Clutch pedal proper free play?

I just changed the clutch master cylinder in my 360 Modena 6 speed manual. As most (or at least some) people know, there must be a certain minimum amount of clutch free play to insure that the pump ports are properly uncovered with the clutch engaged (pedal up) but the free play should not be excessive because that will, in the worst case, prevent the clutch from fully disengaging with the pedal depressed.

On other cars, this free play may be measured at the master cylinder rod or at the pedal itself. My digital copy of the shop manual shows information about how to set the clutch pedal height and pedal throw (which I have done) and how to adjust the safety switch. For some odd reason it does not address the proper amount of clutch free play.

Does anyone have this specification? I would prefer information on the 360, but I’ll be happy with the specification for any other Ferrari with a conventional transmission and a hydraulic clutch system. I’m guessing a millimeter or two at the pump rod will be about right, but it would be nice to hear it from Enzo.

 

I don't think I made myself clear. The clutch pump (clutch master cylinder) has a reservoir connected to the cylinder when the clutch pedal is in the rest position (up). There are several different internal designs from simple ports to fancier relief valves built into the piston face. But in every case that I know about, they operate the same way. The port must be open so that the fluid can flow between the reservoir and the cylinder. When the fluid heats up, it expands and flows into the tank and, of course, the tank supplies the necessary fluid to the system. The piston must be fully retraced, or nearly so, for this port (or valve) to be open as it should be when your foot is off the clutch pedal.

When the pedal is depressed, this port (or valve) first closes to isolate the reservoir from the cylinder and this will allow pressure to build up and send fluid to the slave cylinder. The distance the pedal travels before pressure builds is the total free play. There are several slave cylinder designs, including the combination slave and throw out bearing used by Ferrari which happens to be a constant contact type. It doesn't matter what's downstream, the master cylinder operates the same basic way, or at least every one I've worked on works that way.

Normally there is some slack between the piston and pedal. With a millimeter or two of play in the linkage between the pedal and piston, one can insure that the piston is fully retracted and the port is open. In other words, if the clevis and rod assembly is adjusted so that the piston is partially pushed in with the pedal up, bad things happen. The danger is that the port between the cylinder and reservoir will be blocked with the pedal in the resting position and the clutch can "pump up" and slip as the fluid gets hot. Plus, fluid can't flow into the cylinder from the reservoir to refill the cylinder as necessary. A tiny, but measurable, amount of free play will insure this problem doesn't exist. The requirement make sure the clutch pump piston is properly retracted has nothing to do with the throw out bearing or slave cylinder design.

In my experience, a tiny bit of free play is how you insure the clutch pump piston is properly retracted.

 

Yes, that's exactly what I'm interested in. Unfortunately my clarifying post really didn't clear things up. Plus I'm afraid it sounded as if I was saying your mechanic was somehow wrong. I'll try again.

You could argue that there are three components of "free play". As the pedal is depressed, first there is the very small slack between the pedal mechanism and the master cylinder piston made up of the clevis pin clearance and the clearance between the operating rod and the piston. Some people call this "clevis play" and it's this measurement that I'm interested in. It is typically very small, a mm or so measured at the rod.

Second there is the small movement accompanied by very little pedal resistance (other than the return spring) as the port between the cylinder and reservoir closes during the initial part of the stroke.

Third is the clearance or slack between a conventional throw out bearing and the diaphragm fingers which keeps the conventional throw out bearing from constantly riding on these fingers. Before the clutch can disengage, this slack must be taken up by movement of the slave cylinder. This third component does not apply to the Ferrari "constant contact" TOB. The constant contact design produces a clutch pedal with less overall free play. It's what your mechanic was talking about as he was explaining why the Ferrari system results in a pedal feel a bit different from some other cars.

 

Assuming the 355 is similar to my 360, you should be able to do a little checking even if you don't fancy yourself as a Ferrari mechanic. The 360 has an adjustable stop for both ends of the clutch pedal travel. The manual advises to adjust the upper stop so that the clutch pedal is aligned with the brake pedal. Then it says to adjust the lower stop (when the pedal is fully depressed) so that the movement measured at middle of the foot rest plate is 111.8mm (the 355 may be different). This pedal stroke easy enough to check yourself.

The manual goes on to warn that an excessive stroke which uses the entire travel of the master cylinder can give the feeling of stressing the pressure plate springs. Perhaps you have this situation because one (or both) of your pedal stop screws is maladjusted. You should be able to trouble shoot it using a tape measure, even if you feel more comfortable having a mechanic adjust it.

 

Thanks for the data. See my thread on what I learned when changing my clutch pump.

I settled on one mm "clevis play" measured (eyeballed actually) at the operating rod.