw (bottom right). By carefully unscrewing the hex bolt (with my safety glasses on) and pumping the shock a bit I got rid of the nitro pressure.



    The lockout adjuster is easily removed by untightening this little hex bolt:


    Below you see the steel pin that at its bottom has the cam that control the three step lockout mechanism. The red rebound adjuster know can be removed after untightening the small hex bolt to the left, but I first removed the hex bolt just above the eyelet and cerefully removed the little spring and ball inside (which creates the clicks for the rebound adjuster). When working in this area, beware of the 1.5mm steel ball that sits bewteen the steel compression/lockout adjuster cam and the compression/lockout adjuster rod (which we will see later).



    Here is only the lockout adjuster pin. To remove it one might have to put a screwdriver into the hole and push away the rebound metering rod.


    The damper body unscrew fairly easily by just clamping one of the eyelets in a (soft) vice and using an adjustable wrench on the other eyelet (obviously with care taken). Here we are looking down into the damper body and can see the floating piston separating oil and nitro.


    Below you can see the valving assembly and the compression/lockout valve (the washer to the right). As you might see, even when fully closed some oil can float into one of the compression circuits by means of the four slots facing the side of the valve body (you see two in the pic)


    In the damper shaft you can now see the hole for the rebound circuit. The flow is metered by a hollow metering rod inside the damper shaft controlled by a cam on the red compression adjuster mechanism. In this pic, also note the washer below the valve body. This is a special little tricky compression valve, we'll see more of it later.


    The bottom of the valve body shows two set of holes. Four inner ones (which get a bit of oil flow even in lockout position) and 24 outer ones (which I guess is the high flow (speed) circuit. Both circuits are controlled by their own shim stacks as we will see.


    Here is the compression adjuster/lockout valving. The rod passes inside the hollow rebound metering rod and contacts the cam on the adjuster via a 1.5mm steel ball.
    Notice also the washer to the left of the valve body. Note that there is nothing but internal pressure (=nitro charge) and possible oil flow that pushes the lockout valve into its place. The adjuster cam only controls the minimum distance (three positions) to the valve body. The role of the little spring is still somewhat unclear to me. It contacts the (mounting bolt for the) valve body and lifts the lockout washer just short of 1mm away from the valve body (the big lockout washer can not move on its rod, rod + washer moves as one). It's unclear to me what happens in practice in the full lockout setting. if the shock takes a hit, the oil flow will surely push the lockout onto the valve body. But I'm not sure if the little spring is strong enough to keep the lockout slighty open "at idle", ie if the spring is strongern than forces from the nitro pressure.


    To remove the valve body I first removed the lockout assembly (the one above) and then made a special little tool to fit two of the holes in the body. I just drilled two 2.5mm holes in a piece of delrin plastic and used two drill bits as pins. At the other end, the square end of the air piston/ upper oil seal head can easily be clamped in a vice. But beware of the corner where the oil set screw is located...thin material around here, it seems)


    Now we see the whole valving assembly. It seems to have three compression circuits (+ rebound, which goes through the shaft). The bottom row are the shim stack for the 4-hole circuit. Small diameter shims means they are rather stiff, ie provide firm damping.
    The middle row of shims covers the 24 outer holes. This is the main circuit, I'd say. Large shims can bend away pretty far from the valve body and allow high flow if forces are high enough. The big shims goes towards the valve body. I suppose some tuning can be done by rearranging this stack, fx putting a smaller shim in between some of the big ones etc.

    An interesting note is that the first (4-hole) circuit is covered by the shims for the second (24 hole) circuit. So even when the first circuit opens, oil still has to push open also the shims for the second circuit. The last washer in the 4 hold circuit is also a stiffer one, effectively limiting the amount this circuit can open. So my guess is that this must be some sort of low speed circuit, maybe with the no 1 purpose of offering some "give" in the lockout setting.

    The upper row are the shims and special washer for the third circuit. We mentioned the special washer before. Oil flow for this circuit comes up though the damper shaft/valve retention bolt (upper right in pic below) and escapes into the special washer by means of some holes in the sides in the valve retention bolt (you can see on of these holes). The shims for this circtuit hence sits below the special washer, ie between the washer and valve body. Oil from the cavity in the washer bends the shims downwards, oil can flow down and then up around the washer. This flow is independent from the the circuits. With the main (24 hole) circuit seemingly being quite heavily damped (three big washers right at the bottom, then some smaller ones), a possible role of this circuit is to provide a subtle (lightly damped) flow at small hits before the main valve opens but then not seriously affect things at bigger hits (since the this valve circuit overall hardly can handle that much flow).


