JS Motorsport Technical
Slippery Turcite Fork bushings and damping upgrades

For your forks to ride smoothly with out bottoming or topping out you must install the slippery JS Turcite bushings along with the aluminum sleeves. Turcite is the best material available for fork bushing and will outlast Teflon. It is also more than twice as slippery as Delrin AF.  Plug the 1/8” diameter oil escape hole in the bottom of the fork tube (just above the lower bushing) or you will not get the necessary oil cushion to prevent “top out”. Thread the 1/8” hole with a 6-32 tap and use the provided set screws (coat the threads with JB weld epoxy or red loctite to keep them in place). Make sure the set screws are not proud of the surface or installed too deeply.  For details on the aluminum sleeves (see damping sleeves).

fork spacer

Be sure to check for wear in the aluminum caps. New caps measure .380″ ID so its better to drill them to 13/32″ and loctite (red) in a sleeve of .378″ ID .015″ wall K&S brass tubing (available at hobby shops). The damper rod shaft should measure .375″.  Clearance must be tight or you won’t have enough rebound damping.

damp cap sleeve


To improve rebound damping. Place the damping cup on a socket. Tap carefully with a hammer and a tapered carpenter plumb bob to swell the open end to .677 – .678″  (.680″ or more will be too stiff). If you over expand the cup then you must shrink it back down.  The inside of the damper tubes should measure approx .690″ max.

With a sleeved cap and the cup flaired to .678″ you can pull the damper rod up by hand (immersed in container with 20 wt oil) in about 3 to 4 seconds and this is what you want.

damp cup flairing


Generally speaking you do not need compression damping on the street because too much compression damping will give a harsh ride (the stock setup has no compression damping). If you have damping tubes with holes below the conical section then you need to cover those holes with JS alum sleeves to to achieve a hydraulic bump stop and avoid bottoming “clunk”.   Then drill only one hole in the middle of the conical section as in the photo below right.  The value in compression damping is reducing front brake dive and bottoming in racing – but you will also feel more bumps when cruising.  A single 1/16″ hole per tube in the conical section will give you noticable compression damping (smaller than 1/16″ will cause unwanted cavitation).  I recommend a single 1/8″ hole per conical section which gives minimal high speed compression damping and is good for the street. A 3/32″ hole is a compromise but you will still feel more bumps.

It takes about one second to compress the damper rod fully by hand with the 1/16″ hole (compared to zero stock compression damping).

damp tubes

If you already have 1/4″ holes in the conical section then you won’t have compression damping and that is fine. But if you want compression damping then silver solder a couple brass plugs into the holes. Be careful not to insert the plugs so deep that they interfere with the damper rod cups (they must not protrude on the inside). Smooth them off (they must not protrude outward more than the largest diameter of the conical section) and drill only one of the plugs with a 1/16″ to 1/8″ drill bit as mentioned above  (see image below).

damp compression

To avoid leaks, open the ID of the fiber washer that locates at the bottom of the damper tube to .655″ Drop the washer in place at the bottom of the alum slider.

damp tube washer oring

Place a 5/8” OD x ½” ID x 1/16” Oring on the bottom of the damper tube. You must have the fiber washer installed in the slider as described above to avoid crushing the Oring.

damp tube bottom with oring

To prevent leaks at the drains. Make a brass washer with .4” OD x .312” Id x .050’ thick. Install an Oring with 3/16” ID x 5/16 OD x 1/16” thick.fork drain plug

Use 15 or 20 weight fork oil and stock fork seals (not leak proof seals which have too much sticktion). Now you’ve got oil tight, modernized, stickion free forks with improved dampening.