Rebuilding 89-97 Katana GSX600/750 ForksPREFACEBen2go and I spent an afternoon rebuilding several sets of forks. With the continued interest in others swapping to Kat forks, we felt that a FAQ on rebuilding them was in order. Rebuilding the Kat forks is very easy, much easier than the GS. This guide may look involved but the many steps were necessary to make it clear. Make sure you keep the parts for each fork separate from the other, especially if you are not replacing the bushings.
I am not familiar with the 98+ forks since they require a different wheel and have not looked into using them. They should adapt to the GS the same way as the 89-97. Rebuilding them may be similar, but will require a tool to hold the oil lock from turning (like the GS) as they do not have the set screw to hold it in place. The SV650 has a similar fork design and may even use the same tool.
This write-up was done during the rebuild of several sets of forks, which is the reason for the various colors of the lower tubes.
KATANA FORK BASICSThere are some differences in the Kat 600 & 750 forks and here is a breakdown as I know them. This is just a guide and I will update it as I find more information.
88 (750) no damping rod adjustments
88, 97 (750) no lower set screw for the oil lock
88-97 (600/750) same caliper mounts
89-96 (600/750) same internals
97 (600/750) different internals (same as 98+)
88-97 (600/750) dual 4-piston calipers
98+ (600/750) dual 2-piston calipers
88-97 (600) 3-Piece + bars
88-97 (750) 1-Piece + bars
TOOLS & SUPPLIES6mm allen wrench
10mm socket
12mm wrench
12mm socket
24mm (15/16”) wrench, socket or adjustable wrench
Small flat blade screwdriver
Bench vise
Fork seal driver
Safety glasses
Cardboard box, flattened (to cover work surface)
Rags and paper towels
Note: No special tools are required to disassemble the 89-96 forks. The 88 & 97 forks may need a tool to keep the oil lock from turning.
PARTSFork Oil
Fork Seal, 2 required (part # 51153-08D00)
Dust Seal, 2 required (part # 51571-47D50)
This is also listed as correct part number. It looks like the ones used on USD forks. I have them on one of my bikes and they work just fine.
Dust Seal with exposed spring (part# 51173-15D00)
Additional Parts (as required)
Guide Bushing, 1 per tube (part # 51152-48B31)
Slide Bushing, 1 per tube (part # 51121-48B31)
DISASSEMBLYNOTE: Steps to remove the forks from the bike have been omitted.
Prior to disassembly, examine the outer tubes (fork lowers) closely for damage. If there are any deep scratches or dents, look carefully at the inside. The damage may have bad enough to transfer to the inner surface. I had this happen to one set which damaged the slide bushing and now the lower is ruined.
An old cardboard box was used to lay all of the parts on during the rebuild. A good idea since oil will drip off of the parts and also provide a clean work surface. This picture is of the right tube assembly, which does not have the axle spacer and clamp bolt found on the left fork assembly.
It is a good idea to break loose the fork caps with a 24mm wrench while the forks are still attached to the bike. They do not need to be removed, just broken loose so they can be removed without clamping them in a vise later. If they are already off the bike, clamp the inner tube in the lower triple clamp and secure it in a vise. I have a wooded vise so this does not damage or scratch the triple clamp. A couple of thin pieces of wood in a metal vise will achieve the same results. A 15/16” (or adjustable wrench, if careful) will work the same as a 24mm wrench to remove the caps. Be careful if using an adjustable wrench. They can slip and round over the corners.
The left fork has an axle spacer which needs to be removed before the damper rod bolt can be accessed. Remove the pinch bolt and tap small screwdriver in the slot to help with removal of the spacer.
Clamp the outer tube by the caliper mounts in a vise to break the bolt loose. The damper rod bolt is set in fairly deep. A cheater bar can be used on the short leg of the allen wrench to break it loose. Applying the proper torque when installing is not possible with the cheater bar setup.
I could not find an extended allen socket at any store, so I made my own by cutting off the short leg of an allen wrench and used epoxy to secure the long leg in an extra 6mm socket.
Remove the adjuster caps and pull out the attached damping adjuster rod. Use caution when unscrewing the caps they are under spring pressure. It is best to wear eye protection. Apply some pressure to the caps while turning. This helps to remove them without risking damage to the threads. Do not remove to small screw inside the damper adjustment dial. It holds the cap and rod assembly together. I have not had a reason to take them apart.
Next remove move the spacer, washer and spring. A bent wire was used to pull the washer and spring out.
Time to drain the oil. Pour out the old oil in a container to be recycled. Pump the inner tube slowly several times to help drain some of the oil trapped at the bottom. Watch out for oil squirting out.
With most of the oil drained, remove the damper rod bolt and copper washer (do not loose this washer) from the bottom. Tip the top of the fork down slightly to slide out the damper rod and rebound spring (if they have not already fallen out). There will still be some oil trapped, so be careful. Now remove the oil lock set screw with a 10mm wrench and copper washer (do not loose this washer) to drain the oil trapped at the bottom. You can set this aside to drain while disassembling the other fork or continue on with this one.
Remove the dust seal. Use a small screwdriver to pry it up. Move around the circumference while lightly prying up is best. Use caution not to damage the paint unless you are going to repaint.
Remove the clip that holds down the fork seal. Use a small screwdriver or a pair of needle nose pliers. Wear safety glasses, this may snap loose.
