Hydraulic clutch tube size?

I'm plumbing from the clutch master cylinder to the slave cylinder on my Miura project. I started down the path of using 3/16" SS tube and -3 AN fittings. I then noticed the slave cylinder has -4 AN fittings on it. I googled it and found mixed answers on use of 3/16" versus 1/4" tube for clutch system. It seems logical to me for a mid-engine car with the tube running from footbox to rear of car that a 3/16" with less fluid to compress would work better. But alternatively, maybe a 1/4" tube would let the clutch dis-engage and engage faster because it can move fluid faster.

The master cylinder is 3/4" and it's a McLeod internal HTOB so I don't know the size of it.

So, is there a "best practice" on hydraulic clutch tube size for mid-engine cars? Or will either size tube work?
 

Ian Anderson

Lifetime Supporter
I had the 3/16 and the Renault slave is something like 26mm so a lot of fluid required.A 1 inch master?
for street use I found it ok, fast changes I am sure at times it did not release as fast as I wanted.

if I was doing it again I would run the 1/4 inch

Ian
 
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Definitivly dash 4 tubes and fittings
Stroke of the slave cylinder is linked essentially to diameter of circuit tubes and diameter of master cylinder
No need on a clutch circuit to have hight pressure like on brake circuit ; it's more aquestion of volume and spedd
Usually movment of clutch is largely compensed with fork ratio or hydraulic ratio ( when a right master cylinder is choosen !!)
 
"But alternatively, maybe a 1/4" tube would let the clutch dis-engage and engage faster because it can move fluid faster. "

Exactly. A small and long tube will restrict flow. The problem will show itself when the clutch pedal is released and the clutch plates slip because of the restriction.
 
On the surface, hyd clutch may seem like brake circuits, but they're is a bit of difference.

Brakes do move some fluid, but they operate much more on pressure.
Clutches operate on volume. The pressure is fairly low and much closer to constant. Where this difference will be most noticed is in the time for the clutch to engage as it doesn't have the pressure, and has a lot more volume to move to release a clutch as opposed to releasing brakes.

So, yes, I would go with the bigger 1/4 tube.

edit: to a large degree, you could learn to anticipate the clutch release by the fraction of a second we're talking about and compensate this in your driving, but but you'll not achieve as quick an engagement and risk unneeded clutch slippage. I think it better to go bigger.
 
I’ve had -3 clutch line on the cobra for years and had to live with a heavy clutc. Just changed to a -4 and it’s reduced the effort so worth doing.

as others have said, the clutch is all about moving fluid rather than pressure.
-4 all the time.
 
Thanks for the information/feedback!! I've now ordered up a roll of 1/4" SS tube, fittings, and hose. So much for the good old days of not having to spend a bunch of cash for DIY car building. It cost about $85 for just the clutch line parts. If I'd bought this stuff a couple years ago, I bet the bill wouldn't have topped $50. Oh well, it's a good thing I didn't wait longer as the prices never come down once gone up.
 

Neil

Supporter
Thanks for the information/feedback!! I've now ordered up a roll of 1/4" SS tube, fittings, and hose. So much for the good old days of not having to spend a bunch of cash for DIY car building. It cost about $85 for just the clutch line parts. If I'd bought this stuff a couple years ago, I bet the bill wouldn't have topped $50. Oh well, it's a good thing I didn't wait longer as the prices never come down once gone up.
I rebuilt my 250 GTE years ago. Every time I ordered parts I found that the prices had increased substantially. Their explanation was "It's the exchange rate". I believed that until the exchange rate went down and I still heard the same excuse. :(
Auto parts prices always seem to ratchet up.
 

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Taking this discussion a step further, the need for the 1/4" or -4 size is really needed as the clutch is released, particularly with diaphragm style clutch's where the diaphragm is flattened or even goes over center in the release position at higher RPM or if RPM is kept high during a fast shift, centrifugal force plus the high RPM will hold it in that flattened position as the clutch is released and if small 3/16 lines are used the restriction can be enough to prevent the clutch from engaging until RPM is reduced. Also check the master & slave cyls have holes at the point where the lines attach that are at least the same size as the bore of a 14'' line so they dont become a restriction themselves. My first Boss 302 race car with a twin plate rivetted liner plate's shredded itself for just that reason, Bell housing was chokka with lining material, looked like a couple of cats had a major bunfight inside.
 
Taking this discussion a step further, the need for the 1/4" or -4 size is really needed as the clutch is released, particularly with diaphragm style clutch's where the diaphragm is flattened or even goes over center in the release position at higher RPM or if RPM is kept high during a fast shift, centrifugal force plus the high RPM will hold it in that flattened position as the clutch is released and if small 3/16 lines are used the restriction can be enough to prevent the clutch from engaging until RPM is reduced. Also check the master & slave cyls have holes at the point where the lines attach that are at least the same size as the bore of a 14'' line so they dont become a restriction themselves. My first Boss 302 race car with a twin plate rivetted liner plate's shredded itself for just that reason, Bell housing was chokka with lining material, looked like a couple of cats had a major bunfight inside.
Im going to assume nobody is trying to procure some 14 inch dia pipe for their clutch line for their car, very important RH slash missing from the text
.
 
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