JS Motorsport Technical
More Norton Fork upgrades

The 1st thing to do is 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 .375″ ID .015″ wall K&S brass tubing (available at hobby shops). 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 .680″ (.685″ = stiff rebound dampening). The inside of the damper tubes should measure approx .689″

With used stock components and 20 wt oil you can pull the damper rod up by hand in about 3 seconds. With a new top cap and the cup flaired to .680″ you can pull up the damper rod in about 5 seconds. With a new cap and the cup flaired to .685″ you can pull up the damper rod in about 9 seconds.

damp cup flairing


If you have these type of damp tubes then its easy to cover the holes below the conical section with JS alum sleeves and drill only one 1/16″ diameter hole in the middle of the conical section (below right). This will give you some compression damping. The stock setup offers no compression damping whatsoever. You don’t want too much or you will get a harsh ride. Only one 1/16″ hole in each tube works out just right (smaller than 1/16″ will cause cavitation).

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

damp tubes

If you have damper tubes with holes in the conical section. Silver solder a couple brass plugs into the holes. Be careful not to insert the plugs so deep that they interfere with the damper rods (they must not protrude on the inside). Smooth them off (they must not protrude more than the largest diameter of the conical section) and drill only one of the plugs with a 1/16″ drill bit as mentioned above. There must be no other exposed holes.

damp compression

To avoid leaks, open the ID of the fiber washer that locates at the bottom of the dampner 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

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. 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.

fork spacer

Use 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 rebound and compression dampening.