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"Off Road", April 1, 2013






We wanted to take a look at what lies inside a typical reservoir shock and the steps needed to tear one down and get it back together. FOX recommends racers rebuild shocks after each race and non-racers rebuild about once a season for optimal performance.

The internals of a reservoir shock used on a coilover and the main body internals used for a bypass shock are the same. A coilover has a coil spring over its exterior and can provide the means to support the vehicle at ride height. A bypass shock is used in tandem with a coilover shock or other suspension spring and serves only as a suspension damper. It utilizes some number of oil bypass passages that allow the shock to provide different damping rates over its range of shock travel to improve how the suspension reacts to small and large obstacles.

The main shock has a damping piston with valve stacks on both sides. One stack controls the compression rate and the other stack controls the rebound rate. Additionally, these shocks use a piggyback or remote reservoir to house additional shock fluid and an internal floating piston (IFP) that separates the shock oil from a volume of high-pressure nitrogen gas. The nitrogen serves to prevent air bubbles, or cavitation, from occurring in the shock oil.

Joe Moore at FOX took us through the teardown, inspection, and rebuild of a bypass shock to show us what all is involved and pointed out a few tips along the way. FOX offers a concise text manual for this rebuild. Our photos here should help you further understand the various parts teardown and assembly during the process.

1 Shocks come into FOX for service in all conditions. Some simply need a refresh with clean shock oil and new seals. Others may be in for a revalving or other repair due to some damage that occurred while in use. We’ll take a look inside this two-tube 2.5-inch bypass shock.

2 A vise with some form of padded jaw works well to hold the shock during much of the rebuild process. Disassembly started with loosening the small setscrew in the bearing cap and loosening the cap using a spanner wrench. Nitrogen pressure should be discharged from the shock before completely removing the bearing cap.

Once the bearing cap was unscrewed and slid up the shock shaft, the exposed internal snap-ring could be removed from inside the shock body.

3 Joe tied a shop towel near the top of the shock tube to catch oil runoff and then pulled firmly on the shock eyelet to remove the shaft assembly from the shock body. The old shock fluid in the body should be disposed of.

4 Here you can see the damping piston assembly along with the valve stacks. The piston lock nut was removed and all of the piston components pulled off the shock shaft.

5 Joe laid out all the piston parts in order. He then measured and recorded the thickness of each of the valve stack shims. This is always done to keep track of the valving and any changes. If the shock is being revalved, this is the time to swap out the shims as needed.

6 This aluminum cylinder is the bearing housing that was slid off the shaft. It has both outside and inside O-rings that need to be replaced. Care should be taken not to scratch any of the sealing surfaces when pulling O-rings or disassembling parts.

7 The piggyback reservoir was removed by removing the hose clamp and pulling the internal snap ring that secures it in place. Then the snap ring and reservoir end cap on the opposite end were removed. At this point, the IFP can be seen inside the reservoir tube.

8 Joe pushed the IFP out of the reservoir using a blunt hammer handle. Metal tools should be avoided here to prevent scratching the sealing surfaces.

9 At FOX, all the internal components are cleaned in an ultrasonic cleaning bath. DIY rebuilders can use common parts solvents that won’t attack aluminum. The idea is to ensure everything is clean before installing the new seals.

The insides of the shock body and reservoir are also thoroughly cleaned in a part washer.

10 Joe cleans and polishes shock shafts by spinning them against 400-grit sandpaper with a cordless drill. This smoothes the plated rod without harming it and then allows the rebuilder a clear way to examine the shaft for nicks or other damage that could harm the new seals.

11 A new bearing cap wiper seal was pushed into the bearing cap. This is best done by lubing the seal and pushing it into the cap groove using a hard, blunt tool handle.

12 The bearing housing received new lubed O-rings.

13 The FOX guys use this handy rounded tip fitted over the end of the shock shaft to allow them to slide the bearing cap and bearing housing onto the rod without risk of damaging the internal seals.

14 The valve components were placed back on the shaft with the compression stack toward the shaft. The piston nut gets coated with antiseize and should then be torqued to 30 ft-lbs. A nut that spins on easily by hand should be replaced.

15 Sometimes the interior bore of the shock or reservoir can have built-up oxidation, other foreign residues, or a very slight wear ridge. Joe used a 320-grit sanding flap wheel to lightly hone the bore to clean this reservoir and provide a smooth sealing surface.

16 A new O-ring was slipped onto the IFP with a little Maxima waterproof grease added to make assembly easier and help with initial sealing.

17 A new IFP wear band was wrapped around the IFP as it was slid back into the reservoir body.

18 With the reservoir slid back onto the reservoir tube, the snap ring can be replaced. The reservoir was secured to the shock body with the hose clamp. On a shock with a remote reservoir, there would be the flexible hose here in place of the hard tube used on this piggyback shock.

19 Joe used a non-metallic hammer handle to push upward on the IFP to move it up in the reservoir body. Then the reservoir end cap with Schrader valve was reinstalled with the snap ring. The reservoir was pressurized to about 10 psi. (Not all shocks have the IFP “topped out.” IFP depths vary depending on the shock. These depths can be downloaded from the FOX website.)

Fresh fluid was added to the shock body and Joe worked the shaft assembly back into the body. The FOX manual details a few steps that allow complete filling of the shock oil and proper bleeding for both piggyback and non-piggyback shock types.

20 With the fluid primed into the shock and the air bubbles worked out, the O-ring on the outside of the bearing housing was greased and the housing slipped into the shock body. Excess fluid is pushed out during this process, ensuring no air pocket is left behind in the shock. This must occur with the shock shaft at full extension with the rod guide touching the internal spacer or top-out plate.

21 Next, the shock assembly was nearly completed with install of the snap ring and bearing cap. A 200-psi nitrogen charge was applied. This holds the bearing in place as the bearing cap is fully tightened and secured with the setscrew. The rebuild was complete.

22 Before disassembly, the adjustment screw lock nut is loosened and the adjustment screw turned fully inward to measure and record the number of turns it takes to bottom the screw. This screw adjusts how far the valve will open when the bypass is in use.

Next, the blue (compression) or red (rebound) cap is loosened, and the plunger and adjustment screw are removed as a unit. Behind the plunger are a small spring and a bypass piston. The spring and piston serve as a one-way valve so each bypass is only active in one direction. The piston can be removed with a small magnet, or by tipping the shock upside down.

23 Here you can see the two plunger assemblies. Each bypass plunger has two O-rings that need to be removed and then replaced with fresh ones once the parts have all been cleaned.

24 Joe installed new O-rings using a little fresh shock oil for lubricant. He pointed out how he carefully slips the O-rings onto the plunger using a pick tool while working the tool around the circumference. He cautioned about rolling O-rings onto piston parts as this can twist the seal and not allow it to seat and seal properly.

25 With the new O-rings installed, it was time to put each bypass piston, spring, and plunger assembly back into their respective bypass tube. The tubes were cleaned and inspected when the shock body was completely stripped apart.



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