Topic: Fork Oil or Transmission Fluid?

[GizmoDuck]

So there are a ton of fork oils out there but my Clymer manual says to use 125cc of automatic transmission fluid in the forks of my 1973 CB350F.  What should I do?

[70CB750]

I dare to say you will not know the difference.  Use what you got.

[flybox1]

So there are a ton of fork oils out there but my Clymer manual says to use 125cc of automatic transmission fluid in the forks of my 1973 CB350F.  What should I do?

I use ATF in my 750 forks, and used ATF in my 350F forks. 
For flushing the old fork oil, use mineral spirits, several times....and then refill.

[70CB750]

I used ATF, after that heavy oil - 15W40 -  because of short tubes and now I run Bell Ray fork oil - if there was a difference, I never knew.

[MCRider]

ATF runs about 8W, for a softer ride. It often has additives for seal preservation, too.

But, its personal preference, IMO

[snowfighter2]

I use ATF. Like Prokop says, use what you got. Remember this:

Clean oil is better than dirty oil.
New oil is better than old oil.
Any oil is better than no oil.

[lrutt]

ATF,

I keep a gallon jug that I put all partially used qts of trans fluid in. That is what I put in forks. Has served me well for.......like 50 years. I know I sure liked the way it changed the front end response on my 01 Ducati. all the old bikes are perfectly happy with it as well.

[Muckinfuss]

ATF is hydraulic fluid.  Engine oil is not.  Forks and shocks are hydraulic systems.  Engines are not.  ATF is designed with two important modifiers for your consideration......conditioners that preserve the sealing parts and anti foam additives to keep air out of the liquid....air compresses...the fluid does not.  That way the dampening works. Air entrained fluid doesn't dampen very well.  If Bel-ray and other fork oils are inspected, you will most likely discover they are hydraulic fluid based. 

That said.....as the others have mentioned.....you probably wouldn't know the difference no matter what you use unless you were at the race track and about 10 laps into a twenty lap race.

[MCRider]

ATF is hydraulic fluid.  Engine oil is not.  Forks and shocks are hydraulic systems.  Engines are not.  ATF is designed with two important modifiers for your consideration......conditioners that preserve the sealing parts and anti foam additives to keep air out of the liquid....air compresses...the fluid does not.  That way the dampening works. Air entrained fluid doesn't dampen very well.  If Bel-ray and other fork oils are inspected, you will most likely discover they are hydraulic fluid based. 

That said.....as the others have mentioned.....you probably wouldn't know the difference no matter what you use unless you were at the race track and about 10 laps into a twenty lap race.

Point taken. And I agree.  But, many manuals do specify 10W-30 motor oil in forks.   

[GizmoDuck]

Point taken. And I agree.  But, many manuals do specify 10W-30 motor oil in forks.   
[/quote]

My manual says if you have a CB350/CB400 to use ATF and if you have a CB500 to use 10W-30.

[PeWe]

Fork oil available as 10W-30 too.
http://www.motorex.com/index.cfm?oid=1147&lang=en&webtoolDbTemp=eintragDetail&eintragId=98

Edit: It would be fine to know more about the fork oil viscosity and know which to use. Which parameters affect that?

[ofreen]

I noticed less stiction and better compliance compared to ATF when I went to PJ1 fork oil years ago. Whether that makes a difference they can feel to a given individual is hard to say. I see a lot of people running on under-inflated tires and seemingly oblivious to it. But ATF works and it is cheap.

[Muckinfuss]

you may want to differentiate between science and marketing.  10w-30 never hits any viscosity above the 10 weight when used in forks because it's never heated up to the point of rheopectic change (thickening) such as would happen in a crankcase of a running engine.   ATF formulas currently run in the 7 to 10 weight equivalents on most off the shelf products with the main difference being in the modifiers/additives.  ATF doesn't shift weight equivalents due to heat or cold.  ATF for my Toyota truck was specified to be changed on 28K intervals and the ATF in my Mercedes....never.  The base is the same, the modifiers aren't.  But marketing hasn't figured out a way to sell one over the other.  I find MC people are more gullible to the marketing....in general.  Quite frankly, there was a lot of hip shooting and voodoo snake oil in the marketing of the entire motorcycle industry during the 60's and 70's.  Today....if my manual said to use 10w-30 for fork oil, I'd probably rip that page out and throw it away unless I had a lot of 10w-30 sitting around with nothing to use it in.

[przjohn]

When they made these bikes nobody even thought about fork oil weight until MX bikes started getting suspended in the late 70's. Run what you got you got. 10-30 would do just as fine as ATF or even Canola oil. These forks are a long way from today's cartridge forks, basically a couple of holes that are, "Dampners"