On 11/13/2026, Dave Shattock with MGA Twincam YM1/901 wrote:
"I needed to rebuild my master, and as soon as I got it off it was obvious it was the type you show as non standard, so I thought you might be interested in its internals.

Its history is that I needed a new cylinder in 1970 or so, and asked my local sports car garage to change it. Where they got it from I've no idea.

Apart from the external difference you highlight, the cylinder itself is about 111mm long and at the far end where the port from the reservoir is, the bore is reduced over a length of about 20mm but I've no way of measuring the reduced diameter.

The valve closing this port during braking is different, I guess because of this reduced bore while, I guess because of the reduced length of the cylinder, The return spring is only 76mm as opposed to I think 88 in the original cylinders and 100 in the revised ones.

You show the early type piston with just a ring type seal with the later type adding a cup seal. This piston as you can see has the cup type only.

Hope this is of interest, and now I have to figure out how to use my full rebuild kit to service it!