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MG MGB GT V8 Factory Originals Technical - Clutch master cylinder - V8 or MGB?

I notice that V8 clutch master cylinders are different from MGB ones. The only visual difference is that the reservoir is more squat on the V8 one. Are there any internal differences? Does the V8 cylinder have more grunt?
Marc

I am using a McLeod Hydraulic bearing with the original clutch master cylinder on my Rover 3.5 with a BW T5 5 speed. Doesn't seem to be any problem. Just got back from the drags and it seemed to work just fine

Bruce
Bruce

Thanks for your reply, Bruce - but what is the difference between the two?
Marc

No slave cylinder required.


Bruce
Bruce

The previous post about the McLeod throwout bearing did not address the original question. I have a Factory V8 and have just gone through this. The MGB master cylinder has a 3/4" bore, whereas the Factory V8 master cylinder has an 11/16" bore. I was not getting full clutch disengagement using an MGB master, so I called up Brown and Gammons for a V8 spec one, thinking it would give me more fluid displacement. Turns out the V8 cylinder is smaller diameter, so it would give me LESS displacement (and a harder pedal).

Turned out my problem was a combination of a small leak in the hard clutch hydraulic pipe (this was an original pipe and had some surface pitting which weakened the wall), plus some odd scoring in my slave cylinder, resulting in leaks. This was very unusual since my slave cylinder was a new Lockheed unit with only a few miles on it when it failed (causing me to lose my clutch the morning of the car's first show since the restoration!). I refurbished and reinstalled the old slave cylinder, plus fabricated a new clutch pipe. This solved 90% of my problem (more on this later). Note that the V8 slave cylinders are the same as 1275 midget, but after this I would always disassemble the cylinders and check the inner walls carefully before installation - I would guess some swarf was left in the cylinder during manufacture, causing the scoring.

You will remember I ordered a new V8 master cylinder from Brown and Gammons? Well, when it arrived I found that it had a cheesy plastic reservoir instead of the proper metal one. Someone here on this BBS said that he received one of those for his Factory V8 and that the plastic reservoir fouled the firewall when he tried to install it. The cylinder is still on my workbench, after replacing the aforementioned parts, the MGB master cylinder is working just fine.

The last 10% of the problem is a weak 2nd gear synchro, which can be directly attributed to human error. The tranny was rebuilt by a professional as part of a tech session, but after I arrived home I discovered a brand new synchro amongst my used parts - seems the guy reinstalled one of the old synchros by mistake during the rebuild! The used synchros didn't look all that bad, so we left it in. I will live with it until next winter when I will haul the engine out again and schlep the tranny back to the guy. C'est la vie!

Cheers,
Paul Kile
Paul Kile

MGB and V8 M/C and slave bore sizes differ as follows:

MGB M/C: 19mm
MGB slave: 32mm
V8 M/C: 17.8mm
V8 slave: 25.4mm


Paul Hunt

Using the figures provided by Paul Hunt, I calculated the area of the bores as follows.

MGB M/C: 19mm 283.385
MGB slave: 32mm 803.84
V8 M/C: 17.8mm 248.7194
V8 slave: 25.4mm 506.4506

Not knowing the actual distance the master cylinder moves when the clutch pedal is pressed, I made the following calculations using the easy figure of 10 mm of master cylinder travel.

MGB M/C: 10mm
MGB slave: 3.5

V8 M/C: 10mm
V8 slave: 4.9

MGB M/C: 10mm
V8 slave: 5.6

V8 M/C: 10mm
MGB slave: 3.1

All this says is for every centimeter of master cylinder travel; you will get the above number on millimeter travel by the slave cylinder. It doesnt take into account the leverage of the clutch fork to give the amount the pressure plate moves. The shorter the distance the slave cylinder moves in relation to the master, the easer the pedal pressure. Im sure the combination chosen by the factory was used to get the ideal clutch fork travel without requiring too much pedal travel or too much leg effort.

Before starting my conversion, I measured the clutch slave cylinder travel to be inch, or 12.7mm or 3.63 times the distance listed above for 10 mm of master cylinder travel. I never measured the master cylinder travel, but it calculates to 36.3mm or 3.63cm, which is 1.43 inches. That sounds like a long way for the master cylinder to move, so my calculations may be off. Still, this may help with the original question of the reason for the different parts. The final calculations for the different combinations using the master cylinder travel of 36.3mm (1.42) follow.

MGB M/C: 36.3mm
MGB slave: 12.7mm (.5)

V8 M/C: 36.3mm
V8 slave: 17.8mm (.7)

MGB M/C: 36.3mm
V8 slave: 20.3mm (.8)

V8 M/C: 36.3mm
MGB slave: 11.3mm (.4)
George Champion

Begs the question why the factory chose to have greater pedal effort on what is a heavier clutch anyway.

As well as any fork differences wear in the pedal/clevis/mc pivot can have a great effect, I was losing about 1/2" of travel at the pedal rubber on the roadster.

PaulH.
Paul Hunt

mgb roadstermay have accepted greater pedal effort rather than choosing greater pedal effort.

There are four places that factor in the final pedal effort.
1. The strength of the pressure plate springs.
2. The leverage of the clutch fork.
3. The volume ratio of the master to slave cylinders.
4. The leverage of the clutch pedal.

