More Master Cylinder

[decorn33]

I am still VERY slowly accumulating all of the parts needed to convert from single leading disc to double leading on my '84. I have my disc, nicely refurbished by Truedisk LLC, I have my right side Brembo caliper and pads, I have my splitter, I have the miscellaneous brackets and brake line tubes I need. My last investment was to be new brake lines, then I realized that my master cylinder may not be the right size. I currently have a 12mm. Do the LS model and the European models that came with dual disc typically run a larger size?

[SoloLobo]

My LS has a 14mm, but I would go smaller for better leverage.

Try the 12 first and change if you don't like the feel

[thrang]

Just had a look on my LS and its a 14mil.

[decorn33]

Thanks everyone, I will put it together with the 12 and see what it feels like.

[Tony Smith]

Best of luck, I'd be happier if you were using a 13mm which takes like a dick to water to twin ATEs or "small" Brembos. My own experience is that 12mm leaves you with just a little too much lever travel and that the lever binds on your fingers before full brake pressure is reached.
 
You can gain some additional "space" by going to the early 1979-80 brake lever, a mod that requires a few minutes work with rat tailed file inside the brake lever perch - but given the cost of the blasted lever and the cost of a M/C kit I would buy a 13mm kit and have a machinist sleeve the m/c to suit.

[Barry]

To put some numbers on it by calculating the piston area, if you wanted to retain the same lever feel and travel of the single disk set up then the master cylinder would need to go up to 17mm.

But most if not all would want to sacrifice some lever travel for a bit more power so somewhere between 12mm and 17mm.  14mm is just under half way between the two.

If you are looking for 2nd hand master cylinders the earlier single disk R65's had a 13mm cylinder and as you can imagine it has a very firm lever which presumably is why it was changed to 12mm in 81.



  

[montmil]

...which takes like a dick to water...

The mental image is disturbing. [smiley=drowning.gif]

[Tony Smith]

FFS! I meant DUCK to water.  

[Barry]

We know you did !

They are next to each other on the keyboard

[decorn33]

Just got back from a long jaunt out into the wilds of Pennsylvania around the Susquehanna river - still running my single disk. Lots of corn, and lots of cows. Based on your suggestions, I think I will look around for a used 14mm. Just can't spring for a new one from Moto Bins these days.  Thanks for all of your input.

[Tony Smith]

We know you did !

They are next to each other on the keyboard

I find as I get older I am missing keys with with depressing frequency. I learned to touch type 0ver 40 years ago so the key positions are well ingrained with "muscle memory" or more correctly proprioception. That this is now failing me is a clear sign of approaching age related decrepitude and I am not happy about it!

[montmil]

We know you did !

They are next to each other on the keyboard

I find as I get older I am missing keys with with depressing frequency. I learned to touch type 0ver 40 years ago so the key positions are well ingrained with "muscle memory" or more correctly proprioception. That this is now failing me is a clear sign of approaching age related decrepitude and I am not happy about it!

OK, so we're straying off-topic a bit so I'll add something:

Master Cylinder        -there.

Now then... There are a few local BMW riders in my area. None of us are what anyone would ever call Spring Chickens. We're more like Geezers on Beemers, aka: Sons of Arthritis MC.