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

http://bmwmotorcycletech.info/front-fork-oils-amounts.htm 
section 54-10B
Copyright 2018, R. Fleischer

Discussion of Oil types and characteristics.

The following article SHOULD BE USED WITH:  http://bmwmotorcycletech.info/viscosity.htm.  That article has information that REALLY SHOULD BE READ...either NOW, or, after reading the below article!

I prefer Spectro's fork oils and suspension fluids.  They are of good quality, 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 better off with a real fork oil like Spectro's ....especially the full synthetic or part 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 other reasons I won't bother to get into, I highly recommend you stay with one manufacturer, this is particularly so if you are trying different viscosity grades.

Back in early Airhead & K bikes days, the oil BMW used was "BMW red fork oil".  That oil was actually a military hydraulic oil, you could find its full specifications using a search engine for:  MIL-H-5606- (probably -E now).     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 of finding the information, 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).
Maximum 600 Cst at -40 C  ....too cold to bother about for motorcyclists (unless very crazy!).

You can use those two viscosities, 4.9 Cst and 13.2 Cst, to compare (perhaps!), with other manufacturer's oils specs ...& with the information in the following article on this site:
http://bmwmotorcycletech.info/viscosity.htm

DO NOT DEPEND ON MANUFACTURER'S SAE NUMBERS ...USE MY CHARTED INFORMATION in that above LINK, for oil viscosity information!

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 heavy to 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).

UNDER NO CIRCUMSTANCES CHANGE OIL VISCOSITY TO COMPENSATE FOR FAILURE TO REMOVE AS MUCH STICTION IN THE FORKS AS POSSIBLE!

A fairly high viscosity index is generally a good thing for fork oils.  The viscosity index of an oil is simply how fast or how slow, with temperature rise and fall, the oil thickness changes.  There is an article on this website on actual viscosity and viscosity index values for oils; here is the link again, (it also has other oil information):  http://bmwmotorcycletech.info/viscosity.htm.  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).

Nerdy:
Oils get thinner (viscosity decreases) as they heat up, and the reverse is true as oil temperature decreases. A reasonably decent argument can be made that it would be nice for fork oils if they got thinner as they got colder.  Multigrade oils don't really do this like you might think, they simply change less in viscosity over a temperature change.  Quite an improvement in fork action might be possible with oils that worked in reverse to their normal characteristics.  Still, some improvement ...in some respects ...will be possible by using multigrade engine oils, assuming they are compatible in other respects, with fork innards.   Just since early-2017, several new oils have come onto the market due to changes in car engine manufacturer's specifications, in the ever-constant attempt to reduce friction.  One of those oils is rated at 0W16.  We already have had very widespread use of 10W30 and 5W30.  I think that some 0W16 or other quite low viscosity engine oils are going to end up in front forks, and possibly when rebuilding rear forks.  I have not yet done testing, but am intrigued by the idea of getting a nice smooth semi-soft ride, that still performs excellently over big and tiny road irregularities.  Stiction must be considered.  Meanwhile, the best we can do is to get a fork or shock absorber oil that has a reasonably high viscosity index.


Comments and chart of oil filling amounts:
NOTE:  These are pretty much standardized filling amounts and methods.  If you are an advanced-knowledge person with experience in suspension tuning, your methods may well be different.

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 very small amount of oil, hardly many drops, during the rebuilding; thus, very little volume of oil is left in the forks before you fill them with the proper oil amount.

2.  BMW has revised oil amounts for some motorcycles, which is in my charts.

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.  Measure with the motorcycle on the center-stand, front forks fully extended (that only means the tire is not touching the ground).

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.

8.  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.

 

Drain & refill, cc (cubic centimeters)

Overhaul; slight assembly oil only, otherwise dry

PRE-Airheads:

 

 

1968-1969 U.S. models

265

280

pre-1968

150 to 170

 
     
Airheads:    
/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

220

225

R45, R65 to 1985, R65LS, R80ST

190

195

R80 from 1985, R65 from 1986

300

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

320

 
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

410

420

     
     
Classic K bikes:    
K1

380

400

K75 standard

325

 
K75 Showa, as on K75RT, etc

410

420

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

270-290

 
K100 (8 valve)

325

 
K100 (8 valve) RS, RT, LT, standard

355

 
K100 RS (16 valve)

380

400

K1100LT, K1100RS

Left 350; Right 400

400, 400

     
     

A chart of fork and suspension fluids information:
This table is sorted by real viscosity (centiStokes), not by nominal weight. Note that the nominal weight is ... um... not really in the same order as the real weight. Viscosity Index (VI) is how stable the viscosity is over a wide temperature range. Bigger numbers are better, but not as critical as you may think, unless, perhaps, being on the racetrack.