Design probably starts with the clutch pressure plate, heaver as you say (I recall reading the V8 used the same clutch parts as the MGC but I haven
George Champion

My V8 has MGB master and V8 slave - probably like most V8s, I imagine. The clutch pedal is very heavy - more so than even on my Landrover.
Does heavy pedal mean more work and more strain on the seals? I can live with the clutch as it is but I want a setup which is kindest to the parts involved so that replacement intervals are less frequent.
Marc

George - true, maybe the V8 combination of cylinders and fork results in 'heavier than the 4-cylinder but lighter than the V8 clutch in a 4-cylinder'. That would imply the V8 has shorter travel at the clutch plate than the 4-cylinder. Might that account for the V8 clutch dragging when not used for a while?

Marc - yes. The pressure felt at the pedal has a direct relationship to the fluid pressure.

PaulH.
Paul Hunt

On a lighter note, using a RV8 clutch and SD1 box, without changing any other parts, has made the clutch pedal lighter and should handle any extra torque.

Paul
Paul

Marc, the combination of parts you are using gives the most slave cylinder travel but the most effort. Swapping either or both cylinders would ease the pedal effort but may not fully release the clutch.

Paul Hunt, if your clutch drags on a factory V8 with all stock parts, I would suspect the parts are worn as you mentioned above putting it close to the limit. Perhaps a bit of air in the lines prevents complete disengagement and working it forces the bubbles out. Im grasping at straws!

I dont know how much the clutch forks move to press on the pressure plates on either the stock MGB or V8. It can be calculated out if you have the dimensions of the forks. My conversion uses American parts; Borg/Warner clutch and pressure plate, Buick style bell housing and Buick clutch fork. To fit in the tunnel the fork has been cut down to give no mechanical advantage and I warn anyone I let drive it that it requires both legs and an arm to press the clutch pedal. As both ends of the fork are the same length and I know I get full clutch release only if the slave move its end I know the release bearing travel is also . That doesnt answer the question when using British parts.

My after market pull style slave cylinder no longer contracts so now my clutch drags. Paul Kyles revelation that the Midget slave is the same as the V8 and the combination would provide the required travel I could regain proper clutch action with a little more engineering. Of course I would still have excessive pedal effort do to the shortened fork, so I should just use the McLeod hydraulic bearing recommended by Bruce.
George Champion

George - it's more complex than that. When the clutch is dragging I can ease off the pedal and it will not engage any further until it reaches the 'normal' biting-point i.e. the travel is constant. Roger Parker has said that the clutch material can absorb moisture when not used for a while, and indeed after a few trips the problem vanishes until it is parked up again for several days.

PaulH.
Paul Hunt

I installed one of the McLeod cylinders. At least I guess it is, bought it from Dan LaGrou at D&D. Anyway I've been using silicone fluid in the clutch and brakes for about 20 years, and the instructions said not to use silicone with the new cylinder. Without launching a new discussion about the pros/cons of various brake fluids, I decided to test the seals. They had sent an extra set, so after purchasing a bottle of good quality silicon fluid I measured the seals, using Swiss made dial calipers, and immersed them. One month later I measured them again and there was no measurable swelling and no noticeable softening or other deterioration. I'll continue to check them occasionally and if there is any degradation I will post it, but at this point I feel the silicone fluid will work quite well with it.

As for the geometries, I originally cut out the area under the gas pedal and welded in an inverted box to allow room for the clutch yoke, which I shortened as much as I felt was prudent. (D&D bellhousing) The MGB slave cyl. was entirely too big. So I tried various others before settling on one with something like a 11/16" bore as best I can recall. It's on the shelf so if I remember I'll check it this weekend. Mounting was complex, I had to shorten the gas pedal, and then there was this box under it. Not necessarily a bad thing since it made a good heel rest, but it took a little getting used to. Actually, my first MG took some getting used to so I guess that's a minor point.

I suppose the real point is that it isn't necessary to fret with all these concerns about geometry, clearance, engagement or lack thereof, and pedal pressure if you are willing to put the cyl in the bellhousing. Then there are three things to decide: exactly how far out to screw the adjusting ring (instructions provided), whether to use the 3/4 or 11/16" master, and where (if) to mount the bracket for the lines. Other than that, it reportedly works just fine, but then again this is with American hardware. With the Brits YMMV.
Jim Blackwood

Jim, after trying various slave cylinders you switched to the McLeod unit. Are you now satisfied with it? How does the pedal pressure feel? What was D&Ds price?
George Champion

Pricey. About $245 if I remember right. I haven't hooked the lines up, because I wanted to see what would happen with the silicone. I won't need to use the clutch for another couple of months yet. I'm using the 3/4" master so I expect to get good disengagement at the price of a slightly stiffer pedal. They recommend a pedal stop too. I had a pretty light clutch before, but didn't have anything extra on the stroke. I'll post my impression as soon as it is connected and bled. May do that this week sometime.

The MG-V8 newsletter had an article on this awhile back. Several people have done the swap and most seem to like it. The ones who didn't pretty much had leakage problems with some early units. If you want the back issue check with Dan Masters, he can point you in the right direction I expect.
Jim Blackwood

Paul
I am not sure if you had a typo but it seems you have the pedal pressure and bore size reversed. An 11/16 master cylinder bore will displace less fluid than a 3/4 in bore. The result less clutch travel and a LIGHTER pedal. Obviously the opposite effect follows for the slave cylinder. A larger diameter slave cylinder takes more fluid to produce the same movement as a smaller cylinder - hence a larger slave cylinder will produce more mechanical advantage ( a lighter pedal). Taken to the extreme of a small master cylinder and a large slave cylinder, you could end up with a light pedal but inadequate travel to disengage the clutch. I know, you already knew all this! fwiw
Barry
Barry Parkinson

This thread was discussed between 26/05/2001 and 04/06/2001

MG MGB GT V8 Factory Originals Technical index

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