Fork Oil Weight & Air Gap. 1977 CB400F

[robnobrakes]

Can anyone recommend the correct fork oil weight and air gap, when rebuilding the forks on my 1977 CB400F.  My Haynes manual is not very good!

Thanks.

[Bryanj]

Air Gap?

[totty]

Air gap = Oil level in my understanding.

I used 20w and put in the quantity the manual said, 10 or 15 might have been more appropriate.

[steven400/4]

i put 162ml in each leg never had any problem marked it up on a babes bottle

[Bryanj]

60 yers old and too many years in the trade to talk about and i never heard of an oil level being described as an Air Gap!!

[Lynx]

I've been biking 31 years and have always set up my fork oil using the air gap. It is usually quoted with forks fully compressed and springs out. The gap from top of fork stanchion to top of oil. I just use a metal tape measure. Occasionally it is quoted with forks extended. So you need to check for that. Having said all this. I don't know what the air gap is for the 400.

[Bryanj]

That's 'cos Honda don't quote it

[Bryanj]

If I was still working on the bikes that would be possible, all i have now is my own 500 fours and the ocaisional outsider. Personaly I have never had a problem with pumping out all the old oil and addding the amount shown in the manual from s measuring jug

[SteveD CB500K0]

Why don't they put drain plugs on modern bikes.

I have to remove the front wheel and then the fork legs to "drain" my Tiger. I could turn the whole bike upside down I suppose...

[Bryanj]

I think its because they don't expect the oil to need changing till the seals go, or was it walruses

[K2-K6]

Wasn't that the Beatles?......the walruses I mean............

[UK Pete]

crazy lyrics , I am the eggman, they are the eggmen. I am the walrus, goo goo g'joob.  LOL!

[Bryanj]

something to do with LSD and I don't mean old money

[Oldbuthopeful]

Sorry to be the one to bring this debate back down to earth!!!.....................    I was about to appeal for help with a similar problem when I came  across all these words of wisdom and thought my prayers had been answered without having to put the question.

Not so.  All this talk of weights and air gaps leaves me floundering.  All I  need to know is  -  how many fl.ozs. of ATF should I top up my 350F
with after having drained each leg via the drain plug?    My Clymer manual says 4.2ozs per leg, but when I poured in the measured amount, I found I had almost no travel on the forks.  So I deduce that 4.2ozs is too much.  Could this in fact be the amount needed when filling the  legs from empty after a stripdown?

When I had my 550, I seem to remember Mr Haynes giving figures for both 'filling from empty' and 'topping up'.
Has anyone by any chance got comparable figures for the 350Four please?

[K2-K6]

As the damping action is at the base of the fork, the oil level for damping only has to cover the valved section to avoid cavitation and causing a loss of damping.

Looking at the damper rod design of this type of fork it appears that the upper level for damping purposes is not highly critical......but too high a level will most certainly give you a hydrailic lock, in that the top of the oil reaches the fork leg cap and so prevents any further compression of the suspension taking place.

If the level is unknown, then it would seem logical to wrok it backwards to get a decent start point.......so if you take the total suspension travel (4.5inches?) and then double it to give you a minimum air gap with the forks fully extended.......using a syringe and piece of plastic pipe, mark off 9inches on the tube, insert it into the fork leg from the top until the mark is level with the stanchion, then draw off any excess to make both legs equal.

You should be able to verify with the fork caps still off by pushing each leg in turn up and down to make sure the damping is effective over the full range before riding it.