Brand / weight / model nominal weight Reported cSt @40C (centiStokes) Reported cSt @100C (centiStokes) Reported Viscosity Index (VI) Recaclulated Viscosity Index (VI) Viscosity/ Temperature Coefficient (VTC) VT (cSt@40/cSt@100)
Red Line (like water, Clear)   5.50 2.30 344 343 0.58 2.39
Red Line (extra light, Blue)   9.80 4.00 402 403 0.59 2.45
Spectro, Golden (ultra light)   10.40 4.40 385 440 0.58 2.36
RockShox (2.5w Light cartridge (Torco 5) 2.5 10.75 4.00 325 340 0.63 2.69
Torco RFF 5 5 10.75 4.00 325 340 0.63 2.69
Bel-Ray HVI (3 wt) 3 11.00 3.85 296 297 0.65 2.86
Kayaba (K2-S shock)   11.30 4.00 310 310 0.65 2.83
Showa Suspension Fluid SS-25 (2.5wt. Stock Shock, OEM) 2.5 12.50 3.90 239 239 0.69 3.21
Silkolene Pro RSF (2.5wt) 2.5 13.60 5.83 464 465 0.57 2.33
Maxima Racing Shock Fluid (light, 3wt) 3 14.00 5.20 378 370 0.63 2.69
Torco RSF Light (purple)   14.50 4.70 288 286 0.68 3.09
PJ1 Cartridge Pro For Fluid (3.5 wt) 3.5 14.60 3.60   134 0.75 4.06
Castrol Fork Oil (10wt) 10 15.00 4.00 150 179 0.73 3.75
RaceTech (Motorex) (Light US1) 4.25 15.20 4.40 222 226 0.71 3.45
Penske (lightweight shock fluid)   15.50 6.30 434 434 0.59 2.46
Kayaba 01 (zero-One Fork)   15.60 3.45 150 96 0.78 4.52
Yamaha 01 Fork oil (Kayaba)   15.60 3.45 150 96 0.78 4.52
Showa Suspension Fluid SS-05 (Stock fork, OEM)   15.65 3.60 116 113 0.77 4.36
Maxima Bicycle Fork fluid (5wt) 5 15.90 3.51 150 98 0.78 5.53
Maxima Fork oil (5 wt) 5 15.90 3.51 150 98 0.78 4.53
Maxima Racing Fork fluid (85/150, 5 wt) 5 15.90 3.51 150 98 0.78 4.53
Motul Fork Oil (Very Light ) ??   16.00 3.50 100 95 0.78 4.57
Rockshox (52 Medium, Hydracoil) (Torco 7) 7 16.10 5.05 285 280 0.69 3.19
Torco RFF 7 7 16.10 5.05 285 280 0.69 3.19
Motul Shock Oil (2.5-20)   16.10 6.20 400 403 0.61 2.60
showa Suspension Fluid SS-7 (5wt) (Apolo) 5 16.44 3.76 135 119 0.77 4.37
Spectro, Golden Spectro cartridge Fork Fluid (85/150, Very light)   16.90 3.82 150 119 0.77 4.42
Honda Pro HP Fork Oil (5wt) 5 17.00 3.80 140 115 0.78 4.47
Pro-Circuit PC-Oil   17.00 4.10   149 0.76 4.15
Bel-Ray Fork Oil (5 wt) 5 17.10 4.10 146 147 0.76 4.17
Kayaba G5 5 17.70 3.50 72 72 0.80 5.06
ELF Moto Fork oil SYN (2.5) 2.5 18.00 4.00 121 122 0.78 4.50
Motul Fork Oil, Comfort (Light 5) 5 18.20 4.00 124 119 0.78 4.55
Motul Fork Oil, Factory Line (Light 5) 5 18.30 4.20 138 138 0.77 4.36
Red Line (Light, Yellow)   18.40 7.10 407 409 0.61 2.59
Agip Fork (2.5 wt) 2.5 18.50 4.37 152 153 0.76 4.23
Bardahl (52 Fork Oil)   19.30 5.10 212 212 0.74 3.78
Bel-Ray HVI (5wt) 5 19.50 6.66 345 345 0.66 2.93
Torco RFF 10 10 19.90 5.70 260 256 0.71 3.49
Motul Fork Oil (Light)??   20.00 6.00 260 280 0.70 3.33
Spectro, SX "400"   20.90 8.90 400 460 0.57 2.35
Maxima Racing Shock Fluid (Medium, 7wt) 7 21.00 7.30 349 358 0.65 2.88
