I would be very surprised that all the connections are not metric on the MX5 :confused:

I just double checked in the data bases I have for brake lines and in-board brake pipes : everything is always metric M10x1.00 all around on all Mazda (except in the 70's, some models use M10x1.25, but it is not your concern :p )

The 1/8 thingy you saw can be either 1/8x28 BSP (parallel thread) or 1/8x27 NPT, (tapered thread). In both case, "1/8" has nothing to do with the diameter of the thread in inches, it is just an appellation (1/8 BSP is 9.73mm out. diam. on the male part, and 1/8 NPT is 10.29mm)

But again, I would be extremely surprised that one of these 2 threads has been used on the MX5!

You can start by carefully checking the diameters of the ports of your M/C. Here are some useful data's:
M10x100: male OD = 10mm, female ID = 9mm, 10 threads on 10mm
1/8 BSP: male OD : 9.73mm, female ID = 8.56mm, 28 threads per inch
1/8 NPT (tapered) male OD (middle of the threaded part)=10.29mm, female ID=8,41mm, 27 threads per inch.

If you find a port which is not M10x1.00 on the MX5 brake circuit, I would be very interested to know where, and what thread it is please :)

Thanks, and good luck ;)

Sometimes jap pipe ends do not have the small unthreaded portion at the end of the fitting so the normal 10x1 mm will not seat properly and some have OP 2 concave on the pipe just to confuse the issue a little more