Master Cylinder Markings

[davep]

This photo illustrates the three master cylinders that were used in the 914.
17mm 914/4
19mm 914/6
20mm 914/6 GT

[ChrisNPDrider]

Dave - what is the exact orientation here? I haven't been under my car for a while in front...would this number be on the top of the cylinder or bottom (and visible from underneath the car w/o a mirror).

Is that 20 pretty rare? I've never heard of it. Cheers

[davep]

The marking is on the bottom, however, since it is aft of the flange tht pusts it inside the pedal cluster. There is no way to see the marking without removal from the cluster.
The 20mm was used on the 911S and possibly on the 911E with the aluminum S calipers, so it is not exactly rare. However, I believe they were NLA'ed a long time ago. Since these only last a few years before requiring a rebuild or replacement most would be long gone now. I do have two, and one of those is quite different.

[ChrisNPDrider]

I can clearly see a "20" on the bottom of mine in the front, underneath with the steering rack cover plate removed. It "feels" like a 19mm....and works great!

[davep]

Unfortunately, unless the number is in the position shown in my photo, it is only a casting number, not a size designation. Far too many people have made this mistake. That is one reason why I posted this photo.

Did you know that all of these are standard, inch, size parts in 1/16" increments?

17 = 17.46 = .6875 = 11/16"
19 = 19.05 = .7500 = 12/16" or 3/4"
20 = 20.64 = .8125 = 13/16"

[Dave_Darling]

I had thought that the size number was on the upper surface of the MC when it was installed in the car?

--DD

[davep]

Nope, those are all on the bottom. The holes for the reservoir lines are on the other side as you can see by looking at the 20.