Marzocchi forks cap screw.

78jotadave

78jotadave

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Palm Desert, California. (near Palm Springs)
Exactly what is the purpose of the screw in the top of the fork caps on Marzocchi forks? (as seen in the photo on pg48 of Tim parkers Green book). I have heard that it is used to bleed air out of the forks, how and why would you do that, I have never been able to unscrew them?
 
The telescopic action of the forks would, ever so slowly, introduce air past the oil seals into the system when the fork extends. This air increases internal pressure, resulting in harsh fork action. The purpose of the bleed screw is to let excessive air escape once internal pressure overcomes the valve spring.

More often than not, the ball, spring and screw are corroded to a horrible mess and rarely seal the bleed hole reliably, allowing the fork to work sort of satisfactorily. This type of valve is also found on the 35mm Ceriani forks fitted to the 750s.

Modern forks make use of a purposely induced air cushion and are generally completely sealed. Marzocchi M1R can be pressurised through a Schrader valve atop each stanchion but in reality actually pressurise themselves in use and excess can be let off easily through the valves.

To check if the bleed valve is working, the hex recess can be filled with a little soapy water and the forks pumped several times in quick sucession, bubbles should show in the liquid. If it's blocked, nothing. If the valve isn't sealing properly, the liquid will quickly disappear... :oops:

piet
 
As Piet said, as previously stated, I removed mine, drilled out the holes and fitted Schreader valves, not sure if it was worthwhile but I haven't blown a seal in the fifteen years or so since I did it.
 
Legs said:
As Piet said, as previously stated, I removed mine, drilled out the holes and fitted Schreader valves, not sure if it was worthwhile but I haven't blown a seal in the fifteen years or so since I did it.
Click to expand...
Schrader valves are intended to keep pressure in? Did you fit them upside down or do you regularly release the pressure that has built up?
 
No Tippie, fitted them upside down. Wouldn't have bothered but the original valves were so 'agricultural' I couldn't leave alone.
After I did the job I assembled the forks without springs or oil and compressed the assembly, made a noise just like a bicycle pump so I reckon the forks operate with a bit of a vacuum thus giving the seals a bit of an easier time. Same affect as the ball in the crankcase breather system, keeping the pressure in the crankcases down, stopping unseemly puddles on the ground.
(It's Italian so it doesn't have to mark it's territory like a Pommy bike ;) .)
 
sfcpiet said:
The telescopic action of the forks would, ever so slowly, introduce air past the oil seals into the system when the fork extends. This air increases internal pressure, resulting in harsh fork action. The purpose of the bleed screw is to let excessive air escape once internal pressure overcomes the valve spring.

More often than not, the ball, spring and screw are corroded to a horrible mess and rarely seal the bleed hole reliably, allowing the fork to work sort of satisfactorily. This type of valve is also found on the 35mm Ceriani forks fitted to the 750s.

Modern forks make use of a purposely induced air cushion and are generally completely sealed. Marzocchi M1R can be pressurised through a Schrader valve atop each stanchion but in reality actually pressurise themselves in use and excess can be let off easily through the valves.

To check if the bleed valve is working, the hex recess can be filled with a little soapy water and the forks pumped several times in quick sucession, bubbles should show in the liquid. If it's blocked, nothing. If the valve isn't sealing properly, the liquid will quickly disappear... :oops:

piet
Click to expand...
Hi I have dismantled a pair of 35 mm marzocchi and a 38 mm front fork and there is a ball and spring under the screw in good order but with just the screw in I cant blow air thrue even with compressed air so have does the valve work there is now were for the air to escape ? ore is it just ment to be a hand operated loose the screw thing you do once a while ? regards niels
 
G'day,
I had the same spring and ball set up in the 38mm marzocchis on my 81' jota. Could not blow through them.
I changed the springs over with some out of biros/pens which are much weaker and all is good.
Cheers.
 
Ktm had a valve with a round knob on top you can press with your thumb , added it to my superduke and a couple of kwasakies and a Honda if my memory serves just drill and tap , just bleed the air after the trip to the twisty bits and suspension is no longer air damped
 
It's not air damping - that is performed by the fork oil. Most (modern) suspension setups use an air gap to employ air pressure in the spring function. Maybe it adds too much pressure and causes fork seal leakage - not sure. But there's no point setting up air gap and then venting all the air out of the system using the spring ball valve.
 
I have always been a bit puzzled by those valves. I’m sure mine are well seized/blocked and I have never had seal issues. And my GPz1100 had Schrader valves to put a few psi air preload in.
 
I love how some say add air, and some say remove air. Just like Ecanomics, both are Tech that confuse the masses. My lame guess is that no air, well, equalised atmospheric air pressure is at least a stable state to deal with. A while back, I experimented with air gaps and the same oil weight, from 100mm to 180mm, and didn't notice any dramatic change in my Ceriani forks.
 
After spending most of the day cleaning the shop to find the (Marzocchi -RGS) fork cap that launched itself into the deepest corner, I opened the screw that was probably last touched by someone at the factory... inside was a spring with a rubber cone inserted into the spring...no ball. Changing to a ball would affect the spring height but I'd rather hear about options. If you were to make a new cap what would YOU do instead?
EDIT 1)I'd guess the rubber item is not available 2) What is the O-ring size? It looked in good condition so I must have replaced it at last serviceIMG_0555.jpg
 
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