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MG MGB GT V8 Factory Originals Technical - R 380 Clutch slave cylinder

I'm planning to install an R380 gearbox with a Rover 3.9 V8 in a 1978 mgb roadster; can anyone advise on the correct clutch slave cylinder? I've been assuming that I need an SD1 cylinder with an SD1 clutch, but is there anything significantly different about the R380's dimensions which means this combination won't work?
Or should I use an RV8 slave and clutch, but how would that tie in with the standard MGB clutch master cylinder?
Steve Musgrave


Assuming you are using an SD1 flywheel and bellhousing use the SD1 clutch, lever and slave cylinder. It will work with the standard MGB or GTV8 master cylinder. The R380 box is a direct replacement for the LT77; there is no need to change the clutch. Id avoid anything unique to the RV8; parts are either NLA, scarce and/or very expensive.

Geoff King

I'm using a one inch slave from a Range Rover.This is the older, larger cast iron one.
Correct me if I get any facts wrong someone, I've lost the orrigional data and so will have to work backwards. All I can remember is that I needed half an inch of travel and that I couldn't get the one and one sixteenth slave I wanted so comprmised with the RR one (which also has the same hydrolic hose threads as the MG).
To move the one and one sixteenth slave one half an inch you need Roughly 1044 cubic mm ( if I've done my sums right, pi by diameter by distance). This must be what the master moves.
You can work out the size slave you need by seeing the mimimum travel in the fork you need to dissenguage the clutch plate. Use a clamp or twist a screwdriver in a piece of cord (or something) to move the fork in a controlled manner so you've got a chance of measuring it.
You then use the largest diameter slave that will give this travel. Largest diameter because this will reduce the leg effort required. A kind of leverage gained though cross sectional area differences, ie the slave is wider than the master so moves less distance with the same force(amount of fluid displaced)

Idealy you could find out the diameter of your master and the distance it moves to work out exactly the volume it's moving to optimize your slave size. Could even measure it directly.


That doesn't seem too clear I'm sorry, I'm talking about the volume of "hydrolic fluid" displaced. 1044cubic mm.

Actually, Peter's formula is close - but not quite. Volume of a cylinder is pi times radius squared times height (or distance). So, a 1 1/16 inch dia cylinder moving 1/2 inch displaces
pi X (1.06 inch/2)squared X .5o inch = 3.14 X .53 inch X .53 inch X .50 inch = .441 cubic inches. Using 16.39 cc/cubic inch, then .441 cu in. = 7.23 cc. If you want cubic mm, multiply that by 1,000.

Wayne Pearson

You're right Wayne, embarrassment plus, what was I thinking, brain failure.
area of a circle, Pi radius squared
circumference, 2 * pi* r


I'm using a 3.5 SD1 + 5-speed with all stock clutch components including slave cylinder.

Stock MGB master cylinder (non-boosted) and have no problems, not even a leak.

Gears shift smoothly and no slippage.

Simon Austin

This thread was discussed between 27/04/2005 and 29/04/2005

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