Rebuilding the forks is one of the best, most cost-effective upgrades that can be done to our old bikes. Vintage motorcycles especially can benefit from fresh parts, replacing springs and oil seals that have been distorted and worn over the decades. Although the 1993 FZR 600 is only 20 years old, the suspension (both front and rear) was horrible, and was in desperate need of some freshening up…
Up on the jacks she went. There is a trick to using a solid bar between the oil filter lip and the exhaust, so that the pipes do not need to be removed. This supports the bike while getting the front up on the jacks, giving you clearance to remove the wheel and forks. With all the other mods going on, the front end had been torn down already, so it was a simple matter of loosening the bolts, taking off the brake calipers and handle bars, then removing the forks from the triple tree. Always loosen (but do not remove) the fork caps while in the triple tree, because it provides better leverage than when the forks are off the bike.
The front of the bike will then look like some type of monster-mechanism from The Terminator…
Once removed, the fork overhaul is quite simple. Drain all the oil by removing the drain screw with the bottom of the fork sitting in a bucket. Press down on the fork, depressing it into its travel several times, until there is no more oil exiting the drain hole (some FZRs do not have this drain – in which case you will have to empty it entirely by tipping it into a bucket). Then remove the fork cap, and turn the fork upside down into the bucket. The spacer and the spring will slide out (be sure to grab the small flanged washer that sits on top of the spring). Let the oil drain.
There is a tool that needs to be made to secure the damper, which is easily done with a 3′ length of threaded 5/8″ bar, and 4 matching nuts. Put 2 nuts on each end, with the second nut flush with each end of the bar. Use the inside nuts to tighten the outside nuts. This will be the tool you use to hold the damper from rotating, when you loosen the allen-head bolt inside the bottom of the forks.
Once the forks are empty of oil, put the fork in a bench vice (be sure to use a rag to protect the fork), carefully insert the homemade tool into the inner tube. Slowly rotate the tool, until you feel the nut fit into the slotted top of the damper. Now hold the end of the tool with a large crescent wrench, while loosening the allen-head bolt inside the bottom of the fork. To get better leverage, you may need to use a pair of vice-grips on the allen wrench. You will feel the seal break loose. When removing the bolt, be sure to pull it out slowly, so that you do not loose the copper, crush washer that helps seal the bottom of the fork. Remove the tool, and with the bolt out – you will be able to slide the damper out from the top end of the tube.
With a small screwdriver, remove the dust seal where the inner tube meets the outer tube. Inside there will be an odd-shaped snap ring – which prevents the oil seal from coming out. With the small screwdriver, pry it out (put your hand over the end of the inner tube, to catch the ring when it flies off the end).
Next, with the fork still in the vice, pull several times on the inner tube, deliberately and straight away from the outer tube, until the bearing and oil seal break free. This will allow you to remove the inner tube from the outer tube (base of the fork).
Lay out all the parts on a table. You can clean them with your favorite cleaning product, soak them in hot soapy water, or simply wipe them down with a clean rag. If you are replacing the spring, you may need to modify the length of the spacer. I installed Progressive Springs, and needed to cut the stock spacer down to 3.75″. Be sure to make the ends true – which makes re-threading the fork cap MUCH easier.
A decent trick to cleaning the inside of the tube, and the fork lower, is to tie some string around a good shop cloth and stuff it down into the area to be cleaned. You can move it around with a screwdriver, but be careful not to damage the sides. Then use the string to pull it back out of the tube.
Put the fork back together the way it came apart. Place the clean damper back into the inner tube, so that it lines up with the bottom hole, and can travel through to poking out the end. Back into the vice, place the inner tube back into the outer tube, and line up the bearing (race) with the mouth of the outer tube. To install the bearing back into place takes some gentle tapping. I used a small punch, being sure not to touch the walls of the tube. Damage to the walls of the tube will cause oil leaks.
Once the bearing is back in place, the new oil seal can be placed into the mouth, tapping gently until it rests firmly against its seat, then install the clip. I use the old seal to tap against, so that you don’t damage the new seal. The dust seal is last.
With the fork back together, but before installing the spring or spacer, collapse the fork (fully depressed) and pour in your favorite fork oil (THIS is an affordable oil I often use that can be purchased through Amazon – MotoPreserve may get commissions for purchases made through links in this post). Be aware that if you are installing an aftermarket spring, the amount of displacement will increase, so the quantity of oil should now be less than the manual prescribes. If the spring manufacturer suggests to measure based on a distance from the top of the tube, as opposed to the typical volume method, make a measurement tool from a piece of wooden dowel, and mark the prescribed distance from the top of the tube to the top of the oil.
After the oil is at the correct level, pull the inner tube to the top of its travel, and install the spring. Make sure the spacer is cut to the directed length, and place it on top of the flanged washer, sitting on top of the spring. To install the fork cap, place the fork on the ground, and have a friend help hold it in place. With a socket on top of the cap, lined up with the top of the fork, press down. Have your friend spin the fork tube beneath the cap, and gently thread the cap into place. It is important to not cross-thread this cap. The threads are very fine, and mis-threading can cause major problems! Go slow, and stop if you feel resistance. This means the threads are incorrect. Remove the cap and begin again.
If you don’t have a friend to help with the cap – you can carefully replace the forks into the triple trees (be careful not to spill any oil), tighten the bolts, and use their leverage for replacing the cap.