Spectro Fork Oil (5wt) 5 21.60 4.40 119 114 0.80 4.91
Agip Fork (5wt) 5 22.00 5.20 175 178 0.76 4.23
Silkolene Fork Oil (5wt) 5 22.30 4.40 106 107 0.80 5.07
Motul Fork Oil, Factory Line (Medium, 5-10) 7.5 22.30 6.20 254 253 0.72 3.60
Elf Moto Fork OIL SYN (5) 5 23.30 4.80 130 129 0.79 4.85
PJ1 Fork Tuner (2.5wt) 2.5 26.10 4.90   111 0.81 5.33
Spectro, Golden Spectro Cartridge Fork Fluid (125/150, Very Light) (Marzzochi)   26.10 5.25 150 135 0.80 4.97
Spectro, Golden (Very Light)   26.40 9.90 400 397 0.63 2.67
Maxima Racing Fork Fluid (125/150), 7wt) 7 26.70 5.36 151 138 0.80 4.98
Silkolene Pro RSF (5wt) 5 26.70 9.46 372 372 0.65 2.82
PJ1 Fork Tuner (7.5wt) 7.5 27.50 5.10   113 0.81 5.39
Castrol Synthetic Fork Oil (5wt) 5 28.00 5.70 151 149 0.80 4.91
–hlins (Shock)   28.70 9.40 340 340 0.67 3.05
Bel-Ray Fork Oil (7wt) 7 29.00 5.20 110 108 0.82 5.58
Maxima Racing Shock Fluid (Heavy, 10wt) 10 29.00 9.40 334 336 0.68 3.09
–hlins (High Performance)   30.25 10.18 352 352 0.66 2.97
Red Line (Medium, Red)   30.40 10.80 369 374 0.64 2.81
Agip Fork (7.5wt) 7.5 31.00 6.40 163 165 0.79 4.84
PJ1 Fork Tuner (5wt) 5 31.20 5.40   106 0.83 5.78
Fox Shox (Torco RSF Medium)   31.82 9.57 300 309 0.70 3.32
Torco RSF Medium (Purple)   31.82 9.57 300 309 0.70 3.32
Maxima Bicycle Fork Fluid (10wt) 10 32.00 6.28 151 151 0.80 5.10
Maxima Fork Oil (10wt) 10 32.00 6.28 151 151 0.80 5.10
Spectro Fork Oil (10wt) 10 32.80 5.60 111 107 0.83 5.86
RaceTech (Motorex) (Medium US2) 10 32.90 7.20 200 192 0.78 4.57
Bel-Ray Fork Oil (10wt) 10 33.00 5.90 123 123 0.82 5.59
RockShox (10wt) (Torco 15) 10 33.00 8.40 240 247 0.75 3.93
Torco RFF 15 15 33.00 8.40 240 247 0.75 3.93
Shell Advance Fork 5 5 33.05 7.09 185 185 0.79 4.66
Kayaba G10S 10 33.20 5.70 111 111 0.83 5.82
Bel-Ray HVI (10wt) 10 33.50 10.00 306 308 0.70 3.35
PJ1 Fork Tuner (10wt) 10 33.90 5.70   106 0.83 5.95
Red Line D4 ATF   34.00 7.50 198 197 0.78 4.53
Mobil 1 Synthetic ATF   34.00 7.60 199 202 0.78 4.47
Silkolene Fork Oil (10wt) 10 34.90 7.05 169 169 0.80 4.95
Motul Fork Oil, Comfort (Medium 5-10) 7.5 35.30 6.00 115 114 0.83 5.88
Bardahl (10w Fork Oil) 10 35.50 6.90 158 158 0.81 5.14
Showa Suspension Fluid SS8 (10wt) (Apollo) 10 36.47 7.24 200 167 0.80 5.04
Silkolene Pro RSF (7.5wt) 7.5 37.00 12.00 322 339 0.68 3.08
Shell Advance Fork 10 10 38.97 7.44 161 161 0.81 5.24
Castrol Synthetic Fork Oil (10wt) 10 42.00 7.50 150 147 0.82 5.60
RockShox (15w Extra Heavy) (Torco 20) 15 42.80 10.40 250 245 0.76 4.12
Torco RFF 20 20 42.80 10.40 250 245 0.76 4.12
Agip Fork (10wt) 10 45.00 8.30 164 162 0.82 5.42
Elf Moto Fork Oil SYN (10) 10 45.40 7.60 135 134 0.83 5.97
Spectro Fork Oil (15wt) 15 45.60 7.20 119 118 0.84 6.33
