Marzocchi top cap

[Garbrand]

Hello all, I am restoring my (originally from England, then move to Germany and now happy in the Netherlands) RGS and am currently working on the Marzocchi front fork, among other things.
I can see a screw, a spring with rubber and a very small hole in the top cap.
What is the function of this construction? I cannot find it in the parts book or workshop manual.
Regards Garry

 

[wdietz186]

It is a vent to prevent pressure buildup in the tube when the oil/fork heats up in use. One way [sort of] valve to let pressure out and not suck air in.

 

[Garbrand]

But does that screw seal it again, or can air still escape through it?

 

[wdietz186]

The screw allows air to pass, not a lot but enough. You don't need to crank it down hard.

 

[Garbrand]

Okay, thank you for the explanation. Learned something new again.

 

[Gazman]

I wondered what that was for

 

[Garbrand]

In the meantime, I’ve had a look using AI. It turns out that Paioli used the same system.

On the Ducati 500 (both the legendary 500 SL Pantah and the earlier parallel-twins like the 500 Sport Desmo and GTV/GTL), this specific Paioli front fork is equipped with an ingenious but often misunderstood system.
For these motorcycles, the small spring and ball bearing at the top of the fork cap definitively function as a mechanical one-way valve / breather valve. Therefore, it is not a click-system for an adjustment screw.
This valve works as follows on your Ducati 500:

• How it works: Underneath the small screw on top of the fork cap, the spring pushes the ball bearing down against a seat. This seals off a small passage that connects directly to the inside of the fork leg.
• Function 1: Pressure relief (Anti-blowout): When the fork compresses hard during riding, the air above the fork oil is compressed. Friction causes the temperature to rise, and air pressure builds up quickly. If this pressure gets too high, it overcomes the force of the spring. The ball bearing is pushed upward, allowing the excess air pressure to escape in a controlled manner through the threads of the small cap screw. This protects your fork seals from blowing out.
• Function 2: Vacuum prevention: As soon as the fork rebounds quickly, the ball bearing immediately seals the port hermetically again. This prevents false outside air (or moisture and dirt) from being sucked inside, ensuring stable damping.

Important maintenance tips for the Ducati 500:

1. Watch out during disassembly: When you loosen the small screws on top of the fork caps (for example, to change the oil), the spring often shoots out immediately and the ball bearing easily rolls away across your workshop. Keep a magnet handy and disassemble it very carefully.
   
   
   
2. Cleaning: After several decades, some fork oil mixed with dirt can cake up around the ball bearing due to the oil vapor. This can cause the ball bearing to stick (making the fork too stiff or cause pressure buildup) or stay open (causing fork oil to sweat outwards). Thoroughly clean the chamber, the spring, and the ball bearing with brake cleaner during a rebuild.
3. Correct assembly order: The ball bearing goes in first, followed by the spring on top of it, and finally, you screw the small closing screw back in