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MG MGB Technical - Clutch pipe size

Whys is the clutch pipe from the master cylinder 1/4" copper and not 3/16" like the brake pipe?
Graham Gilmore

I got all new copper pipes, mine is 3/16!
Pat Gregory

Did you buy the pipe ready made Pat or was it your choice to have 3/16"?
Graham Gilmore

They are definitely 3/16. It is something to do with the amount of fluid that needs to flow because of the slave cylinder bore.
We always used -4 pipe on the clutch systems on race and rally cars, instead of -3 for that reason.
Colin Parkinson

The pipe came in an Automecs kit, Full brake, Clutch & fuel pipes all in copper.
Pat Gregory

You now have me confused Colin. My understanding was that-3 is 3/16" and -4 is 1/4". Why does a race/rally car need a different amount of fluid flow to operate the clutch? If you want to change the force required at the clutch pedal then you would change the master cylinder bore size.
Graham Gilmore

Quick thoughts.

Wouldn't the full relationship be between master, slave and pipe?

Do different models of MGB have different size master, slave and pipes?

Have you checked out Paul Hunt's excellent mgb-stuff website for more info?

Have you got access to a copy of the book - Original MGB with MGC and MGB GT V8: The Restorer's Guide to All Roadster and GT Models 1962-80

Nigel Atkins

4-cylinder MGBs have different master and slave bore sizes to V8s, same as 1500 Midgets and the earlier models.

The V8 slave is completely different to the 4 cylinder slave, so maybe that dictated the slave bore, and it was compensated for with a different master bore. Also the V8 clutch is 'heavier' for obvious reasons, so maybe it also compensates for that to some degree with slightly less release-bearing travel. But then if you look at the relative bore sizes I have come across, the V8 appears to give MORE slave travel then the 4-cylinder, which would make it heavier.

None of which is anything to do with the clutch having (originally!) a larger bore metal pipe. What may be a factor is that when using the clutch the whole volume of the master cylinder is displaced each time the pedal is pressed. Whereas with the brakes - when correctly adjusted, there is relatively little fluid displacement, hence less fluid movement, so a smaller bore can be used.

I believe it is to do with the amount of fluid that needs to be moved in ashore space of time.
If the diameter is too small, there may be a delay in the clutch re-engaging when you take your foot off the pedal.
Dave O'Neill 2

Sorry guys, I got it wrong!! It's an age thing. The clutch pipe is 1/4 in.

Graham, I honestly cannot remember the reasons, but I was taught to do it many years ago, and always use 1/4 on the clutch, whatever the car (race and rally).

Colin Parkinson

The clutch hydraulics operate at a relatively lower pressure than the brake system. The clutch needs volume displacement for the slave cylinder to travel 3/8 to 1/2 inch as a result of depressing the clutch pedal say 3 1/2 inch. If the clutch pipe was too narrow bore you wouldn't get the volume of fluid through. That's my theory!
C Harvey

Plenty (most) cars run the smaller pipe but it seems older cars run it larger but then a lot of older cars ran large brake pipes as well---I'm thinking it's just a sign of the times thing where it has been found you get a much solider brake pedal feel and easier to bleed with smaller tubing and it has gradually gone that way for both clutch and brake out of convenience
One thing I had thought with older clutches, is that if the line was small and the pedal released quickly, maybe with the possible lower quality seal of the m/c cup there could have been the possibility of air being sucked past the cup if the fluid couldn't return fast enough----needing a larger pipe to assist flow

But as far as an MGB goes, just changing the pipe size will do squat because if I remember correctly the port holes in both cylinders are only about 1/8" and would control the flowrate anyway
William Revit

You can't get air sucked past the pressure seal - in an MGB brake or clutch master at least, as there is fluid behind it anyway. There is a secondary seal that retains that fluid.

The way they work it has to be like that, as when the piston is pushed into the cylinder the bypass hole that lets fluid into the system, ends up behind the pressure seal, and fluid would run in there if it wasn't there already, from a second passage that is behind the pressure seal when the piston is released, as in the attached. This a brake, but the clutch is the same.


In the case of the brake master there is a one-way restrictor valve (see attached) in the outlet which does restrict how fast the fluid can return to the master, but not how fast fluid can flow out to the brakes.

When applying the brakes fluid pressure pushes the semi-circular spring out of the way to open a large port and allow free flow. When the pedal is released the spring closes the large passages leaving just the small groove, arrowed, which restricts how fast fluid can return.

This does create a negative pressure in the cylinder, and because of that fluid is sucked from behind the pressure seal. With the pedal released as fluid flows back through the restrictor the excess is pushed up into the reservoir through the small bypass port.

It's this valve that allows you to 'pump up' the brakes if you have air in the system to be able to develop full pressure if the pedal hits the floor on the first operation.


Good point(s) Paul--I agree it would be impossible to suck air on this type of cylinder---now I can't see why there would be a need for a 1/4" pipe at all when other cars operate quite well with 3/16"
William Revit

Ah well, custom and practice - "It's the way we've always done it", which was part of the downfall of British mass manufacturing.

This thread was discussed between 17/09/2018 and 19/09/2018

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