    Finally for now, Specialized sources special size of Fox shocks for many of their bikes. This Triad shock is 190*45 (7.5*1.75). How do they get away with this... Well by just using standard shocks and modify them, it seems. Below you can see an 5mm (aprox 0.25") travel reducing spacer. So it is really just your standard 190*50mm (7.5x2.0") unit. It should be possible to construct such a spacer that could be mounted without disassembly of the oil end of the damper (by having two thinner washers with a "slot" that could be pushed on the rode from the side, the rotated to lock them on the rod and then with a bolt or two securing them together).


    I'm still fighting getting the upper eyelet assembly off the shock shaft which is what I have to do to get the rebound metering rod out and replace its seals/orings (where the leak probably is since oil is coming out where the adjuster knobs are located.

    If anyone has some trick for getting the upper eyelet assembly and shock shaft apart (I have clamped the shaft in soft (wood) jaws and heated the upper assembly to loosen the red locktite inside, but still no luck.

    Comments welcome.
    Last edited by Ola H.; 05-12-2013 at 05:49 AM.

  2. #2
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    To unscrew the shaft without damaging it you can make a shaft clamp from a piece of aluminium or other soft metal.
    Drill a hole through as close as you can to the size of the shaft and then saw a slot through it. Use these in the vice to get enough grip without damage.

    Alternately, you can try wrapping a piece of inner tube around the shaft so it grips your wooden blocks better.
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  3. #3
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    Great info and pics. Thanks for sharing.

  4. #4
    PMK
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    I'm going to post what's below and it will no doubt upset some, but it may save a lot more a bunch of money, frustration, and downtime.

    While it is nice to see information about this, and not even taking warranty stuff into the discussion, you have just opened up a Pandoras Box for a lot of people.

    I am all for seeing quality DIY maintenance, but having been a suspension geek for over 30 years, I know these are not your basic damper.

    By comparison, a motocross rear damper, any quality brand, Showa, KYB, Ohlins, are fairly straightforward to work on with no option circuits like Pro Pedal, or lockouts.

    These MTB dampers are more complex, much more difficult to properly bleed, and Fox has the infamous needle fill for the gas pressure.

    I have rebuilt and tuned loads of various MTB dampers. Removal of the center pin with the lockout plate secured to it was not such a good idea. The "O"ring at the opposite end, that is a fluid seal, normally is ejected when the pin is reinstalled.

    Yes you will need to remove the upper end fitting. Dougal is correct in stating an aluminum shaft block. Good luck with a wooden block, it has never worked well for me, plan on machining a true shaft block from aluminum. Also, fire up the propane torch and heat the fitting pretty good. While clamped very firmly, and don't let the shaft slip or it will be ruined, unscrew the end fitting. Obviously the heat will likely trash all the "O"rings, at least the urethane "O"ring will melt.

    I've done two dampers this week that required that much disassembly, both were RP series.

    I would rather rebuild 5 motocross dampers than one Fox Pro Pedal damper.

    BTW, you're only 25% done, reassembly and proper servicing is another entire book, not a chapter.

    BTW, anyone that has plans to pull apart an RP series with Pro Pedal, when you remove the piston assembly and shims, be very careful as you remove the piston retaining bolt. There is a spring for the Pro Pedal, under that spring is microscopic (about 1mm ID and 2mm OD) shims that add preload (PP threshold setting). Don't lose any of it or it won't work properly.

    Again, I'm all for folks doing their own work, but this is not for most.

    Great post and topic though, one I love to see and have no problem following.

    PK

  5. #5
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    Quote Originally Posted by PMK
    ...By comparison, a motocross rear damper, any quality brand, Showa, KYB, Ohlins, are fairly straightforward to work on with no option circuits like Pro Pedal, or lockouts...
    This fox propedal stuff looks more complicated than the WP PDS with a piston "and a half" and its associated complicated bleed process

    I wanted to mod the comp and rebound shim stack on an RP3 that I use on my daughters 24" FS bike to better dial it in for her very light weight but when I look at what's involved even I don't feel inclined to open it up - and there is nothing on a bike or motorbike that I have not fixed, rebuilt or re-engineered.

    Not wanting to derail the thread but are there any other manufacturer's shocks that are completely user serviceable?

  6. #6
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    Quote Originally Posted by PMK
    ...
    Removal of the center pin with the lockout plate secured to it was not such a good idea. The "O"ring at the opposite end, that is a fluid seal, normally is ejected when the pin is reinstalled.
    ...
    I see your point: not for most. But for me it's mostly a learning process. I just NEED to try now when the possibility opened up, so to say.

