Topic: Fork Oil for CB400F Which Weight (viscosity)

[sbutler]

The shop manual for my 1975 CB400F list ATF fluid as the proper fluid for the front forks.  Looking at the viscosity of several ATF fluids (prior to the recent use of low viscosity ATF), these published viscosity are in the range of about 32-36 cSt at 40C (104F).  This looks to be the same range as for 10W Fork oil, so it looks like 10WFork Fluid would be the correct replacement.

Note, SAE 's 10W specification deals with low temperature pumpability not viscosity but the two are related.  Not to sure if there is much difference between ATF and Fork Oil, both are low foaming hydraulic fluids.

The big difference is that Fork Oil is available in different weights (viscosities).  Since the older Honda motorcycles are typically under-damped, has anyone tried 15W Fork Oil and was there an improvement?

[Tracksnblades1]

The shop manual for my 1975 CB400F list ATF fluid as the proper fluid for the front forks.  Looking at the viscosity of several ATF fluids (prior to the recent use of low viscosity ATF), these published viscosity are in the range of about 32-36 cSt at 40C (104F).  This looks to be the same range as for 10W Fork oil, so it looks like 10WFork Fluid would be the correct replacement.

Note, SAE 's 10W specification deals with low temperature pumpability not viscosity but the two are related.  Not to sure if there is much difference between ATF and Fork Oil, both are low foaming hydraulic fluids.

The big difference is that Fork Oil is available in different weights (viscosities).  Since the older Honda motorcycles are typically under-damped, has anyone tried 15W Fork Oil and was there an improvement?

 Most agricultural hydraulic oils are 10w…CaseIH HyTrans (rated for hypoid, transmissions, wet clutches, and hydraulics) is what I use in our Honda CR dirt bikes front forks. It’s always red and never turns black..Nor does it in their transmissions with wet clutches.. John Deere’s HyGard is a very good and is rated the same for rearends, trans, and hydraulic like all modern tractors the swaps spit (uses the same in all their systems).Caterpillar, Case, New Holland, Adco, etc all have good 10w oils too. They all warranty their products so their oils are good and usually reasonably priced compared to motorcycle lubricants.

The newest hydraulic oil offering are starting to show some multi grade choices, semi synthetic probably but I don’t know for sure.  I’m thinking the newest HyTrans might already be a multi grade. Guessing your front forks would never reach the temperature that would make it change though..

[Tim2005]

10W is fine, stock 400f damping is quite acceptable for road use. Maybe try 15W if you've installed stiffer springs though

[HondaMan]

If you ride long distances more often than not, try a multi-weight version. I still use 10w30 non-detergent in my (750) forks. Lawn mower oils are non-detergent: if you use detergent oils it will make a thin film of it escape past the seals and collect on the lowers, looking like it "blew a seal". It didn't...

[leovich]

I use standard ATF oil on the 400, damping is good with it. I tried 15w but they become too hard and not enjoyable on the road. ATF oil seems to have some sort of hydroscopic habit and I think it’s good because if some moisture goes inside (and 400f don’t have the fork gaiters to protect against that) it can be absorbed by the oil instead of stratify inside the fork so when you change the oil you remove moisture too

[Deltarider]

ATF usually is around 7W. That's what I have in my CB500K2-ED forks, prescribed by Honda. 10W I did not like. Didn't have to replace a seal yet, thanks to those wonderful gaiters.

[Bankerdanny]

Are you a bigger guy? 10wt fork oil is the equivalent of ATF (I don't believe separate fork oil was a thing back in the 70's). Personally as a bigger guy (about 250lbs) I have run 15 wt in my 550 and my 750.

[bryanj]

Correct there was no specific fork oil in the 70's, atf was used as it didnt "foam" as much, original spec was engine oil

[Deltarider]

Are you a bigger guy? 10wt fork oil is the equivalent of ATF (I don't believe separate fork oil was a thing back in the 70's). Personally as a bigger guy (about 250lbs) I have run 15 wt in my 550 and my 750.

I am a bigger guy too, 195cm with a modest 87kg though. But... because of my length, I tend to sit more to the rear. It's for that reason 7W is best for me.