Remove the oil seals. While holding the fork upright and compressed, grasp the inner tube in one hand and the outer tube in the other. Pull the outer tube down quickly while holding the inner tube in place. You will have to do this several times to remove the seal. The upper tube may pull through the guide bushing in the outer tube. Remove the seal spacer and the guide bushing from the outer tube.
Inspect both bushings for wear or other damage. Replace as required. Use caution when handling the bushings. The coating can be damaged easily and may need to be replaced. The bushing on the left was damaged by the dent in the outer tube in the earlier picture.
Clean all the parts before re-assembling. Use caution to avoid scratching the inside of the outer tube. This is where the slide bushing and oil seal makes contact.
If you plan on repainting the outer tubes, now is a great time. You can always paint them once reassembled. Just leave off the dust seal and tape off the fork seal and inner tube. I would at least strip the old paint now to avoid any chemical damage to the oil seals.
ASSEMBLYInstall the slide bushing on the inner tube. Use caution not to bend the bushing or damage the coating. Next, slide the rebound spring on to the damper rod. Now gently slide the guide rod with spring into inner tube. The damper rod will stick through the bottom of the inner tube.
Use a wooden broom handle (or something similar, I used a piece of PVC pipe) and slide it inside the inner tube against the damper rod. Hold this so the damper rod is pointing up and set the oil lock on the damper rod. Make sure the oil lock indexes on the D-shaped tip of the damper rod.
Slide the outer tube over this assembly. Look through tapped hole where the set screw goes and rotate the outer tube until you see the notch in the oil lock. Apply a small drop of lock-tight and install the set screw with washer. Torque this screw to 7-9 ft-lbs.
Apply a drop of lock-tight to the damper rod bolt. With the washer installed, snug the bolt in place. Now you can remove the broom handle. Clamp the lower tube by the caliper mounts in a vise. Torque the damper rod bolt to 21.5 ft-lbs. Remove from the vise.
Now the guide bushing needs to be installed. With the fork upright, slide the guide over the inner tube down to the outer tube. Tap it in place a little to get it started. Lay the seal spacer on top of bushing and use the seal driver to press it in place. My seal driver was not long enough to set the bushing completely. I had an old bushing and double stacked them, then with the spacer on top was able to set it completely.
Slide the fork seal down on top of the seal spacer. If the tubes have any nicks or other potentially damaging marks, use a plastic bag over top of the inner tube to acts a barrier between the seal to prevent damage to the seal. Once past the nicks, remove the plastic bag. Set in the seal with the driver.
I used a split seal driver which can be used on a small range fork tube diameters. The split design is needed for USD forks. They cost a little more but are more universal.
Install the circlip on top of the oil seal. Make sure the clip is completely seated in the groove.
Now the dust seal can be installed. Slide the dust seal over the tube and press it place. A small rubber mallet can be used to lightly tap it to seat it. Be careful to avoid damage.
Here is a picture of the other type of dust seal, which is commonly used on USD forks. This is an actual listed part (51173-15D00) for some Katana forks.
Pour in the fork oil. Stock fork oil volume is 491ml. The level can be adjusted for the rider’s preference or if different springs are installed. I filled the 15wt oil to a level 5” (127mm) below the top of the tube. Measuring the oil level from the top is the proper way, instead of by volume.
I made a simple oil level gauge tool with a syringe, brass tube, silicone tubing and a block of plastic with a hole drilled the size of the brass tube. The hole in the plastic needs to be a snug-slip fit for the brass tube so it can be moved and stay in place without a set screw. You just have to set the plastic block to the level needed and suck out the extra oil. I purchased the brass tube and silicone tube at an R/C airplane hobby store for around $3. They had syringes (for about $12) there as well but I already had one.
Extend the forks completely and place the spring inside the inner tube. Put is slowly so not to splash out any oil. Drop in the washer and then the spacer. The spacer may need to be shortened or replaced if it needs to be longer, depending on if new springs are installed or to adjust preload. The caps extend into the tubes by 3/4” (19mm), which is also the recommended sag for correctly sized springs. You are using the correct springs for your weight, right? With the .95 Sonics springs selected for my weight and riding (these are going on a track bike), the stock spacer needed to be shortened by 1-1/4” (31.75mm). In this case, the spacer is flush with the top of the fork tube.
Install the damper adjuster rod and cap. The adjuster rod has a D-shaped tip that will fit inside the damper rod. Turn the cap until they fit together. Do not force it as the adjuster rod can bend easily.
When installing the caps, apply light pressure while turning. This will limit the amount of wear on the threads of the aluminum cap. Also it is a good idea to turn the cap backwards until you feel a click. This is when the threads slip past each other and they will engage immediately, allowing the threading much easier and less likely to cross thread the softer aluminum.
With the fork reassembled, push it down several times to make sure it moves smoothly. If there is any grinding or binding take it back apart and inspect everything for the cause. Change the setting of the damper adjustment and you will feel a difference in the forks motion.
Repeat the rebuild with the other side. Install the axle spacer in the left fork with the shoulder to the inside and tighten its clamp bolt. Now they are ready to be installed on your bike. The Kat forks are longer than the GS forks, IIRC by 20mm. Depending on how you modified the triple clamp to fit the GS, they may stick up higher above the top clamp plate. This can be good if you want to have clip-ons about the top clamp plate.
Enjoy your new suspension.
