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Discussion Starter #1
Hey all,

I am completing some maintenance to my 1975 Honda CB125S to get it road ready. While I had the forks off to replace the rubber gators, I lost some of the fluid when my dog decided to check in on what I was doing and knocked them over.

Question #1 : Do you have to have Fork Oil? Or have you seen success with a 10W30 or 5W30 motor oil? What about Hydraulic jack oil (I have a bottle of it on the shelf)?

Question #2 : Anyone know the quantity per fork?

Google search turned up a wide variety of information. I have ordered a manual, but but thought I'd reach out on here while I waiting for it in shipping.

Thanks in advance.
 

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I would use atf if anything. I think thats what they used before fork oil on the older bikes (like yours) anyway. You might want to call a dealer to see if they have fork oil for your bike though
 

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ATF or Spectro Fork Oil are what I use.
 

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You only need a small bottle of fork oil, since they won't take much. Check the manual for the amount and viscosity just to be sure. It will only cost a few bucks for the proper oil, so it's worthwhile to do the job right the first time.
 

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If it's an original Honda manual, it will say ATF, but of a kind that's no longer available. I use 10 weight fork oil in my '70 CB450, and it is fine. The correct volume is what's important, though.
 

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Discussion Starter #7
If it's an original Honda manual, it will say ATF, but of a kind that's no longer available. I use 10 weight fork oil in my '70 CB450, and it is fine. The correct volume is what's important, though.
Would you consider your CB450 to front end to be soft or stiff? In between? Someone else also mentioned they use 10W, but said I may want to go heavier if I want a bit of a stiffer ride. I'm a bigger guy (5'10", 270lbs.) so am wondering if this isn't something I should do? Thoughts?

I guess it's not that hard to change if I don't like it, just need to find what works I suppose.
 

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The designated weight of the oil affects damping, both on impact of a bump, and on rebound. This should be balanced against the springs; note that the air volume above the oil also acts as a spring, so you can tune the suspension by changing the oil volume, once the viscosity is chosen.

I like a slightly stiffer suspension, and the brand of 10W I used fits that bill. You should know that the viscosity index is a relatively broad description, so 10W from brand A may be the same as 7.5W or 15W of another. Some brands have different lines of oil, and the viscosity index is not the same between them. Bottom line: try one from one brand and give it a ride, even adding or removing a small quantity. Since fork oil wears out, just like other oils, and you use so little, you can try another weight or brand after several months to a year, just to pick the one you like. I first tried 15W oil, and couldn't get the damping to balance with the springs, and dropping to 7.5W just wasn't as secure feeling as I wanted. BTW, I'm ~5'7" and weigh about 170# when dressed for a ride. Good luck on your journey - tuning suspensions is a lot like tuning carbs.
 

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One argument I've read for a dedicated fork oil vs ATF is that ATF tends to vary a bit, being somewhere between 5w and 15w. Dedicated fork oils are somewhat more predictable.
 
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Fork oil works best IMO. But you may not notice the difference, unless on a bike with very good suspension, and light enough that you can feel it. And I mean subtle changes.
You may or may not notice the difference on your bike.
I always used fork oil with the race bikes.

On my XS11 which weighs 555 pounds dry, and has forks that are too thin, with crappy damping, I use a heavier motor oil, and more than the amount stated in the manual. I also use more air pressure.

Like Wintr, I also like stiffer suspension. On my Suzuki, I use fork oil of the correct amount, with the stated air pressure.
I switched the tyres to hard centers and soft sides, and that gave the desired results. But I can feel the difference with this bike, just as I can feel an increase or decrease of 2 pounds pressure in the tyres.

Some bikes it is hard to tell if a tyre is flat.

Unkle Krusty
 

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Discussion Starter #11
My Haynes manual finally came in. In case anyone else stumbles on this thread, here's what it says (of course, as it has been said, can be varied to fit your riding style)

Fork Oil (no specified wieght) or Dexron ATF

Table 5 of Chapter 3 gives us capacity:

Drain Only/Change: 4.1-4.4 oz OR Rebuild/Reassembly: 4.4-4.7 oz

The book covers all Honda OHC Singles 100-350cc from 1969 - 1982 so if anyone has any questions, hit me up and I look up the info you need.

Thanks all for the info over the last few days!!
 
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