Master Cylinder - Engineers??

[davejw]

Hello,

I am looking for someone who can line-bore my BMW master cylinder and fabricate a new piston. I have new Brembo calipers and am looking for more pad travel.

Thanks,

Dave.

[Trooper2]

I'm not an engineer by a long shot but that type of brake modification just doesn't sound safe to me.

[davejw]

It's quite simple and the work required will balance the amount of fluid pushed through the sytem with the size of the pistons in the calipers. The original diameter is 1 inch and I'd like to increase that between 5 and 10%

Dave.

[steve_d]

Is this something you have come up with yourself or do you know it can be done.

The first concern is that your machining will cause a thin wall section somewhere. There are often a variety of cast or drilled passages that your lager bore could break into or worse not quite break so you don't know of the danger.

These drillings I speak of are there to provide supply and return paths for the fluid and they may joint the bore at an angle. When you overbore, the drilling will join the bore in a slightly different position, this change could significantly change the way the brakes operate.

You will need larger seals which will be difficult to source.

You have no idea what clearances you should design for the fit between bore and piston. You also do not know how deep to make the groove the seal sits in. Get any of this wrong and the seal will turn inside out with total brake failure.

I think I'm trying to say DO NOT DO THIS your life and the lives of others are at stake.

Steve

[rlk500]

I would have thought it far cheaper and safer to source another m/c from an alternative bmw. For instance it's quite common to upgrade the m/c on the e30 m3 for a larger version from the 750.(I did this on mine, when I upgraded to AP's). The fittings are the same so it's a simple job.

[MR2Mike]

I think people are being overly dramatic here. As long as the wall thickness of the existsing master cylinder is adequate, and facilities can be found to machine it out to the desired size and with a suitable surface finish then I don't see any problems. A case, as always, of using a few brain cells.

[correlejco]

As a professional automotive engineering designer, specialising in hydraulic systems, I would concur: DO NOT DO THIS.

We are not being melodramatic. I did in fact do something similar many years ago on a Lockheed master cylinder when I was young, enthusiastic and foolish. The feed expansion port in the cylinder wall was just behind the piston cup on the original cylinder; when I bored it out (from 3/4" to 7/8", everything looked perfect. I fitted Lockheed 7/8" piston and seals. What I didn't realise that the port was slightly oblique so the seal lip went over the rim of the new hole by about 40 thou. Worked perfectly for the first 50 or so brake applications, Then the seal lip tore off agianst the sharp edge and the pedal went to the floor. No dual braking on this system = severe brown trouser moment. I could have killed myself or someone else and it would have been 100% my fault. I'm sure there are many other similar pitfalls. Vowed never to modify braking systems without thorough engineering study first.

If the car is used on the road, insurance will be invalidated by such a mod for sure. And I don't want you behind me!

[lanan]

I agree completely. If you really feel you need to achieve a harder pedal which will reduce the brake pad clamping pressure for the same brake pedal pressure.ie, You will have to push the pedal haredr to get the same retardartion. Get a new cylinder of the size you want.

[Avocet]

Agree with pretty much everyone else - master cylinders are pretty dicey things to modify - especially if you don't know exactly where the ports are and where the galleries go. You might get at least some of your 10% back by switching to "aeroquip" type braided flexible hoses. The rubber ones expand a bit when you brake and these don't. That said, I really would be very cautious about playing with the brakes at all. Does the car have ABS? If so, I know you can upset the phasing of the ABS pulses by altering the volume of the system, the lengths of the pipe runs and / or the elasticity of the components. Certainly if it were my car, I wouldn't do this!

[GreenV8S]

Sounds like the original poster has made substantial changes to the calipers. The issue of master/slave piston travel is an important one, but to start with I'd be far more concerned about getting the static and dynamic brake balance right.