Under tank Master cylinder size

[jagarra]

Today I dug out an under tank master cylinder for a dual disk application. Decided to take it apart and put a kit in it. Had to use the hydraulics from a porta power unit to push the frozen piston out from the unit. When I measured the bore, it is 15.59 mm in diameter. I am guessing that makes it a 16mm cylinder master. When I checked the parts fiche, they only showed a 14mm, 16mm and a 17mm. So is what a I have considered a 16mm or something different??

[melville]

I think they call that a 16mm. But I'm wondering if they're built to an inch standard. My '14mm' master for a single disc measured 14.28mm or nearly precisely 9/16".

[jagarra]

Melville,
I know it is a bit confusing as I would expect to see a 16mm measurement somewhere in the assembly. But every measurement is below that figure. I ordered a 16mm rebuild kit and a small brake hone. When you get to that size hone the options are very limited. Had a lot of crud and rust in bore, soaked it in carburetor cleaner and then in Evaporust. Looks pretty good now.
I have had much of the parts for a dual disc conversion for sometime. Maybe this winter I'll make the conversion.

[Rob]

When I rebuilt my master cylinder, I honed it with a dowel rod slotted on one end for a piece of fine wet 'n dry, and chucked it in the hand drill.

I don't remember any problems. I didn't have to buy a new cylinder afterward!

[Ken in Oklahoma]

When I rebuilt my master cylinder, I honed it with a dowel rod slotted on one end for a piece of fine wet 'n dry, and chucked it in the hand drill.

I don't remember any problems. I didn't have to buy a new cylinder afterward!

I've done the same thing with good results. The difficulty that can arise is when there is a etched out dimple near the rear seal of the master cylinder. That dimple needs to be sanded out, and the only way I could think of to do it was to bear down on the honing in that area--and hope that the back seal of the master cylinder is flexible enough to cope with the slightly lumpy bore. The results I've had have been mixed. It seemed like it would 'fix' the master cylinder only to have the leak reappear. That get's old fast.

It strikes me that in the airhead world there must be somebody who can rebore the master cylinder and then insert a stainless steel or similar 'rust resistant' sleeve in it. Stainless steel would be 'resistant' to the water that gets into the braking system all too often. (Actually, one is too often).

Unfortunately both the new master cylinder casting and the kits are atrociously expensive.

Ken

[jagarra]

For a bit there I thought I may have to have the MC sleeved. I cleaned the bore with a gun cleaning jag, then dropped a bore light down into the unit. when you have light coming from the bottom outward, you really can see the imperfections. I saw a good size rust area in the bottom surface of the cylinder, but with the light I could see it was just past the finished portion of the bore where the seals ride. So I guess I will give it a try and see what happens.

This outfit will sleeve your MC, http://www.whitepost.com/

[Airbear]

Gerard, there is some debate about the best size of MC for a twin disc setup. ChasBMW (he posts here and on ADV) swears by a 13mm on his 90S, but I'm not sure if he is using the under-tank MC or a bar mounted version. There has been a lot of discussion on ADV > Old's Cool > Airheads along the way. Maybe worth a search.

I don't know if you want to stay stock (under-tank) or not, but I switched to a bar-mounted MC with great results on my single disc.

viewtopic.php?f=1&t=5461&hilit=Magura

The conversion works really well with the 1974 model and is less expensive than the kit for the original. I am still loving the difference in braking performance and feel.

That said, you may already be on the right path. If the MC bore is too big I guess you can get it sleeved down.

[Rob]

I don't know if you want to stay stock (under-tank) or not, but I switched to a bar-mounted MC with great results on my single disc.

viewtopic.php?f=1&t=5461&hilit=Magura

The conversion works really well with the 1974 model and is less expensive than the kit for the original. I am still loving the difference in braking performance and feel.

I don't think you are alone in this club, Charlie!

[jagarra]

Charlie, I have thought about the handle bar mounted MC, in fact I have the Magura one in my wish list on Amazon. I am concerned there isn't enough room on the handle bar to mount the unit. Since I have already ordered the kit I am committed to try the under tank style one more time.
According to the ratio chart for single piston activated calipers, such as the ATE, the recommended ratio is 12.1--14.1. with a 16mm master and 40mm calipers I fall into the 12.5 area. Looks like the bottom of the range, but it may work. I have a 14mm under the tank now, but it only is a single output, I also have a couple of 38mm calipers that are in my pile of BMW parts. I already ordered a new set of braided lines from Spiegler, so more stuff on the way for the under tank style.

[George Ryals]

Buy some fittings and make up a splitter for the 14 mm mc. Use your 40 mm calipers. You will have a little more lever travel but much better hydraulic leverage. Braking will be really good.