R65 front master cylinder bore?

[Blackjack]

Anyone know what it is?

This is pure laziness, cause I could pull it apart an measure it, I know.

But, if you already know....

Also, I've not ridden the bike, I'd like a bit of feed back on just how "wooden" and 1970's the (ATE) brakes feel.

I'm slightly tempted to use a set of Kawasaki ZX *** R discs (assorted diameters, 5 stud fixing) and some disc adapters (R65's having ridiculously deep bells) along with some decent modern 4 pots and associated m/cylinder.

But nowhere near as tempted by the idea as I am intimidated by the cost....

Dual disc by the way.

[Bob_Roller]

There should be a two digit number cast on the bottom side of the master cylinder, if you have a dual disc, it's most likely a 15 mm master cylinder .

[darrylri]

But beware - a dealer installed the wrong replacement master cylinder on my 1981 dual disk bike.  That one should be 16mm.  One silly millimeter longer, but it makes a huge difference in how the lever works.

And yes, the ATE brakes are very wooden.  They work ok, but when you reach the braking point, the lever doesn't move any more, even though added pressure does increase the braking force.  So, there's no real feedback.

[Julio A.]

I have both the Brembo and ATE left side caliper, they look the same mechanically and dimensionally, is one better than the other?

The 16mm Master cylinder is the rectangular one right?

[Barry]

The 16mm Master cylinder is the rectangular one right?  

16mm is for a twin disc set up. With a single disc I imagine it would be truly awful.

Single disc should be 13mm 78 - 80 and 12mm 81 - on

[darrylri]

For dual disk, it's 16mm from 78-81, 15mm 82-on.  Same years for ATE vs. Brembo.  But the master cylinder is round 78-80 and rectangular 81-on.  So, 81 is a weird year, with the bigger bore, the rectangular MC and the ATE brakes.

[Blackjack]

Thanks chaps, that's what I needed to know.

I'm on the hunt for something a little cleaner looking for the cafe racer.

I was thinking that a 14mm was worth a try, based on previous experience, so I googled 14mm master cylinder, just in case I can't find one in the workshop.

Turns out that there's quite a few threads on Advrider about fitting smaller m/cylinders to BMWs.

Here's one of them, if you're interested,

http://www.advrider.com/forums/showthread.php?t=403427

[Barry]

That's a good thread from advrider. I think I would also consider 14mm on a twin disc set up but not smaller

We also had a discussion here on master cylinder sizes recently and there is some info that might help with your decision.

http://www.bmwr65.org/cgi-bin/yabb2/YaBB.pl?num=1290616014/3#3

[Blackjack]

14mm would seem to answer the case.

But if I should happen to find a 13mm, then I might chuck it on and see what it's like.

[zebedee]

The ADVRider thread was probably me asking much the same questions over there.

My 1984 single disk has a 12mm master cylinder. I believe the factory master cylinder for a twin disk setup, eg: R65LS is 15mm, but apparently other sizes in 1mm increments exist too.

A smaller master cylinder diameter will give you better leverage; eg: less force but more lever travel for the same braking outcome.

Obviously you do not want to run out of lever travel!

[tvrla]

I prefer the 13mm with the dual discs. And the ATE calipers work very well when everything is set up properly - fresh fluid and decent pads. And make sure the pistons aren't corroded or pitted.

Don't let anyone tell you the ATE brakes don't work well! They do! I used to think the single disc ATE systems were horrible until I rode one that was set up properly.

By the way, the Master Cylinder size is cast into the bottom (underside) of the outlet near where the brake line attaches.

[Julio A.]

Hey, My R65 does have the 16mm reservoir yet it only has a single disk.
 

What do you mean by awful? reduction of braking power?

[Barry]

Hey, My R65 does have the 16mm reservoir yet it only has a single disk.

What do you mean by awful? reduction of braking power?
 

Julio it's all about the ratio of master cylinder area to caliper piston area which amplifies the force you apply at the lever.
The difference in area between a 12mm and 16mm master cylinder is huge the 16mm being almost double. Therefore with a 16mm master cylinder on a single disc set up you have much less amplification and you are going to have to pull literally twice as  hard on the lever to get decent braking. The lever is also going to be really solid and  devoid of feel.

I think you need to change to a 12mm.

[Bob_Roller]

Or get a second brake disc .

[Blackjack]

Julio,

That sounds like you're saying you have the rectangular reservoir?

I don't know, but I'd imagine that BMW made single disc versions with the rectangular reservoir, but they would have had a 13mm cylinder bore.

Not trying to be pendantic, just that if you're messing with brakes it's best to be crystal clear about what's being said...

On a different note, while personal preference comes in to it, riding ability probably dictates how useful it all is. Watch the back wheel..

http://www.break.com/index/biker-falls-over-highway-mountain-railing-1973929

Scared of the front one you think?