Topic: Fork oil Or ATF ?? Opinions?

[1974cb550k]

I have a k5 750 and i was looking for different opinions on what to use for fork oil. I have a service bulletin somewhere that states that honda switched to ATF in the late 70's, But im sure there is something better out there. These are brand new tubes with new seals so its not a leakage issue. Im just looking for the smoothest ride. I upgraded to progressive springs as well. Just looking for a few opinions. Thanx

[bender01]

I run ATF because I dont have a motorcycle shop close to me. I have one thats open from 3 pm til 6. I am never off from work those times.  The auto parts stores dont sell fork oil either. I replaced the tubes and seals on a 74 750k this spring and i am happy with ride. Especially since I dont have one leaking.

[ofreen]

I used ATF way back when.  Many years ago, I tried 71/2 wt PJ1 fork oil on the advice of someone.  I noticed immediately that small bump compliance was better with the fork oil, just as I was told it would be.  Less stiction.

[TwoTired]

Both ATF and fork oil have the stuff to keep the seals from drying out.
Both have additives to keep foaming at bay.

So, it is a matter of how fast the fluid can flow through the internal valving.  Thick oil makes the damping action slower to respond.

ATF averages about 7 wt for viscosity.  For my 550's I like 10Wt.  For the 750s I like 15wt.

You'll feel the road bumps more in the bars with heavier wt oil, and it will take the tire longer to regain full contact with the road on rebound.  Some prefer the solid feel heavy weight oil provides.  Of course, too light an oil will allow the tire to oscillate (bounce) during rebound from bump.

FYI, if you mix 10 Wt oil with 15 Wt oil 50/50, you get 12.5 wt oil.     If you feel the need for fine tuning.

If you routinely ride where the forks bottom AND top out in their travel, you probably need a thicker oil.  If it mostly bottoms, or mostly tops out, then you need a spring change (weight or rate).

Cheers,

[CoachDoc]

The owner's manual for a '74 550 specifies ATF in the forks, and this is what I have always used. IMO keeping it fresh and at the correct levels is more important than debating what to use.

[Lamp]

Take these owners manuals with a grin of salt. They were written 40 years ago. I'm running 15w fork oil in my 750 and I'm very happy with it.

[trueblue]

I'm using 15wt fork oil in my 650 and it works well

[Dimitri13]

I run 10w in my 550 but it doesn't matter because my springs are way past their expiration date.

[Deltarider]

I consider going back to ATF. I sit further back which reduces front load. Therefor I think 7wt is better for me.

[Lostboy Steve]

I run 10w in my 550 but it doesn't matter because my springs are way past their expiration date.

lol +1 I gotta get new ones myself. I run 10wt. I cant get my left fork to stop leaking oil tho. I have checked the tubes over and over, they are smooth. No idea what I'm gonna do. Changed seals twice.

[HondanutRider]

I run 10w in my 550 but it doesn't matter because my springs are way past their expiration date.

lol +1 I gotta get new ones myself. I run 10wt. I cant get my left fork to stop leaking oil tho. I have checked the tubes over and over, they are smooth. No idea what I'm gonna do. Changed seals twice.

Usually replacing the seals should clear leaks if the tubes and slides are clean and not marred. 

I don't know what other problems it would introduce, but there is often an additive for ATF transmissions that you can find, that stops leaks by causing the seals to swell or become soft again.  A long time ago I had a British sports car with leaking front shocks (leaver in action), that instead of trying to replace I topped up with progressively heavier oils to no avail - all the way up to pure STP which was like thick syrup.  No matter what grade of oil I added they would be fine for a while and then leak all out.  In desperation I finally topped it up with some plain ATF and this additive - and the leaking stopped!  The front shocks seemed fine and the MGB handled just fine.  Now I'm not saying what it would be like on a motorcycle, but I do have this DOHC CB750 with a leaking front fork, so I'm thinking of trying it.

[chewbacca5000]

Do a search for 35mm film others have used it to clean the dirt out from between the fork and the fork seal with good results. 

Here a link that runs you through it.  http://forums.sohc4.net/index.php?topic=71117.0

Good Luck!

[MCRider]

Per HondaNutRider: A shot of "StopLeak" works wonders. Drain and Fill 60% ATF/40% StopLeak.

[Tanqueracer]

As long as we are on the subject of front forks , does anyone know the free height of the front fork springs? It's supposed to be in the manual somewhere but dang if I can find it. 1975 CB 750F. I'm 6'1" and 240 lbs and the forks are always mushy and almost bottom out when stopping. Currently using Amsoil ATF.
Thanks