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Tim Phillips
(@tsp)
Active Member

My bike is a 1980 R100S. I bought it about a year ago and rode it most of last summer after attending to all the minor problems it had. One thing I didn't do and don't even feel it absolutely needs as it rides great, is changing the fork oil. I have looked at Snowbum's pages and there are recommendations for what fork oil to use but none for the procedure of changing it. On other bikes I've had (all non BMW), there was always a plug to remove at the bottom of the fork to drain. Mine has none except a "cap" that looks like it threads into the end of the fork. Is this what I remove to drain it? A how to write up would be helpful, as even the ever available You Tube doesn't have any videos of the procedure. Any pointers?
Tim

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Posted : 03/08/2016 20:51
Donald Beverage
(@6208)
Member Moderator

I have a 1980 r100s also. The large nut ( CAP NUT )on the bottom of the fork leg . There is a rubber cap on the bottom. Remove the rubber cap. In the middle of the larger nut is a small nut. Get a 10 mm socket and remove the nut. Know u can remove the large nut and drain and clean the cap nut. You will have to order a new crush washer for the cap nut . Part number 31 42 9 963 486 crush washer. Also order the ring ( buffer ring ) 31 43 1 231 314.

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Posted : 03/09/2016 09:52
Richard Whatley
(@wobbly)
Member Moderator

My bike is a 1979, and may differ slightly....

► Place the bike on the center stand. Pop out the rubber plug on the very bottom of the fork leg. Up inside the leg is a 13mm hex nut and wave washer which get completely removed. Using a 4mm Allen (hex) key, push the protruding threaded fastener up into the leg. At about 3/4 inch insertion it will stay in the pushed up position, and oil will start to drain. This is not all the oil. You must then pump the forks by holding the front brakes and pushing the bike forward/down. Oil shoots out in a 12" long line so use a long drain pan, similar to a paint roller pan. If you have 2 pans, then you can drain both sides simultaneously.

When the leg(s) starts huffing and puffing air, the fork is empty. Pop the threaded fastener back down from it's resting place. Replace the wave washer and hex nut and hand tighten. It is nearly impossible to get any torque on the nut because the fastener rotates, but you need surprisingly little to effect a really good seal. Maybe 4-5 ft-lbs max. Replace the rubber plug.

► Filling is about as easy, but has its own issues. Pop off the black plastic cap on the top of the fork stanchion. Using an 8mm Allen (hex) key, remove the bolt and sealing washer. The problem here is to get 250cc of fork oil down that little hole without spilling it. Funnels DO NOT work. You'll need a big syringe or squeeze bottle, either with a very small hose that you can insert all the way into the stanchion. This work goes slowly. After inserting the oil, reinstall the bolt, seal and plastic cap.

► ATF is the correct viscosity for these older Airhead forks. However, due to availability issues of seals and other internal neoprene parts, it is best to use a synthetic oil to help preserve seals and o-rings as long as possible, such as BelRay 7W.

If you are experiencing "nose-dive" during braking then you can experiment with 10W fork oil, but the real solution is a set of progressive fork springs (~$100). The 10W oil will certainly slow the dive, but the general ride becomes firmer at the same time. However, oil is always cheaper than fork springs, and certainly worth a try.

► The main reason to change fork oil is not the number of miles, as most people might suspect. Mileage is always a good reason, but the main purpose is to rid the system of water. Fork oil needs changing once a year (riding season start is suggested) minimum, but more often if you wash with high pressure hose, ride in the rain, or store you bike in conditions that promote condensation.

As in every where else on the bike, an incorrectly used high pressure water hose is the greatest threat to long life.

🙂

This post was modified 2 days ago by Richard Whatley

[color=blue]Don't hide 'em, Ride 'em !!
#15150[/color]

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Posted : 03/09/2016 16:12
Tim Phillips
(@tsp)
Active Member

Thank you both. I'm going to plan on doing this weekend. Riding season almost upon us in the far northern regions of Wisconsin and I want to be ready when all the ice and snow is gone!

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Posted : 03/09/2016 21:35
Harlan Juster
(@12361)
Active Member

@wobbly This is the best description of the procedure for changing fork oil I've been able to find. Thanks.

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Posted : 09/15/2020 18:40
Richard Whatley
(@wobbly)
Member Moderator

Glad it helped.

[color=blue]Don't hide 'em, Ride 'em !!
#15150[/color]

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Posted : 09/16/2020 06:27

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