    About the o-rings. I figure one of these (lockout rod/rebound rod or rebound rod/damper shaft) is what is shot anyway, so they will need replacement, but thanks for the tip.

  7. #7
    PMK
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    Quote Originally Posted by TigWorld
    This fox propedal stuff looks more complicated than the WP PDS with a piston "and a half" and its associated complicated bleed process

    I wanted to mod the comp and rebound shim stack on an RP3 that I use on my daughters 24" FS bike to better dial it in for her very light weight but when I look at what's involved even I don't feel inclined to open it up - and there is nothing on a bike or motorbike that I have not fixed, rebuilt or re-engineered.

    Not wanting to derail the thread but are there any other manufacturer's shocks that are completely user serviceable?
    If you have been inside a WP PDS, we know they, meaning WP, botched it up by going a 2 piston design. The Ohlins and works WP are the 1 1/2 piston setups.

    Yes, this is more difficult to build, bleed, tune than a WP, and the results are not near as noticeable, with a downside failure being steep.

    Other manufacturers yes, or if you can fit it in her frame, buy the girl a Fox DHX 5.0 air. These are nice to work on and an easy service with some basic damper bleeding skills. Also, gas pressure is filled via the boost valve as they call it, and is shrader not needle.

    By the sound of it tigman, you know where to find me, send a PM and maybe your kid can get dialed in for a fraction of what PUSH charges. Never mind, noticed it was Sydney and I doubt that's anywhere but Oz.

    PK

  8. #8
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    PMK: Since I'm aslo in the market for a shock upgrade to the Triad (the one I'm experimenting with above) I'm wondering if a DHX air might fit a 06 stumpy. Maybe a bit overkill, but to me the possibility to work/tune the shock yourself is worth a lot.

    I see there is a 190mm (7.5") DHX air that one might hope could be reduced to 45mm/1.75" travel like Float series can. But will the piggyback fit? Of course, it will not fit the tunnel through the frame, but maybe it "ends" before it contacts the tunnel. I reckon I have about 110mm/4.2" from the upper eyelet, measured parallell to the shock, see image below.

    PMK or Anyone with a dhx air: is this enough? Other things to consider?


  9. #9
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    I believe I know PMK from KTMTalk and he is right about the RP series of Fox air shocks. They are amazingly complex and there seem to be endless variations of the propedal system over the years.

    As he mentioned the DHX Air is very easy, in comparison, to service. For a light rider it will work very well, but for a more aggressive or heavier rider the DHX Air is not as good as the RP23.
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  10. #10
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    Quote Originally Posted by Ola H.
    I see there is a 190mm (7.5") DHX air that one might hope could be reduced to 45mm/1.75" travel like Float series can. But will the piggyback fit? Of course, it will not fit the tunnel through the frame, but maybe it "ends" before it contacts the tunnel. I reckon I have about 110mm/4.2" from the upper eyelet, measured parallell to the shock, see image below.

    PMK or Anyone with a dhx air: is this enough? Other things to consider?
    I measure my 190x50 DHX5 as approximately 110mm from centre of eyelet to end of BO adjuster. On top of that the dust cap I have on the valve is another 5mm or so, but you could probably use a lower profile one than that if needed.

    Sorry I can't give exact figures at the moment as it's difficult to measure whilst bolted to the frame. Shout if you want more exact values. Looking at your picture though I'd be inclined to say it would fit.

  11. #11
    PMK
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    Ola
    I can not measure for you since one of my loaner shocks is, well being used as a loaner while I was working on a customers shock. His is done and waiting, but he needs swap back to his RP2 so I can get my loaner DHX 5.0 back.

    The loaner is a 7.5/2.0, so it's the one to check.

    Sorry, hope to get it back soon.

    PK

  12. #12
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    I have a DHX 5 coil on one of my bikes (also 7.5x2). Let me know if you're after anything specific.
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  13. #13
    PMK
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    Quote Originally Posted by Scott@GO-RIDE.com
    I believe I know PMK from KTMTalk and he is right about the RP series of Fox air shocks. They are amazingly complex and there seem to be endless variations of the propedal system over the years.

    As he mentioned the DHX Air is very easy, in comparison, to service. For a light rider it will work very well, but for a more aggressive or heavier rider the DHX Air is not as good as the RP23.
    Yes Scott, KTMTalk it is.

    Before I ask about the DHX vs RP, how is Zerodog doing? I have been trying to find time to see if he is interested in making some parts for me.

    So, back to the RP vs DHX. You mention DHX for light guys, is this based on the air spring volume or internal settings.

