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Front Fork Oil discussion; filling amounts; hints.
This article applies to Pre-Airheads, Airheads, Classic K bikes 
section 54-10B

Copyright 2014, R. Fleischer

Discussion of Oil types and characteristics.
See also:

I prefer Spectro's fork oils, or suspension fluids.  They are GOOD, have low stiction, wide temperature range (excellent viscosity index), & the viscosities can be depended upon.   Due to how specified, & lack of stiction fighters & other characteristics, you are MUCH better off with a REAL fork oil like Spectro's ....especially the full synthetic fork oil.    For fork oils & suspension fluids, the various manufacturer's do not agree on measuring viscosity; sometimes they don't do more than give some sort of approximate SAE grade value.  Viscosity measurements & temperatures are vastly more accurate between manufacturer's for engine & gear oils, not so, apparently, for many suspension & fork oils. Because of these various things, AND MORE, I highly recommend you stay with ONE manufacturer if playing with different viscosity grades.

Back in early Airhead & K bikes days, the oil BMW used was "BMW red fork oil".  That oil was really a military hydraulic oil, you could find its full specifications using a search engine for:  MIL-H-5606E.     Oil with a "5606" military specification can be depended-upon for viscosity (but not for some things you really want for your fork oil). To save you the trouble, just the viscosity specifications for that oil are:
MINimum 4.9 Cst at 100 C (generally accepted as 210F).
MINimum 13.2 Cst at 40 C (100F).
You can use those two, 4.9 Cst and 13.2 Cst,  to compare, perhaps!, with other mfr's oils specs ...& with the information in article #52F on this site:

Maximum 600 Cst at -40 C  ....too cold to bother about for motorcyclists (unless very crazy!).


Except for some GS models, all the BMW's (that this article applies to) need a rather thin oil, approximately SAE 4 (that military oil, as above), so do not willy-nilly change the viscosity grade to higher.   Only in the instance of very heavy loads (rider & passenger weights & luggage), or very aggressive performance, is an oil heavier than an actual SAE 7.5 weight of any benefit.  NEVER higher than SAE 10 for the non-GS models (some GS models specify use of different oils, SAE 10 and SAE 15, in right & left fork legs, respectfully).

The GS models have differing fork legs.  The RIGHT leg is for damping compression, the LEFT leg is for rebound damping.  I recommend using the same weight grade of oil in both legs.   The right leg oil will get dirtier much faster.  I suggest first trying a 10 weight oil for a heavy rider, passenger, & loading. You could try Progressive Company springs, which hold up pretty good.  Use about 1 inch of preload.  Try 5cc or so less oil than specified.   You may want to experiment, & with some types of really heavy loadings, go to a higher viscosity oil, 15, and even try 20.   UNDER NO CIRCUMSTANCES CHANGE OIL VISCOSITY TO COMPENSATE FOR FAILURE TO REMOVE AS MUCH STICTION IN THE FORKS AS POSSIBLE!!

A high viscosity INDEX is generally a good thing for fork oils.  There is an article on this website on actual viscosity values for fork oils; here the link is, again, (it also has other oil information):  Once the VI is high enough, you won't notice any difference in performance by going higher, unless doing VERY aggressive riding in VERY bumpy & constantly bumpy extreme conditions, such as RACING, and even then, only noticeable between early (cold) forks and forks warmed by lots of use.

BMW Airheads forks from 1981 are especially sensitive to fork oil foaming.  I suggest you use a quality FORK or SUSPENSION fluid (oil).

Comments and chart of oil filling amounts:

1.  In instances where I know the information, I have listed oil refilling amounts for both a drain/refill; and, a fork rebuild.  A rebuild is where the fork parts are assembled with only a FAINT amount of oil, hardly many drops, during the rebuilding; thus, very little volume of oil is in the forks before you fill them with the proper oil amount.

2.  BMW has revised oil amounts for some bikes. 

3.  The /5/6/7 forks... up to 1977....can be checked for level by inserting a thin rod (I use a full length welding rod).  The oil level should be 437 mm (17-1/4 inches) below the flat face of the fork top nut to the oil level.  

4.  For the /5; /6; and /7 to 1980, with the forks fully extended, wheel off ground, the level is 50 mm above the damper.

5.  For the R45; R65 to 1985; R65LS; R80ST, the level is 35 mm above the damper,  +-15mm.

6.  For the /7 (1981 to 1984); R80GS, R80, R100RT & RS (1981-1984) you can insert a long rod & wiggle it, if needed, so the rod drops to the lowest point the rod will go to.   The level should be 300 mm from tip of rod to oil height shown.  That is 11-3/4 inches.

7.  Mind my comments about using a quality oil with no tendency towards foaming.


Drain & refill, cc (cubic centimeters)

Overhaul; slight assembly oil only, otherwise dry




1968-1969 U.S. models




150 to 170

/5, /6, /7 to 1980

235 (265 is obsolete)

250 (280 is obsolete)

/7 from 1981, R80G/S, R80RT 1983-1984, R100 models from 1981



R45, R65 to 1985, R65LS, R80ST



R80 from 1985, R65 from 1986


R80RT from 1985, R100RS and RT from 1987 (1988?)


R80GS, R100GS, R100GSPD

Left (rebound) SAE 15, 410.

Right (compression) SAE 10, 440.

Left (rebound) SAE15, 470.

Right (compression) SAE 10, 470.

R80R, R100R, SHOWA forks



Classic K bikes:    



K75 standard


K75 Showa, as on K75RT, etc



K75 SPORT; K100 SPORT (8 valve). Both have S stamped on the top aluminum plug


K100 (8 valve)


K100 (8 valve) RS, RT, LT, standard


K100 RS (16 valve)



K1100LT, K1100RS

Left 350; Right 400

400, 400


08/08/2012.  New article.  Discussion section transferred from 54-10A, expanded/revised.  Possible errors.
02/24/2016:  Increase font sizes appropriately.  Move table to left.  Narrow article, left justified. Clean up.  Update meta-codes.
06/24/2016:  Update metacodes, H.L., justify left, layout, colors, fonts, scripts, etc.
01/26/2017:  Clean up.
03/29/2017:  Fix typo (extra 'the') in number 6.

Copyright 2014, R. Fleischer

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Last check/edit: Wednesday, March 29, 2017