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Clutch Hydraulics


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I'm just getting back into a couple of Spridget projects after taking a break. They are both Bugeyes that will have Sebring bonnets, wire wheels, disc brakes, and 1275 engine/gearboxes. I'm not a total novice, having built a '62 Midget racer and tinkering with these cars for nearly 30 years. I think I understand the need to 3/4" bore brake master cylinders and have sourced single action 3/4" wheel cylinders (as used on later Morris Minors), so I've got that sorted out. I'm somewhat confused by information on seeing on various websites offering tech advice and various vender offerings with regard to clutch hydraulics. As I understand it, the early drum brake 948 Spridgets with combined brake/clutch masters had 7/8" bores for both cylinders. Then when the disc brake 1098 cars came out, the bores in the brake/clutch masters both changed to 3/4". Here are my questions:

1) What was the bore of the slave cylinder on the 1098 cars? I'm only seeing 7/8" and 1" for sale.

2) Does a 3/4" clutch master work with a 7/8" or 1" bore slave cylinder?

3) When did the slave cylinder go from 7/8" to 1"?

4) Is it true that silicone fluid should not be used for clutch hydraulics? I read somewhere that the stroke of a clutch cylinder requires lubrication not present in silicone fluid.

Thanks in advance any advice you can provide!


Staff member
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not sure about the various bore issues but, I run DOT 5 - silicone fluid clutch and brakes with no issues. The only thing is that I had to go and find the 'thickest' fluid I could as the more viscous fluid was leaking past the seal.


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I think you meant to say that the less viscous fluid was leaking pat the seals. I've been warned against using silicone fluid on a front drum brake Spridget for this very reason. I was told, "You'll never stop all the leaks". I have done a little research today and apparently the only clutch slave bores on 948 and 1098 cars were 7/8" and all the 1275 cars were 1". So it appears that the 3/4" bores on 1098 and 1275 clutch masters do in fact work with 7/8" and 1" slave bores. I guess clutch cylinders work differently from brake cylinders, possibly due to the increased pedal travel and stroke length.

Joe Schlosser

Jedi Warrior
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I believe that when the 1275 was introduced the single line system was replaced with the dual line MC and a separate clutch MC. That is the main reason the clutch slave was changed to the 1" diameter. I run the 3/4 inch single line MC system with a 7/8" clutch slave on a 1275 with no issues.
I also run the Castrol LMA (forget what the new name is) and bleed the system every year or so as car sits in the winter. Tried silicone once and did not like the pedal feel.
Changing to the dual MC is a lot of work. I know people who have done it for purely safety reasons.
You are very correct that the brake and clutch system hydraulics are very different. Brake MC has a return valve built in to help maintain pedal travel and the clutch is meant to return instantly when the pedal is released.


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Thanks Joe. My '62 Midget racer (1275) has a single circuit, dual line MC as found in the '67 1275 Spridgets. I guess I didn't think or know about all the cylinder sizes because I never checked them when I built it. The clutch always felt great with Castrol LMA. The car has been mothballed for a decade. One of my new projects will have the the same set-up because the scuttle has already been modified by a previous owner to accept the dual cylinder pedal box. My other project will use the single combined cylinder that I plan to have sleeved. It will also have a 1275 with ribcase gearbox. Thanks for the input.
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