    I ask, because our Ventana ECDM tandem had an RP3 on it. The RP3 revalved was actually very good in PP#2, horrible in PP#1, and we never ride semi locked in PP#3. This is a small air can 7.875/2.0 RP3. I want to believe it was not a tandem damper on account of having to run the rebound fully closed on our first ride, fully closed on second ride after a fluid change, but mid clicks after a revalve. I noticed also our bike had an 8mm upper spacer set, where the short travel tandem has a 6mm spacer. After talking with Sherwood, he informed me the long travel bikes use 8mm. We bought the ECDM used with no mention of a shock swap, but who knows. Again revalved it's not bad.

    As for our DHX5.0 air, I got it from eBay, supposedly has only a few rides. I installed it and set up pressures and so forth. Had the wife make some clicker setting changes as we rode. This DHX has the large air can and was typical soft through the mid stroke on spring rate. I swapped it to a small air can, lowered the pressure setting to the same as the RP3, again a few clicker adjustments, and it dialed in very well with no revalve.

    So I ask, RP series vs DHX5.0 air, damping and internals or air spring setup?

    Also, do you have any Fox40 series used forks for sale? We need more fork in regards to tunability, not so much travel, since the fork is now the limiting factor on the ECDM. The limiting factor should be us.

    PK

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    Ouch, very tight then. Kind of steep money to take the chance. A more exact figure would for sure be welcome. I reckon even the schaeder valve with a (minimalistic) end cap would have to fit within the 110mm.

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    Quote Originally Posted by Ola H.
    Ouch, very tight then. Kind of steep money to take the chance. A more exact figure would for sure be welcome. I reckon even the schaeder valve with a (minimalistic) end cap would have to fit within the 110mm.
    Red = 108mm
    Yellow = 3mm
    Green = 6mm

    It'll look a bit off in the image as the camera distorts the angles;

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    Thanks. I'm gonna go home and measure the frame carefully again and start looking or the right sized DHX.

    PMK or someone else: do you reckon it's as straghtforward to limit travel on a dhx air as on a Float (ie that it can be done with a reducer on the shock shaft)?

  17. #17
    PMK
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    The DHX has a larger diameter shaft. The method of a spacer will work though..

    You might consider running that triad can so the spring rate is similar.

    How much you looking to spend, when my loaner is returned I'll likely part with it since it is to short to be a spare on the ECDM.

    PK

  18. #18
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    Quote Originally Posted by PMK
    Yes Scott, KTMTalk it is.

    Before I ask about the DHX vs RP, how is Zerodog doing? I have been trying to find time to see if he is interested in making some parts for me.

    So, back to the RP vs DHX. You mention DHX for light guys, is this based on the air spring volume or internal settings.

    I ask, because our Ventana ECDM tandem had an RP3 on it. The RP3 revalved was actually very good in PP#2, horrible in PP#1, and we never ride semi locked in PP#3. This is a small air can 7.875/2.0 RP3. I want to believe it was not a tandem damper on account of having to run the rebound fully closed on our first ride, fully closed on second ride after a fluid change, but mid clicks after a revalve. I noticed also our bike had an 8mm upper spacer set, where the short travel tandem has a 6mm spacer. After talking with Sherwood, he informed me the long travel bikes use 8mm. We bought the ECDM used with no mention of a shock swap, but who knows. Again revalved it's not bad.

    As for our DHX5.0 air, I got it from eBay, supposedly has only a few rides. I installed it and set up pressures and so forth. Had the wife make some clicker setting changes as we rode. This DHX has the large air can and was typical soft through the mid stroke on spring rate. I swapped it to a small air can, lowered the pressure setting to the same as the RP3, again a few clicker adjustments, and it dialed in very well with no revalve.

    So I ask, RP series vs DHX5.0 air, damping and internals or air spring setup?

    Also, do you have any Fox40 series used forks for sale? We need more fork in regards to tunability, not so much travel, since the fork is now the limiting factor on the ECDM. The limiting factor should be us.

    PK
    PK,

    Zerodog is doing great. We hang out about once a week and work on MTB projects when we have time. Not sure if he is making parts on the side because his full-time job is keeping him busy. He's also been riding a lot of motorcycle trials. Not sure if those things even have damping.

    I can't begin to compare shocks on tandems as I don't own one and don't think I've ever even ridden one. However, on singles I do have loads of experience and testing. The DHX Air has problems in both areas you mentioned. Everyone I've ever seen comes with a large air sleeve and requires very high air pressures (generally 250psi or more) for riders over 150 - 170 lbs. Yes, if you get a small air sleeve and combine it with a low leverage bike like the Ventana you can make it work.

