How to determine master cylinder size?

[madjurgen]

The supposed 25mm MC I received conveniently has the area where the '25' should reside filed off. The holes for the brake lines both measure 10mm so I guess that rules out that its from an e30M3. The thing that bothers me is that in every single picture I can find of a 25mm MC, it has one hump on the underside. This one has two.

I measured the opening of the metal housing. Since its a one-piece cast metal housing I figure the cylinder itself cant be bigger than the opening in order to actually assemble the whole thing. It measures 25.00 exactly. Gustave's site lists the exact number as 25.4 mm. This was done with a digital caliper so dont tell me 'its close enough'.

http://e30m3performance.com/installs/installs-1/25mc/25mc.htm

So what does the 25.4 actually refer to? Can it be measured non-destructively?

I bought this from a person who ordered it from a parts dealer under the correct part number. Im certain that the seller did not do the filing as he has already offered me my money back. I am seriously starting to consider that this parts dealer is selling fake 25mm MCs. Their pictures mysteriously have the hump(s) photoshopped out...

[Massive Lee]

Master cylinders are measured in SAE sizes. The commonly called "25mm" MC is in fact a one inch MC. One inch equals 25.4mm and it is the size of the piston, not the outer diameter. So, to make sure it is what it is, you must measure the inner diameter by removing the clip and the sliding pistons.