Wednesday, January 26, 2005

How to change the oil in a Fox Vanilla RC shock

About a month ago my Fox Vanilla RC fell apart. I was on a ride and noticed something that sounded like a loose pivot in the rear suspension it turned out it was not a pivot but the shock cylinder had started to unscrew from the shock body and all of the oil had leaked out. I had been looking for a reason to get a new rear shock but I didn’t want to do it till the spring. I looked into having someone like PUSH Ind. refill it but it turns out they won’t refill a shock unless they have rebuilt it in the past. Looking at spending $90 on a shock wanted to get rid of wasn’t appealing. So I figured I would take a crack at refilling it myself, how hard could it be right? Turns out not too hard at all. A good bit of searching showed no tips for doing this. So after successfully working on my shock I figured I would write this up for the better good.

Fox says you should never take you’re shock apart. It will void the warranty and you could break it. If you are don’t regularly work on your bike and wouldn’t consider working on your fork walk away now. If you work on you shock and it breaks in the future or blows up on a big landing hucking you into traffic I am not responsible. If you have reservations about anything buy a new shock or send it to someone carrying liability insurance like PUSH.

When I disassembled my shock I found that the o-ring that seals the cylinder to the shock body had been damaged likely during the small about of ridding before I noticed the shock was falling apart. Again no one sells parts for this shock as you’re not supposed to take it apart. So some measuring of the old o-ring and related parts lead me to the replacement being a 27mm ID x 2mm wide Buna-N o-ring available from many places including
McMaster Carr (pn 9262K193).

Beyond the o-ring all you should need is 5wt. shock oil and assorted hand tool. Latex gloves can be nice as there will be oil all over your hands.

Hydraulic system warning:
A shock is a sealed hydraulic system. There is noting you want less then dirt in a hydraulic system. Cleanliness is paramount here. Before you take anything apart (but after you remove the spring) clean the entire shock. Use a tooth brush to get in the threads on the shock body. Make the think look like new. As you are working use clean towels. If you are going to wipe any internal parts do it with a new paper towel that you know is clean.

Start by removing the compression adjuster. To do this first remove the two set screws one on either side of the compression adjuster. One of these has a spring and ball under that provides the clicks when you turn the adjuster. The other set screw is longer and stops the adjuster from unscrewing completely from the shock body. Don't worry about witch side these came out of as they can go back in either side.

After the set screws are out you can remove the compression adjuster. Just turn until it comes out. There should be an o-ring on the adjuster and a small spring and metal cup on the bottom of the adjuster. Don't lose these pieces.

You don't need to worry about an explosion or anything from the nitrogen charged reservoir or ‘losing’ your charge. There is a floating piston in the reservoir that separates the gas from the oil. This will keep all the gas in unless there is a leak around that piston. Then you have bigger problems then we can deal with here.

After you have the adjuster out you can drain the oil from the shock by holding it rod up, reservoir down and stroking the shock. Make sure you turn the rebound adjuster all the way out (faster) it will make it easier to stroke the shock. Once you have all the oil out it's time to take stuff apart!

First you need to remove the cylinder body from the shock base. The cylinder simply threads into the base with the same threads that the preload adjuster runs on. To remove it use a large crescent wrench on the flats at the end near the rod seal or a strap wrench on the cylinder. You should have a pan ready to catch any remaining oil that comes out.

Completely unthread the cylinder and drain the last of the oil completely. This can be done by stroking the piston while holding the cylinder upside down over.

Once you have all the oil out you can go about replacing the cylinder-base o-ring if you need to.

Now it's time to refill the shock. I'm not sure what weight oil Fox uses but I refilled with 5wt and it feels the same as factory. Start with the rod fully extended. Then fill the cylinder about half full with oil. Now slowly push the piston up until there is about 5mm of oil left on top of the piston, there should be bubbling as you do this. Now pull the rod back down, again more bubble. Add oil as need the goal is not to suck any air behind the piston and to get all the air out that is there already. Repeat this until you are confident that all the air is out. Pull the rod all the way out (piston to the bottom/shock extended) and fill the cylinder up to the top.

Next thread the cylinder into the body and tighten by hand. No need to really crank on it now the o-ring will keep things sealed.

Now we need to bleed the last bit of air out. The important thing to know here is that as you compress the shock the available space in the cylinder for oil will decrease. The oil will pass thru the main compression porting to the space behind the piston but that space now contains the piston rod which takes up space. This is why there is the piggy back reservoir with the floating piston. The oil displaced by the rod will flow into this reservoir and compress the nitrogen. This compressed nitrogen is why the shock will rebound slowly even without a spring on it. The nitrogen pushes the oil back into the cylinder.

We can use this rod displacement to our advantage when bleeding the cylinder. If you push the rod in with the shock at about 45 degrees and the open compression adjuster hole up, the air will flow out. Slowly push the rod in until you see oil come up thru the adjuster hole and get near the top of the hole. If no oil comes out it is OK just keep going. Now use a dropper bottle or syringe to keep the adjuster hole filled with oil as you extend the shock by pulling the rod back out. The oil should be sucked back into the cylinder. Repeat this procedure until you get all the air out of the shock and have to rod fully extended.

To reinstall the compression adjuster, fill the adjuster hole to the top (actually to the set screw holes) with oil. This will insure any remaining air is pushed out. Getting the adjuster back in with the spring and the cap on it can be a pain. The oil will hold the spring on but that little cap just wants to fall off. You can use a very small dab of clean grease to hold it in place. I don't think this should cause any big problems when it mixes with the oil. Another option if you don't want to use grease is to suck most of the oil out of the hole and carefully place the cap in the bottom of the hole with the hole side out. Then refill with oil and install the adjuster.

Turn the adjuster about half way to seal things up and use a wrench to tighten the cylinder to the body.

Now cycle the shock. Is should feel smooth and quite. If there is still air in the cylinder you will hear it hiss thru the porting. You will need to remove the compression adjuster and bleed the cylinder some more.

Congratulations you have just saved yourself some money and got to see the innards of you shock. Remember you can now also tweak the oil weight if you desire more of less damping then you can achieve with the standard adjusters or habitually ride in very cold weather.

Final Note:
It may be possible to do all this thru the compression adjuster hole and never remove the cylinder. The problem will be in filling the shock. The best way I can think of would be to use a syringe with about a 3 inch needle on the end, a Teflon needle would be ideal. If you insert this thru the compression adjuster hole and up the small hole that leads to the cylinder cavity you should be able to push oil in while still leaving a passage for the air to escape from.

21 comments: said...

Great write-up man! I'm in a similar situation in that I'm riding a 6 year old shock on a 6 year old bike that I beat the crap out of and would rather not drop $100 on getting my shock rebuilt... the fork needs bushings much, much more ;-)

Anyway, I know it's an old post, but thanks anyway... there's not a hell of a lot out there on this and I sensed it was much easier than Fox represented...


Anonymous said...

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Дмитрий Щегол Goldfinch said...

Great manual thanks!

Now i have almost the same problem an have a little question, hope you may help:

When I took out the compression knob, some amount of oil has just jumped out at a high speed. but in you manual this doesn't happen.

do you add some additional compression to the oil section by wrenching the cylinder to the body? (leaving it half wrenched when filling the shock?)


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jrbooth3 said...

thanks for a write up on shock. I will read the BMW one now as that is also a pain point for me.