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Important note on the use of the correct oil for the damper of the late hydraulically damped front fork

For a given reason, I would like to point out that no brake fluid should be filled into the damper of the late hydraulically damped front fork. The device description D624/1 speaks of "Bremsflüssigkeit" brake fluid, but this primarily refers to mineral hydraulic oil.

The original text from the Gerätebeschreibung D 624/1 is (Issue 25.09.1944, page 49):

Als Dämpfungsmittel (für die Gabel) kommen Ölsorten in Frage, deren Viskosität nicht unter 2,74 °E (Engler-Grad) und nicht über 7,47 °E bei 16 °C liegt, wie z.B.:

(Suitable damping agents are oil types whose viscosity is not below 2.74 °E (Engler-degree) and not above 7.47 °E at 16 °C temperature, as such:)

• Ate-Bremsflüssigkeit, Viskosität 2,74 °E bei 16 °C
• Volo-Bremsflüssigkeit, Viskosität 7,47 °E bei 16 °C

Die Bremsflüssigkeiten dürfen nicht miteinander vermischt werden.

(The brake fluids must not be mixed with each other)

In both cases the word "Bremsflüssigkeit (brake fluid) is used. This often leads to confusion. Because it is not mentioned, that the "Volo-Bremsflüssigkeit" is not a "normal" brake fluid, but a thin mineral hydraulic oil, while the well-known "Ate-Bremsflüssigkeit" is polyglycol based.

There is only a strong warning, not to mix the two liquids.

The use of polyglycol-based brake fluid is indeed possible, but today's collector's vehicles it causes only problems:

• Polyglycol-based brake fluid is hygroscopic (draws water), which causes corrosion.
• Polyglycol-based brake fluid damages the paint.
• Polyglycol-based brake fluid requires special shaft sealing rings. Standard shaft sealing rings (NBR) are not resistant to brake fluid.

For the reasons mentioned, I strongly advise against the use of standard brake fluid in the front fork.

I recommend to use thin mineral oil (hydraulic oil). The original specification was a viscosity between 2.74 °E and 7.47 °E at 16 °C. If you convert that to today's SI units for the kinematic viscosity, you get a kinematic vicsosity of about 54 mm²/s.

This corresponds pretty much to a modern motorcycle fork oil SAE 2.5. Yeah, that stuff is thin.
Such fork oils are available in motorcycle shops and on the Internet in 1/2 l bottles. The filling quantity for a Kettenkrad fork 115 cm³. So a 1/2 l bottle is enough for several Kettenkrads.

So use a modern SAE 2.5 grade motorcycle front fork oil and the Kettenkrad's front fork will work fine.

You should not use a thicker fork oil. That can cause too high damping forces and can cause damage to the front fork.