    For comfort on a tandem I bet it would be good as they have very little compression damping. However, when used as intended on a 5 - 7" travel (single) bike they just can't keep up as the the speed picks up and the terrain gets rough. I believe that this is mostly due to the damping oil being pressed through the rebound rod to get to the reservoir. My guess is they just start cavitating and loose most of their damping.

    Sorry, no used 40s at this time. I'd be concerned with the lack of steering radius you would get from a double crown fork on a super long wheelbase bike.
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  19. #19
    PMK
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    Quote Originally Posted by Scott@GO-RIDE.com
    PK,

    Zerodog is doing great. We hang out about once a week and work on MTB projects when we have time. Not sure if he is making parts on the side because his full-time job is keeping him busy. He's also been riding a lot of motorcycle trials. Not sure if those things even have damping.

    I can't begin to compare shocks on tandems as I don't own one and don't think I've ever even ridden one. However, on singles I do have loads of experience and testing. The DHX Air has problems in both areas you mentioned. Everyone I've ever seen comes with a large air sleeve and requires very high air pressures (generally 250psi or more) for riders over 150 - 170 lbs. Yes, if you get a small air sleeve and combine it with a low leverage bike like the Ventana you can make it work.

    For comfort on a tandem I bet it would be good as they have very little compression damping. However, when used as intended on a 5 - 7" travel (single) bike they just can't keep up as the the speed picks up and the terrain gets rough. I believe that this is mostly due to the damping oil being pressed through the rebound rod to get to the reservoir. My guess is they just start cavitating and loose most of their damping.

    Sorry, no used 40s at this time. I'd be concerned with the lack of steering radius you would get from a double crown fork on a super long wheelbase bike.

    Yes, we found also the need to run what I thought was to much air spring pressure with the large can. In regards to compression damping, we had to back off PP with the high pressure and wallowed or blew through the stroke. Small can cured this.

    Yes, our ECDM is 2:1, which helps in many ways, but should overtax the compression circuit. The 2:1 did let us dial in a very detailed PP setting between detents.

    On the single bike, my guess is that it is not so much a flow into the rezzy on account of the fluid travelling the tube, rather once again platform settings and more importantly, flow restrictions of the PP circuit add unwanted compression spikes. If the damper is cavitating, and it may be, it would be in rebound, thus affecting compression also.

    Have you or anyone removed the platform circuits and built a DHX air as a conventional based damper? This would probably make it decent. Then again, for DH, a coil over setup, with direct flow from body to rezzy is better than shaft flow.

    We already run double crown forks with an ATC. Sadly the damping is not up to the task but may be if I work the cartridge. I'm leaning towards a Kashima Fox40 for the ECDM if I need to buy something new.

    PK

  20. #20
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    Great write up and pics Ola H. Thanks for taking the time to post this info up. It's enlightening.

    P

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    Ola, or anyone, how does the gas charge hold pressure before the cap is replaced?

    Is it charged with a needle similar to the old Vanilla RC?

    Thanks for the pics and interesting discussion.

  22. #22
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    Float series shocks are charged with a needle through a rubber disc at the top of the shock body. The shock is charged after its all assembled and bled. Triads are similar.

  23. #23
    PMK
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    Quote Originally Posted by Ola H.
    This photo shows the gas service port which is a rubber disc, pierced by a needle, located under the set screw on the dampers lower end, next to the mounting eye. The set screw is drilled in the center for needle access. Normally when you look at these they have a plastic plug to minimize tampering.

    The disc setup is somewhat of a nuisance. It's a shame I can't get warranty replacements on needles like tools at Sears.

    The person that posted about servicing the gas after assembly and bleeding is correct. Also, I have never seen it posted, but the depth of the floating piston is critical on initial build and final bleed. To deep and the damper hydraulic locks, to shallow and the damping piston contacts the IFP and cavitates the damper and rides the combined spring and gas pressure with no damping.

    They can be serviced with a few special tools and some knowledge. Just be careful not to do more harm than good.

    PK

  24. #24
    PMK
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    Ola, you might offer these guys that are itchin' to bust into their dampers a shot of the fluid bleed port. This is the small set screw that uses a ball to seal a drilled hole in the sealhead.

    PK

  25. #25
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    Quote Originally Posted by PMK
    Also, I have never seen it posted, but the depth of the floating piston is critical on initial build and final bleed. To deep and the damper hydraulic locks, to shallow and the damping piston contacts the IFP and cavitates the damper and rides the combined spring and gas pressure with no damping.
    Yes I've given Ola some useful information on setting IFP depths.
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