what is the size of the stock brake master cylinder on a 1971 240z? I am looking to buy a new bmc and have seen 15/16" and 7/8" bmc for 240Z's. does it matter which one i purchase? is one better than the other?

7/8's were used up until 78. The 15/16's were used on the ZX's starting in 79 IIRC. You can use either one on a later 240 but you will need to check your 71 and see if it is an early style or a later style. The early style had the reservoirs reversed, which means either using the original style or you'll have to change the plumbing of the lines underneath the master to correct.

15/16's are the master of choice when doing a 4 piston caliper or 4 wheel brake upgrade.

how do you tell if it's reversed? if i take a picture of my bmc and post it up will you be able to tell? if so i'll do that.

Easy to spot.... early masters have the large reservoir in the front and later ones have it on the back (closer to the firewall)....

If you have the early one, be prepared as they are almost twice the price of the later ones, that's why so many people change the lines around underneath..

so i looked at my bmc. i have the larger reservoir closer to the firewall on my 71 z. so this would be the later one right?

so originally my car came with the bmc with the larger reservoir farthest from the firewall?

what would happen if the previous owner had installed this bmc on my car and they didn't reverse the lines and just installed it backwards?

Well, it all depends on what your production date is. If you have a later production date in 71 then it is probably OK, the early cars or the series 1's we usually refer to are the only ones that have the reversed master cylinders.

The only reason I said to check is because during the later 71 model year they were changed. If you have a later one on there now, you're lucky as you will save a pile of money buying a new one.

If a PO had changed it without changing the lines around, you'd probably have all sorts of brake problems, and the smaller reservoir would be overflowing due to the volume of fluid that the front brakes use.