Master Cylinder, what does it mean?

[hoveringuy]

Can someone 'splain this to me...

I removed the MC from my car during my upgrade to an X3 booster/MC. The X3 one is labeled "25 22" front-rear which sounds good.

I'm looking at what I pulled which I thought was a 25mm one, but it's actually labeled "31 20". Hmmmm..... No P/N on it.

I disassembled it and measured the bore as 20.6mm.

So, what does the 31 mean? Do master cylinders even have different bores? Is bias done through some other internal trickery?

BTW, my 320i booster and the master cylinder weigh 9.5 lbs, the X3 one seems featherly light in comparison and is a tad over 6lbs.

[fitswell]

BUMP

urious about this as well. I thought about swapping over the seals from another MC to refurb my aluminum Girling.

[moatilliatta]

Assumed they use the same casting for many range of pistons. Which sucks trying to figure whats what later on...

[Northern]

I wish I knew about the master question, but rear bias is also done with the prop valve below the master on the firewall

[Victell]

Why have two different master cylinder bores AND a rear bias valve? I dont understand why BMW did/does that.