Changing the fork oil, seals, and gators
This page is about the BMW motorcycle models R50/US, R60/US, R69US, R50/5, R60/5, R75/5, R50/6, R60/6, R75/6, R90/6, R90S, R60/7, R75/7, R80/7, R100/7, R100S, R100RS. This page is for changing BMW motorcycle telescopic fork seals and oil. It is not to be used for changing the fork springs. They are their own thing, and I highly suggest that you refer to my page on fork alignment. While changing springs, you can get in big trouble with fork alignment.
The fork seals should be changed when they are found to be leaking. No huge safety problem exists because of leaky fork seals. If they leak enough to lower the oil level, then maybe the dampening function of the forks would be lost. By that time, the whole front end would have been filthy dirty for years.
BMW motorcycle fork seal leakage test
You may test them for leaks often; it takes only seconds. Learn the “feel.”
Grab the boot midway (just above where you can feel the lower fork leg casting) and pinch the rubber gator until you have the fork tube held in your fingers. Now try to slide the boot up and down. A clean boot won’t slip on a clean tube. That means that the seal isn’t leaking. An oily boot will slide rather easily. Check out several similar bikes at the next group gathering to learn the difference. Just go around and try them. I have never tested 5 and found all of them to be dry, and you will find one, or more, that is easy to slide. It is possible that someone replaced the seal without cleaning or replacing the old fork boots, and now the inside of the boot is still oily.
If you want to see the seal for an even better and more accurate inspection, remove the lower clamp and lift the boot up, as shown above. If you see oil, then the seal is leaking.
Draining the fork oil the easy way.
1. This is just for draining the oil quickly. Have the bike on the center stand with the front wheel in the air. If you have a Reynolds stand, you need to put a 2 X 4 under each side to jack it up. The tire needs to be about 2″ off of the floor unless you are going to remove it, and then it only needs to clear slightly. For safety reasons, tie the center stand forwards so that the bike can’t roll forward and off.
The Center stand is tied in the “down” position for safety.
2. Remove the front wheel and set it aside. This is only necessary if you are changing the gators. If you only change the oil, the wheel may stay on and save time. It will be much heavier to lift the “assembly” with the wheel back into place, but it can be done. For a beginner, I recommend removing the wheel. I like to remove the wheel so I can also inspect the brakes. By “the assembly,” I mean the fender, brace, rear fender brace, and fork legs or castings. It can all stay bolted together to save time.
3. Remove the small black rubber dust covers from the bottom of the legs.
Screwdriver touching the cover The dust cover
3. Remove the fork caps on the top.
That is the fork cap on the left and a cap wrench on the right. Do not use a hammer and punch. You can stick two drill bits in the holes and a lever between them. The bits must be held firmly in place.
3. Get two containers ready to catch the fork oil. If you have never done it this way, you are in for a surprise. The oil is going to come out of there in two seconds. You must have the catch containers ready and be waiting. I use ones made out of two old paper oil cans. I have a handle or hook that I hook over the axle. There is little room for the oil to splash as it goes directly into the containers. If you use containers on the floor, get them ready. The oil will splash.
4. Look up under where the rubber dust cover was.
You will see this from below.
Use a 4 mm Allan wrench to hold the center. Use a standard 13 mm offset box end wrench on the nut. This may be a bit clumsy. You can buy a great tool made just for this job.
Now it will look like this.
Hold the Allan wrench to keep the center from turning. Remove the 13 mm nut and washer from each side.
5. All that is holding the fork legs up is jammed on crush washers on each side. You can’t see the crush washers at this time, as they are on the inside. See below. The fork springs are now trying to shove the whole assembly down. If the wheel is off, the boots will keep the assembly from falling all the way to the floor. Bump the fork brace downwards. The whole assembly will drop a couple of inches, and all of the oil will gush out almost instantly.
If you are only changing oil, then lift the assembly back up, and attach the 13 mm nuts and washers. Be very careful not over tighten the 13 mm nut. They strip easily, and maybe it will be the nut and maybe the threads on the damper. Push the rubber plugs back on, then remove the top cap for filling the fork oil.
That is what it looks like once the cap is off. See the messed-up fork retainer bolt? That is from some idiot using a Crescent wrench and letting it slip off. Don’t remove that part, or you will have big trouble. It releases the fork alignment. You will have a huge job ahead of you. You can learn more about that job if you go to my page on fork alignment.
That’s how you can put the oil back into the fork leg. Check your book for the proper amount. It need not be exact, as it isn’t critical. It is better to have 20 ccs too little oil than too much. In my shop, we would pre-measure the fork oil into individual containers ready for future use.
Wasn’t that an easy job?
Many seem to want to replace the crush washers each time the fork oil is changed. In the shop, we didn’t do it. There is no reason; it only adds time and money to the customer’s bill. Some say, “Look at the deformed crush washers,” and they are correct. They are deformed, but the assembly goes back on, and the deformed washer “forces” it back the same way, and it won’t leak. If you want to replace the crush washers, continue reading.
To replace the seals, crush washers, lower bumpers, damper rods, or boots.
6. Loosen the upper boot clamps and slide the boots down and off of the lower triple tree. The assembly can be lowered to the floor.
7. I like to use a helper for this task. One person tilts the bike over about 1/2 way. That gives room for the other person to drop the assembly out and away from the fork tubes. If you have no helper, use a 4 X 6 on the floor on which to lay the cylinder head. The bike is only on its side for a minute or so.
8. Remove the boots and replace them. Replace the crush washers if desired. They must be pried off as they will be stuck. It is a good idea and causes no harm. I seldom replace them, and they don’t leak.
This is a photo of the piston damper rod end. See the “smashed” copper-colored crush washer?
9. The legs are off; this is a great time to test for fork alignment. See the link to my article on this procedure above in step 5.
10. Getting the seals out requires a tire iron, pry bar, or lever, as in a screwdriver. Be careful not to scratch the aluminum. Click on the photo and see how the seal is levered out.
To install the new seal, I use the 3/4″ drive socket that is a bit smaller than the seal diameter. Keep it straight and gently drive it in until it is flush with the metal.
Smear a drop of oil on the inside of each seal before re-installing the assembly on the fork legs. This is a great time to replace the gators. If you are using the old gators, check to see if they have oil on the inside. Clean them out, or my test above won’t work. They will feel oily, and that indicates a bad seal, even though the seal is new and not leaking.
11. Reassemble in reverse order. Before you put the wheel in place, check to see that the axle goes in freely. Do not overtighten the axle nut, or you may ruin your hub.
Tip for a longer boot (gator) life
I install my boots a bit differently than BMW intended. The bike spends most of its life on the center stand. The forks are extended fully. This means that the boots are stretched out most of the time. That causes them to crack in short order. The ones that you see in the photos have been on that bike for 18 years and still look fine. I fasten the lower clamp up to the highest point on the lower leg. It looks a bit funny, but I don’t care. I am lazy, frugal, and will go for longer life.
BMW intended them to be mounted down at that lower ridge, not at the top of the casting where you see it in the photo.
Updated 15 July 2022