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Changing BMW motorcycle fork oil, fork seals and gators

by Duane Ausherman

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 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.   To just change springs can get you 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 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."  

This photo shows how to grab the boot, or gator, midway to feel for oil that has leaked past the fork seal on a BMW motorcycle with telescopic forks.

Grab the boot midway (just above where you can feel the lower fork leg casting) and pinch the rubber till you have the fork tube held in your fingers.   Now try to slide the boot up and down.   A clean boot won't slide on a clean tube.   That means that the seal isn't leaking.   An oily boot will slide rather easily.   To learn the difference, check out several similar bikes at the next group gathering.   Just go around and try them.   I have never tested 5 and found all of them to be dry, 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.   

This photo shows how to pull the boot up a bit to examine the area for leaking oil.  This only applies to BMW motorcycles with telescopic forks.

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 to some extent.  

Draining the fork oil the easy way.  

1.   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 off of it.  

This photo shows how to tie the center stand in the down position.  This keeps the bike from going forwards enough to allow the stand to fold up and the BMW motorcycle to fall over.  This stand is a Reynold's Ride Off Stand.

Center stand tied in the "down" position for safety.  

2.   Remove the front wheel and set aside.   This is only necessary if you are changing the gators.   If only the oil, then 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 that I can inspect the brakes too.   By "the assembly" I mean the fender, brace, rear fender brace, and fork legs or castings.   It can all stay bolted together and it saves time.  

3.   Remove the small black rubber dust covers from the bottom of the legs.  


This photo shows the black rubber dust covers on the bottom of the telescopic forks of a BMW motorcycle.  Remove this cover to get at the 13 mm nut for removal preceeding fork oil change.                      This shows the dust cover after removal.  It can swell up a lot from oil, but it will usually still work OK.

                                                     Screwdriver touching the cover         The dust cover

3.   Remove the fork caps on the top.   

This photo shows the fork cap after removal.  On the right is the tool supplied by BMW tool kit.  This tool lost one of the pins and is useless for this job.

 That is the fork cap on the left and a cap wrench on the right.   If you look carefully you can see that one of the pins on my wrench fell out.   Now I improvise, until I find another one.   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 before, then 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 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.   Some of the oil will splash.  

4.   Look up under where the rubber dust cover was.    

This is a close up view of the bottom of the telescopic fork of a BMW motorcycle.  The nut is 13 mm and must be removed in order to get the fork oil to drain.

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.   A special tool is made by Joe for this job and quite inexpensive.   

This photo shows how the tools are positioned to remove the 13 mm nut.  Remove  both and the forks will drop to allow the oil to gush out in seconds.

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 the jammed on crush washers on each side.   You can't see the crush washers at this time, 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 of 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, attach the 13 mm nuts and washers.   Be very careful to not over tighten the 13 mm nut.    They strip easily and maybe it will be the nut and maybe the threads on the damper, not nice.   Push the rubber plugs back on, remove the top cap for fork oil filling.   

The damage shows at the 11 O'clock position on the large nut.  If you look closely, you may see that each of the 6 points has some damage.  Use the correct tools and this won't happen.

That is what it looks like once the cap is off.   Joe also makes a tool for this job.   See the messed up fork retainer bolt? That is from some idiot using a Cresent wrench and letting it slip off.   Don't remove that part, or you will have big troubles.   It releases the fork alignment.   You will have a really big job ahead of you.   You can learn more about that job if you go to my page on fork alignment.  

The plastic funnel will allow easy filling of the fork leg.  Use a measured amount of oil and close to the same on each side.

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 just isn't critical.   It is better to have 20 cc to little oil than too much.   I pre measure the fork oil in containers when I buy a gallon.   It is 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 really no reason and only adds time and money to the customers 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 at this point.   One person tilt 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 to lay the cylinder head on.   The bike is only on it's side for a minute or so.  

8.   Remove the boots and replace.   Replace the crush washers if desired.   They must be pried off as they will be really stuck.   It is a good idea and causes no harm.   I seldom replace them and they don't leak.  

This shows the crush washer on the damper rod of the telescopic forks on a BMW motorcycle.  BMW suggests always replacing them.  On my own personal BMW, I seldom replace it and they never leak.

See the "smashed" copper colored crush washer? This is a picture of the piston damper rod end.  

9.   While the legs are off this is a great time to test for fork alignment.   See my page on this procedure.  

10.   To get the seals out only 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 level.  

It is very easy to pry the seal out of the fork leg.  Do not scratch the aluminum with the tool, or you may have a leak.

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.  

One may use any properly sized tool to use for tapping the BMW fork seal into place.

Smear a drop of oil on the inside of each seal before re-installing the assembly on the fork legs.   If you are using the old boots that have oil on the inside, clean them out or my test above won't work.   They will feel oily and indicating a bad seal, but it is new and not really leaking.  

11.   Reassemble in reverse order.   Before you put the wheel in place, check to see that the axle goes in totally freely.   Do not over tighten the axle nut, or you may ruin your hub.  

Tip for a longer boot (gator) life for your BMW motorcycle

I install my boots a bit differently than BMW intended.   The bike spends most of it's 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 and cheap and will go for the longer life.  

This photo shows my preference in mounting the fork boot a bit higher on the leg.  This allows the boot to not stretch so much and it will last longer.

BMW intended them to be mounted down at that lower ridge, not at the top of the casting where it is shown.

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This page was last edited: 07/16/2006 - copyright Duane Ausherman
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