Lotus Elan

Clutch Slave Cylinder Diameter

PostPost by: david.g.chapman » Tue Dec 15, 2015 2:36 pm

I have had a new slave cylinder picked up for a song lurking in my garage for some years, and as the one on the car had started leaking I thought I would fit it.

It turns out that when comparing the two, the new one is for a cortina and is 3/4 inch diameter, and my old one is 7/8 inch. All other important dimensions look the same.

If I fit this slave cylinder the effort to operate the clutch will go up, but by how much? Has anyone tried this setup? How does it feel?

As the clutch fluid is incompressible the slave pushrod will move further, at a ratio of radius1 squared/radius2 squared (the volume displaced in the slave is pi x radius squared x length).

That means that if the 7/8 inch pushrod moves, say 10mm, then the 3/4 inch pushrod will move 13.6mm.

This may be OK if the cortina used the same release arm, clutch spring diaphragm etc., otherwise I might do some damage.

Any thoughts?

Dave Chapman.
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PostPost by: john.p.clegg » Tue Dec 15, 2015 3:24 pm

,, so if you're getting 36% more movement,you'll need 36% more effort...

,,,and also you'll need to depress the clutch pedal for 36% less travel for the same engagement/disengagement

John :wink:
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PostPost by: alan.barker » Tue Dec 15, 2015 3:35 pm

that means you can have more freeplay between slave and push rod.
Second idea even with a thick carpet the clutch will desengage fully :wink:
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PostPost by: david.g.chapman » Tue Dec 15, 2015 6:53 pm

Thanks for that.

I think I will give it a try with the smaller diameter. I will leave a 3-4mm gap before the release arm starts to move which will only give a 2mm or so overextension of the release mechanism. Surely it can cope with that.

Mind you, I am due for half knee replacements next year. It could make for interesting physiotherapy!

I can always go back to the old slave cylinder. I have just had a look inside and it seems as if a good clean out and polish will be enough with a new seal kit. There are scratches, but only near the outside end of the bore where the steel piston rubs on the ali.

The part of the bore where the seal runs looks OK.

Cheers,

Dave Chapman.
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PostPost by: ecamiel » Wed Dec 16, 2015 12:20 pm

Careful. You will have increased travel and can easily bend or break the clutch fork.
Ask how I know this!
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PostPost by: david.g.chapman » Wed Dec 16, 2015 4:31 pm

That was on my mind.

I did try it out today with plenty of free play, but the effort is too much, especially in some future traffic queue.

I am going back to the 7/8 inch slave.

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