These cb550 K3 (uk) fork springs

[McCabe-Thiele (Ted)]

What weight is medium? 5w 10w or 15w for instance. I wouldn't use ATF these days in forks, ok in the 70s but fork oil has progressed a lot since then. Personally I'd have put a set amount in one fork leg, measured the air gap and then used that to set the other, it makes sure they have the same amount of oil in each leg.

I used a modern fork oil in my 500 my build log says 160 ml of SAE15 in each leg.

That said Ken modern ATF's have also modernised since the 1970's with many ATF's now being fully synthetic, they also contain plasticisers to maintain seal life.

The problem I see with using an ATF is nowhere does the specification stipulate a universal understood grade/weight  like 10W etc, my Superdream manual quotes ATF with no specific details such as Dexron2 etc.

Having only covered about 30 miles in 2024 I can't give honest feedback about how my forks feel except to say the clonk under braking they had before has gone. They felt okay when braking with the old noises gone. I plan to ride it more in 2025

[nairb]

What weight is medium? 5w 10w or 15w

Here is a pic of the oil used.
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It would be nice to have a warm spring day to find out..... New spark plugs as well. 

[Oddjob]

You may find 10w a little light for damping on these old bikes, it depends a lot on the weight of the rider, I think the standard back in the day was 15w which is odd as most people riding them back then were light. I'd have put a little more in each leg as well, maybe 160cc.

First off, try it and see, if it doesn't feel right you can always alter the amount of oil.

[taysidedragon]

There was a recent post on here that showed that there is no standard fork oil weight between different manufacturers.  If it says Medium on the bottle I would say that is fine to use.
I've used Silkolene 15w fork oil in my 400f and the fork action is ok for me. 👍

[deltarider]

Originally prescribed by Hondawas ATF, which is usually around 7w. I have had 10w fork oil in mine before but I like the 7,5w best.