Rockshox Mag 21 Service

My daughter uses this Mag 21 on her 24" full suspension mountain bike . Its an old fork which is flexy and of limited travel for a full grown adult, but works fantastically for a light young rider and is tunable to boot.

Service instructions

There's a few ways to go about this, but I've found the following steps to provide the easiest approach:

With the fork still on the bike:

  • let all of the air out of both fork legs;
  • remove brakes and brake arch;
  • loosed top caps with the stanchions still held in place with the crown pinch bolts;
  • undo pinch bolts and remove stanchion/fork leg assembly;

For each stanchion/fork leg assembly:

  • remove boot and dust wiper - dust wiper can be removed by prying underneath in a gentle and careful fashion with a small flatblade screwdriver;
  • remove retaining circlip;
  • use shock pump to slowly pressurise shock, as the pressure comes up it will force the seal to pop out - wear eye protection and place the whole assembly in a cardboard packed with foam or rags. It could come appart with some force depending on how much air pressure you need to use. I needed about 40-50psi in each leg to get the seals to pop. Damping oil will go everywhere;
  • drain remaining oil from fork leg;
  • inspect all components for wear and damage;
  • replace whatever needs replacing;
  • perform the long travel mod if you feel like increasing the travel of this fork.


  • reverse of disassembly, but make sure you get the bushing order right - the larger diameter bushing goes higher up the stanchion;

  • use a quality 5wt fork oil to refil the damper, oil heights can be found in the Mag21 user manual. If you don't have that, do a Google search, there are many copies up on the internet.


This the process that I follow to set these forks up:

1. Give the fork a service with new oils and set levels as recommended in the manual. Make sure everything is mechanically A1 before going any further.

2. set the air pressure so you don't harshly bottom out of the largest obstacle in your normal ride.

3. If that pressure results in the fork riding too low in the travel then up the oil volumes in each leg by 5ml at a time to make it more progressive. Alternatively, if you're getting too much initial stiction and can't bottom the fork out then then drop the oil level and air pressure.

4. If the fork feels under-damped then go a heavier weight oil or vice-versa.


The seals in the Mag 21 measure 25.4mm internal diameter and 35mm external diameter.

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