M8 Greyhound clutch problem

[Big D]

Hi Malcolm,

That would be the restoration of my M8 you have seen. Has the clutch on yours ever worked properly?

I to was surprised at the size of the hydraulic tubing for the clutch. As you are probably aware, the clutch slave cylinder on the M8 is quite chunky, and quite a bit bigger than the slave cylinder for the throttle. The throttle uses 3/8" hydraulic tubing though so with the lines on your clutch being even smaller than that at 5/16" doesn't seem quite right. Perhaps the line size simply doesn't allow sufficient volume of fluid to be pushed through to activate the slave cylinder?

[Russell Boaler]

Dealing with fluid flow on a day to day basis I'd say that the size of the tubing is probably due to the amount of fluid that needs to be transferred. If the pipe is small you'd wind up with a pedal that is just slow to depress rather than a clutch that won't fully disengage. It would also mean a lot more pressure being needed at the master cylinder to push the fluid through the pipe (compared to the larger pipe) if depressing the clutch at normal speed. Maybe that increased pressure is causing some bypassing in the master cylinder?

[Malcolm Towrie]

Originally Posted by Russell Boaler

Dealing with fluid flow on a day to day basis I'd say that the size of the tubing is probably due to the amount of fluid that needs to be transferred. If the pipe is small you'd wind up with a pedal that is just slow to depress rather than a clutch that won't fully disengage. It would also mean a lot more pressure being needed at the master cylinder to push the fluid through the pipe (compared to the larger pipe) if depressing the clutch at normal speed. Maybe that increased pressure is causing some bypassing in the master cylinder?

I agree, Russell. The pedal is easy to depress with 5/16" tubing, which is actually quite large for automotive brake and clutch systems, so I have no idea why 1/2" tubing was thought necessary.
I'll check for bypassing of the m/c cup seal by measuring what slave cylinder rod movement I get, as described above.

Malcolm

[Malcolm Towrie]

Originally Posted by Big D

Hi Malcolm,

That would be the restoration of my M8 you have seen. Has the clutch on yours ever worked properly?

I to was surprised at the size of the hydraulic tubing for the clutch. As you are probably aware, the clutch slave cylinder on the M8 is quite chunky, and quite a bit bigger than the slave cylinder for the throttle. The throttle uses 3/8" hydraulic tubing though so with the lines on your clutch being even smaller than that at 5/16" doesn't seem quite right. Perhaps the line size simply doesn't allow sufficient volume of fluid to be pushed through to activate the slave cylinder?

Darryl, yes, it was that thread on your very nice restoration I was referring to. I drove the M8 a few years ago and my recollection is it drove ok, so I think something has deteriorated.
When I saw your reference to 1/2" tubing, I said, that can't be right. I've NEVER seen 1/2" tubing on an "automotive" braking or clutch system. But the parts list confirmed it.
I can see no advantage to using tubing that large in this application. I used a pressure bleeder applying only 15 psi to the m/c reservoir and when I cracked the bleeder on the slave cylinder, I got a FOUNTAIN of fluid coming out. So 5/16" tubing seems to be quite adequate for good flow. And it's not like the tubing causes a hard pedal from squeezing fluid through restrictive tubing. The pedal effort is very light.
I did some measurements on the m/c and slave cylinder. The m/c pushes out 20 cc per stroke. The slave cylinder needs 25 cc to go its full stroke of 1 1/4". So the slave cylinder is stroking 1" when the pedal is floored. Now I need to confirm that I am getting 1" movement of the slave cylinder rod and also if 1" movement is enough to fully disengage the clutch plates.
Malcolm