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Front Fork Oil discussion; filling amounts; hints.
This article applies to Pre-Airheads, Airheads, Classic K bikes
© Copyright 2020, R. Fleischer
Discussion of oil types and characteristics:The following section should be used with: https://bmwmotorcycletech.info/viscosity.htm.
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 oils. 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 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 same as 210°F).
MINimum 13.2 Cst at 40° C (100°F).
Maximum 600 Cst at -40° C ....too cold to bother about for motorcyclists (unless very crazy about cold weather riding!).
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:
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 (like that military oil, as above), so do not willy-nilly change the viscosity grade to much 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 medium-high viscosity index is generally a good thing for most riders for fork oils. However, a good argument can be made that in some circumstances ...a lower VI can have advantages. One needs to THINK about how viscosity changes with heat (not just atmospheric, but the heat produced by considerably bumpy terrain, which affects viscosity, some less or more than others). 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): https://bmwmotorcycletech.info/viscosity.htm. Once the VI is quite high, you might not notice any difference in performance by going higher in VI. But, you MIGHT actually want a change ....with the forks not staying as stiff with a lot of fork movement. HOWEVER, you may not want that ...it depends on YOUR riding preferences. If you are an aggressive rider in VERY bumpy & probably constantly near extreme bumpy conditions, the fork oil will heat up a fair amount. If your forks feel too soft under such conditions, but feel OK when starting out, then you may want to consider a high VI oil, even perhaps an oil that is a bit thicker (sometimes) to start with; and, for sure one that has anti-foaming characteristics. If you think about what I have said here, you may just come to the conclusion that an overly-high (or overly-low) viscosity index is NOT necessarily what you may want. Keep in mind that as the front end gets colder, everything, in general, has more friction (except maybe the tires!).
BMW Airheads forks from 1981 are especially sensitive to fork oil foaming. I suggest you use a quality FORK or SUSPENSION fluid (oil).
Only a bit 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 some riders if their fork oils became somewhat thinner, proportionally, as they got colder. Multigrade oils don't do this like you might think, they do help though, by changing less in viscosity over a temperature change whether higher or lower. Still, some improvement ...in some respects ...will be possible by using multigrade oils. Do consider stiction. It is VERY important that forks, fender mounts, fork braces, etc., all be assembled very carefully to minimize stiction.
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 be different.
1. In instances where I know the information, I have listed oil refilling amounts where different for a drain and a refill; and, a fork overhaul and a rebuild. An overhaul/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, which will be somewhat larger than a drain/refill. When you increase oil quantity beyond specifications (this is sometimes done when tuning the front end), the AIR above the oil level will DEcrease. Air compresses, oil does not. Thus, tuning is affected. BMW's revised oil amounts is also shown in my charts.
2. 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).
3. For the /5; /6; and /7 to 1980, with the forks fully extended, wheel off ground, the level is 50 mm above the damper.
4. For the R45; R65 to 1985; R65LS; R80ST, the level is 35 mm above the damper, +-15mm.
5. 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.
6. Use a quality oil with no tendency towards foaming.
7. 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 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/rebuild; slight assembly oil only
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.
Left (rebound) SAE15, 470.
|R80R, R100R, SHOWA forks||
|Classic K bikes:|
|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)||
Left 350; Right 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. This is a mix of OLD information and later information, that is believed correct, but the manufacturer's have occasionally changed specifications, or changed the SAE rating for a specific oil. In August, 2020, based on what is believed to be reliable information from someone who was in contact with Torco, Torco down-numbered some of their oils as far as (at least) SAE viscosity is concerned. I decided to remove all mention of Torco from these charts. Contact the company if you wish their published data. I have NO recent lab testing on Torco products. Here is a link to someone else's compilation of fork oil weights:
The charting is old, but contains much more than mine, and it has the Torco figures (out of date though)
|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|
|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|
|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|
|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|
|Bel-Ray Fork Oil (5 wt)||5||17.10||4.10||146||147||0.76||4.17|
|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|
|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|
|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|
|Ohlins (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|
|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|
|Shell Advance Fork 5||5||33.05||7.09||185||185||0.79||4.66|
|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|
|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|
|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|
|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|
|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|
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.
07/15/2020: Improve clarity of explanations.
08/18/2020: Again, improve clarity of explanations.
08/19/2020: Add note. Remove all Torco chart entries. Add a link and note.
© Copyright 2020, R. Fleischer
Last check/edit: Monday, December 07, 2020