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Is this a 17mm or 19mm master cylinder?
Thanks for the help!!

I’m not sure you can tell by that code which is probably a casting number. Someone will correct me if I’m wrong.

The bore diameter 17 or 19 is on the casting but unfortunately that part of the casting is on the other side of the sheet metal, inside the pedal box area and you can’t see it without removing the master cylinder.

Even then if the casting is really rusty they can be hard to read.

I recently had on that was super crusty and hard to read. Wasn’t until I disassembled and measured that I knew for sure.

Pretty sure all of the sixes came originally with 19mm

So educated guess is that it would be unlikely to have been downsized by a previous owner

FWIW I have a 19mm ATE taken apart right now and it has “13” in that same location - furthers my suspicion it is a mold number

And I have a 17mm on my bench right now that has "22" in that location. No idea what that means.

Can you get a flex angle mirror to look at the top? That you you might be able to see size.

Both of my cars are 4 banger conversions.