Slave Cylinder/Release Bearing for ZF DS25-1

My transaxle is an eBay-purchase ZF DS25-1 from Utah. I was quite lucky as it turned out to be unused and still in its crate from the mid 60s (don't you just love eBay?? :heart:). After a thorough checkout by a ZF specialist which included flipping it 180 degrees it is now ready to mate to my new custom bellhousing. It will be used with a twin-plate clutch pack and I need to find someone knowledgeable who can translate my required dimensions into a suitable axial release bearing/slave.

The box came with an external slave setup but I'm not sure my puny legs will be up to a stiffly-sprung racing clutch :eek: Or perhaps it won't be as difficult as I think? Anyone using the external slave setup in a GT40 with a racing clutch? Do you think it is worth swapping it for an axial, in-bellhousing slave/release bearing from the outset or would I be wasting my time/money?

 

Rick Muck- Mark IV

GT40s Sponsor
Supporter
I don't think the "integral/external" slave type will impact the effort required, that will be a function of the slave to master bore ratio and the pedal ratio. Some guys love the internal slaves but if you have an issue it's "pull the transaxle" to resolve.
 

Alan Watkins

Lifetime Supporter
Some guys love the internal slaves but if you have an issue it's "pull the transaxle" to resolve.
Further to that point they are also easier to "break"; just once push the clutch pedal all the way down without everything adjusted right and "pop" goes the slave cylinder deep inside your drivetrain. So all in all the external ones are way less hassle and less risky.

There are many discussions of master and slave cylinder diameter choice here so if you do something as simple as search for "slave" you'll probably find them. If your clutch really is unusual in that regard it would be a good idea to find out from the manufacturer what its release pressure is. Armed with that and a calculator you can make plumbing choices that will head off any surprises. I'm pretty sure if you keep the resulting pedal pressure in the 50-100 lb range you'll be fine. As someone else pointed out to me on this forum, when you stand on your toes on your left foot that same set of muscles is supplying your entire body weight.
 

Trevor Booth

Lifetime Supporter
you may also find that the release pressure for a twin plate clutch is less than for an equivalent single plate clutch
 

Alan Watkins

Lifetime Supporter
it would be a good idea to find out from the manufacturer what its release pressure is. Armed with that and a calculator you can make plumbing choices that will head off any surprises. I'm pretty sure if you keep the resulting pedal pressure in the 50-100 lb range you'll be fine.
And going a little further on this issue: My 500 hp big block SPF has a single-plate clutch good for "up to 500 hp" with a release force of 500 lb (McLeod 360850 pressure plate). I currently have a 3/4" master cylinder; 1" slave cylinder, pedal ratio of 4.2 and lever ratio of 1.4, so the overall ratio is 10.4 for pedal pressure of a little under 50 lb. I find the clutch travel through engagement a little to long, and pedal pressure lighter than it needs to be, and so am probably going to switch to a 7/8" slave just to "speed things up." This should increase pedal release pressure to 63 lb which I'm pretty sure will feel just fine.
 
And going a little further on this issue: My 500 hp big block SPF has a single-plate clutch good for "up to 500 hp" with a release force of 500 lb (McLeod 360850 pressure plate). I currently have a 3/4" master cylinder; 1" slave cylinder, pedal ratio of 4.2 and lever ratio of 1.4, so the overall ratio is 10.4 for pedal pressure of a little under 50 lb. I find the clutch travel through engagement a little to long, and pedal pressure lighter than it needs to be, and so am probably going to switch to a 7/8" slave just to "speed things up." This should increase pedal release pressure to 63 lb which I'm pretty sure will feel just fine.
Don´t forget to put in a pedal stop than, to avoid overstressing the pressureplate spring (they do have max recommanded travel)

OTher than that i can second what the others said.

2 plate clutchs can be designed with lower pressure forces to be able to
transmitt the same torque as a one plate clutch ( only a question of friction surface available, which is likely higher on a 2 plate clutch, if the diameter is not drastically reduced)

I have posted more than once an excel chart in this forum (clutchreleasecalculation.xls). Try to look it up ( i´m in HK currently and don´t have it available here). You can input all your values into it and it gives you a good reliable idea about the resulting pedal forces. Below 50 lbs should be ok.

TOM
 
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