If that doesn't work, and it may not if you are trying to disassemble something 20+ yrs. old & never-apart, go to your hardware store and find a 3/4" I.D. X 10 t.p.i. hex nut which uses a nylon thread lock insert at one end. I found that while it is not an exact "tightest-fit" you can use it with an extension and a socket of the appropriate size (use a piece of masking tape to hold the nut in the socket) to index within the 12-point female of the top of the damping rod so you can lock it in-place with a ratchet or a breaker bar when you undo the allen hex bolt inset to the bottom of the slider. As they say, "assembly is the reverse of disassembly." Be sure you clean all thread & mating surfaces and don't forget the copper washer on the allen hex bolt. Interestingly, I tried other 3/4" I.D. nuts w/o the nylock feature and the flats did not fit inside the damper 12-point.
I hunted several hardware stores, including the 'big-box' store,before I found a nut of the correct dimension.
Beats buying a Kent-Moore tool for $$!
As another handy tip, I found a great use for a tool I infrequently use but it works great for re-assembling the downtube caps. Back-off the preload adjuster completely, so the cotter key is against the bottom of the cap. Don't forget your 'silver-dollar' discs which sit on top of the spring before the tube cap goes in-place! Otherwise, your infinitely-adjustible cap preload function has nothing on-which to bear.
Now, I use a 3/8" offset socket drive handle, the kind of lever that has the "Model T" handcrank shape to it. A deep socket will provide clearance for the cap preload adjuster, since for the installation it's all-the-way out (the preload adjuster). Carefully align the cap to avoid cross-threading of the cap or tube, and use the mushroom end of the socket handle to compress the spring while you hold the offset and crank it to tighten. The combination of the deep socket, the large end of the manual socket handle driver, and your tightening action with the offset will make the easiest job possible to seat the cap with great ease.
Almost-done. Now, grab a crows-foot 10 mm open-ended socket and again use the offset handle to tighten the preload adjusters to where you need 'em, don't forget to match the settings on both sides. That should take < 1 minute.
I tried the ratchet and deep socket method before as needed, (and got frustrated from not easily being able to seat the threaded cap in the downtube) and it wasn't until I happened to come-upon this combination of equipment that I was able to make an almost pleasant experience out of servicing my forks.
Incidentially, I tried the rubber seals advertised as 'leakproof' with a lifetime warranty, and 'so-far-so-good.' These are the all-rubber ones with no tiny internal springs or metal bands around the exterior of the fork seal. Be sure and follow the assembly sequence exactly, as there is another flat washer which comes with it which gives you the correct stack height to be held in-place by the seal clip. They also include a dust seal set. I found them on eBay.
I've owned this bike since late '87 when I bought it from a friend of mine who purchased it new from Broward Motorsport in Ft. Lauderdale. I got it with < 3K mi. on it and I have about 24 K mi. now. It runs like a top! I could do without some of the cowling and side-fairing decals, but I have always found the lines of the bike and its white pearl paint to be very appealing. I recently found another FZR and am considering selling this to buy the newer model.
