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Overboring master cylinders
https://www.ausmini.com/forums/viewtopic.php?f=5&t=67469		 Page 1 of 1
Author:  kirby [ Sat Dec 04, 2010 6:03 pm ]
Post subject:  Overboring master cylinders
Would it be sensible to overbore master cylinders in the hope of getting a softer clutch and "better" brakes.

I'm talking tin can type, I understand the later type where larger anyway? (if so how much larger?)
As some of you may know I have upgraded to front discs but don't intend to run a booster.

Would this be a worthwhile en-devour?
Author:  drmini in aust [ Sat Dec 04, 2010 6:37 pm ]
Post subject: 
If you increase the bore of the brake MC you will get a heavier pedal, ditto for the clutch one too.
This is because you are moving more fluid at a given pedal stroke.

For example,
When the 998 Cooper came out, to reduce pedal effort they went down to a 0.625" diameter MC, and got rid of the hydraulic intensifier the 997 Cooper had.
Author:  mini7boy [ Mon Dec 06, 2010 2:31 pm ]
Post subject: 
The good doctor is correct about the effects from increasing the diameter of the master cylinders, and their piston areas, but incorrect about the reason behind it.

Because hydraulic pressure is measured in psi(pounds per sq. inch), the area of the pistons involved, both in the master cylinders on one end, and those in the caliper, wheel cylinder and slave cylinder on the other end, determine just how much effort is required to move fluid or have the fluid move something on the receiving end.

So, a piston of larger area in the master cylinder, with no other changes. will require more effort than a piston with smaller area.

On the receiving end (caliper, wheel cylinder and slave cylinder), a piston of larger area will cause more force to be applied to the pads, shoes or clutch throwout/release bearing for the same amount of foot pressure on the corresponding pedal.

I recognize that this is a rather fine distinction, but it is really the pistons' relative areas which determine pedal pressure required or the force applied at the other end. Of course, the amount of fluid moved is related to the area of a piston FOR A GIVEN STROKE. Of course, this is only the hydraulic side of things and does not take into account the mechanical advantage for the relevant pedal's length.
Author:  drmini in aust [ Mon Dec 06, 2010 6:27 pm ]
Post subject: 
7boy, yes I was fully aware of this and you are correct, I just didn't want to write a long treatise to explain it.
I tried to do it in simple terms...... :lol:
Author:  Kennomini [ Mon Dec 06, 2010 7:28 pm ]
Post subject: 
So a small bore long stroke MC with a big bore short stroke slave or caliper will be the best combination, correct :?:
Author:  drmini in aust [ Mon Dec 06, 2010 8:31 pm ]
Post subject: 
Kennomini wrote:
So a small bore long stroke MC with a big bore short stroke slave or caliper will be the best combination, correct :?:

Yes for brakes, but the pedal will go down further.
As for the clutch MC I had a long pushrod in a 0.7" one, it worked but felt weird having to lift my foot so high.
IMO a 0.75" is best MC for clutches, they still work even with a bit of the usual slop in the linkages.
Author:  mini7boy [ Mon Dec 06, 2010 9:33 pm ]
Post subject: 
drmini in aust wrote:
7boy, yes I was fully aware of this and you are correct, I just didn't want to write a long treatise to explain it.
I tried to do it in simple terms...... :lol:


Doc - I only took the trouble, and novel, to explain it because so many people seem to get it wrong. Many people get it exactly backwards when all they have to do is remember to think in terms of piston area and psi.
Author:  9YaTaH [ Tue Dec 07, 2010 7:29 pm ]
Post subject:  Re: Overboring master cylinders
kirby wrote:
Would it be sensible to overbore master cylinders in the hope of getting a softer clutch and "better" brakes.

Would this be a worthwhile en-devour?


Twud be more sensible to resleeve to standard with a Stainless Steel sleeve and making sure the rest of the braking system is in tip-top condition :idea:
Author:  1962CKD [ Sun Feb 06, 2011 6:55 am ]
Post subject:  PSI or Bar
I thought oil pressure was measured in BAR not PSI, well ya learn something every day! :roll:
Author:  mini13 [ Sun Feb 06, 2011 7:49 am ]
Post subject: 
bar/ psi depends where oyu are in the world, a bit like smanner sizes.
Author:  Morris 1100 [ Sun Feb 06, 2011 10:35 am ]
Post subject: 
I think you would find that 90% of the people on this forum would use psi or kpa for oil pressure.
Show me a 2" Smiths dash oil pressure gauge in bar. :lol:
Author:  MiniBob [ Wed Feb 09, 2011 12:50 pm ]
Post subject:  Re: Overboring master cylinders
9YaTaH wrote:
kirby wrote:
Would it be sensible to overbore master cylinders in the hope of getting a softer clutch and "better" brakes.

Would this be a worthwhile en-devour?


Twud be more sensible to resleeve to standard with a Stainless Steel sleeve and making sure the rest of the braking system is in tip-top condition :idea:


Hi KIrby, Clutch & Snake in Geelong do a stainless m/c re-sleeving service.
Had mine done recently, should last longer than the original m/c lining which is subject to pitting etc as we all know..
Author:  kirby [ Wed Feb 09, 2011 6:27 pm ]
Post subject: 
Any idea on a rough price?
Author:  MiniBob [ Thu Feb 10, 2011 6:14 am ]
Post subject: 
I'll have to dig the receipt out. I'm sure it was cheaper than a new one.
Best thing is to give 'em a call, ask for Mel and say you are a local Mini owner.
They have sponsored our Western tour with a prize before and should be very helpful.
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