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While it is possible to change the fork oil without removing the forks from the bike, I don't recommend it unless you know for sure that they have been serviced within the last couple of years. This means that either you have had them apart or you bought the bike from someone who you know does maintenance to a high standard and he specifically told you he did them.
Some people's definition of servicing things leaves a lot to be desired so if you aren't absolutely sure you should do a thorough job at least the first time. Otherwise, just changing the fluid would be like changing the engine's oil without changing the filter - all the stuff that doesn't come out the drain is in there to contaminate the new oil you put in.
My normal procedure for servicing forks is:
1) Remove the forks from the bike one side at a time and then repeat the rest of the steps for each leg. I usually spread a section of newspaper (several sheets thick) on the clean workbench to make sure no foreign material gets into the working parts.
2) Remove the drain screw and place the bottom of the fork into a container to drain. If the fork has a port in it's cap for air you can use low pressure air to force the fluid out faster. I use a bicycle pump for this. When all of the fluid that will come out has, replace the drain screw finger tight.
3) Loosen the allen bolt in the bottom of the slider but don't remove it yet - leave it finger tight. This must be done before the cap is removed so that the spring pushes against the part that the bolt is screwed into and keeps it from moving. I have on occasion resorted to a couple of sharp blows to the allen wrench to get the bolt started and once or twice I have needed to use an impact driver (the kind you hit with a hammer).
4) Remove the cap. Caution - it is under pressure from the spring. If possible, place a large rag over the cap while you remove it so that any parts that fly out will be caught in the rag.
I clamp the fork tube in a wooden jawed vise for this (my Dad was a carpenter), but any soft jawed vise or any vise with wood placed between the jaws & the fork will do. You could even make wooden jaw covers so that you don't have to juggle the wood & the fork while turning the handle.
For disassembly you could even clamp the fork back into the bike temporarily, but this can be awkward with the fairing in place.
Remove the spring(s) and lay them on the newspaper in order. I always lay them out with the top of the fork on the right so that I remember which order they have to go back in. It really doesn't matter which side you put the parts from the top on as long as you are consistent.
5) Pour an ounce or so of mineral spirits into the top of the fork and pump it vogorously several times to force it through the valves and clean them. remove the drain screws and let the spirits drain out.
A word about mineral spirits: Mineral spirits is the generic name for a family of petroleum distilate solvents that includes ExxonMobil (Imperial oil)'s brand called Varsol. All of the standard warnings for use of flamable solvents apply - don't smoke, no open flames, well ventilated area. I recommend generic mineral spirits, usually marked "Paint Thinner". Get the low odour kind if you can. If you buy Varsol brand you will pay about 25% more than you would pay for the generic product bought from the same store and the only difference will be the ammount of money in your pocket.
6) Disassemble the fork according to the instructions in your shop manual. Lay out all of the parts the same way you laid out the spring(s).
7) To get all of the sediment out of the bottoms of the sliders wrap a small rag around a piece of rod and swab it out. Shine a light down the tube and if you see any more dirt swab it out again. This is the stuff you couldn't get by just draining the oil.
8) Re-assemble the fork according to the instructions in your shop manual, but don't install the cap & spring(s) yet. Wipe as much of the old oil as possible off of each part before you install it. I have never used Loctite when assembling forks and have never had any problems because of it.
If you are installing new seals, use a piece of plastic drain pipe about a foot long that is a loose fit over the stanchion but will fit inside the top of the slider as a slide hammer to drive the new seal in.
9) Clamp the fork in the vise again. Pour in the appropriate amount of oil of your choice. I use a 50/50 mix of ATF and motor oil - 10W40 for summer use and 5W30 for the sidecar machine that I drive to work all winter.
I used to recommend measuring the fluid with a graduated bottle that baby juice comes in, but they never hold enough and you have to make sure you add up what you poured correctly. The last time I bought a clear plastic 300mL measuring cup from a $1 store - it's markings are more accurate and it's spout makes pouring much easier.
10) Re-install the spring(s) & cap. If you are installing some form of added spring preloaders, they should go on the top. Many have simply added an equal length of pipe above the springs with good results. I prefer to add springs. I use valve springs salvaged from an engine I scrapped years ago. If you don't have something like that, ask a friendly auto mechanic or see your local bike wrecker. Some small engine valve springs might work too.
I made a special tool for starting the caps on Honda forks with air fittings. It is sort of a big wooden wrench - a piece of wood a foot or so long and a couple of inches wide (size not critical) with a couple of smaller blocks nailed onto it crosswise near the centre and just far enough apart for the cap to fit between.
To use it, I clamp the upper fork tube into the wooden jawed vise, then bear down on the ends of the tool to compress the spring while turning the tool (& cap) counter clockwise until I feel the ends of the threads pass each other, at which point I start turning clockwise until the cap is threaded in a couple of turns, then finish with a regular wrench.
GX650EI + Velorex 700 (winter), '83 GL1000 (summer)
'84 CB750SC Nighthawk (died - anyone need parts?)
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