Repsol Moto Horquillas Fork Oil (10W20)   46.00 8.40 150 161 0.82 5.48
Maxima Bicycle Fork Fluid (15wt) 15 46.00 8.48 154 163 0.82 5.42
Maxima Fork Oil (15wt) 15 46.00 8.48 154 163 0.82 5.42
Maxima Racing Fork Fluid (235/150, 15wt) 15 46.00 8.48 154 163 0.82 5.42
Castrol Fork Oil (15wt) 15 46.50 8.30 150 155 0.82 5.60
ELF Moto Fork Oil (10wt) 10 47.00 8.30 153 153 0.82 5.66
Spectro, Golden (light)   47.27 16.76 235 370 0.65 2.82
Kayaba G15S 15 47.30 7.20 112 112 0.85 6.57
Silkolene Pro RSF (10wt) 10 47.36 13.69 303 303 0.71 3.46
Silkolene Fork Oil (15wt) 15 48.45 8.50 153 153 0.82 5.70
Bel-Ray HVI (15wt) 15 49.00 13.90 297 298 0.72 3.53
Bel-Ray Fork Oil (15wt) 15 53.00 7.50 103 103 0.86 7.07
Red Line High Temp ATF   53.90 10.00 172 176 0.81 5.39
PJ1 Fork Tuner (15wt) 15 55.30 8.70   133 0.84 6.36
Motul Fork Oil, Factory Line (Heavy, 15-20)   61.00 13.20 224 225 0.78 4.62
PJ1 Fork Tuner (20wt) 20 61.70 8.40   106 0.86 7.35
Spectro Fork Oil (20wt) 20 65.00 8.90 110 111 0.86 7.30
Maxima Bicycle Fork Fluid (20w) 20 65.00 12.90 201 204 0.80 5.04
Maxima Fork Oil (20w) 20 65.60 12.90 201 202 0.80 5.09
Red Line (Heavy)   66.80 16.00 256 256 0.76 4.18
Shell Advance Fork 15 15 67.00 10.37 120 143 0.85 6.46
Silkolene Fork Oil (20wt) 20 67.70 8.75 101 101 0.87 7.74
Castrol Fork Oil (20wt) 20 68.00 11.20 150 159 0.84 6.07
Repsol Moto Horquillas Fork Oil (15W30)   68.00 11.20 150 159 0.84 6.07
Agip Fork (15wt) 15 69.00 11.50 162 163 0.83 6.00
Elf Moto Fork Oil (15) 15 72.60 11.60 155 155 0.84 6.26
Bel-Ray Fork Oil (20wt) 20 73.50 9.50 106 107 0.87 7.74
Motul Fork Oil, Comfort (Heavy, 15-20)   77.90 10.10 111 112 0.87 7.71
Torco Synthetic RTF   82.50 14.33 182 182 0.83 5.76
Motorex Semi-Bath fork oil, 5W40   90.90 14.60 169 168 0.84 6.23
Silkolene Pro RSF (15wt) 15 92.50 19.50 235 233 0.79 4.77
ELF Moto Fork Oil (20) 20 99.00 14.60 150 153 0.85 6.78
Agip Fork (20wt) 20 104.00 15.03 155 151 0.86 6.92
Silkolene Fork Oil (30wt) 30 105.00 11.84 101 102 0.89 8.87
Bel-Ray Fork Oil (30wt) 30 105.00 12.00 104 104 0.89 8.75
Kayaba G30S 30 116.10 16.70 156 156 0.86 6.95
PJ1 Fork Tuner (30wt) 30 116.90 15.00   133 0.87 7.79
Spectro, Golden American 4 (20w50)   155.00 19.60 146 145 0.87 7.91
Credit to the source:  www.peterverdonedesigns.com



Rev:
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.
11/11/2017:  Add comments on new 0W16 and similar oils. Clarify some minor details.  Reduce fonts, colors, and html changes.
04/05/2018:  Add chart, re-arrange, fix html;  and add comments.
05/05/2018:  Expand 3, slightly, to include that the measurement is front tire not touching ground, motorcycle on center-stand.

Copyright 2018, R. Fleischer

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Last check/edit: Monday, June 18, 2018