btw. the pin dimenstions are:

______________
I.............................I
-------.........--------
..........I.......I
..........I.......I
..........\___/

diameter top: 16mm (0,63 inch)
total height: 9,10mm (0,358 inch)
height of flat top: 2,20mm (0,086 inch)
diameter of the lower section: 7,97mm (0,314 inch)

An M8 bolt has a 13mm head (e=14,38mm, see link below), so even if it not 100% matching the easiest way would be to shorten an M8 bolt (I`d suggest DIN920 as these have a thread only at the end) and machine down the head.
Interesting in this case would be how long the non threaded section is, but I guess at the end it doesn`t make any difference also if you use a finethreaded bolt with thread up to the head (DIN961).

http://www.online-schrauben.de/cgi-bin/ ... 931-2-8-35
(on the right, click "M8x1" and then on "technische Info")

I don`t think that the 3mm less top diameter have any negative effect. But there`re also different types of bigger headed M8 bolts like i.e. this type:

http://www.online-schrauben.de/cgi-bin/ ... 921-2-6-16
(on the bottom left "DIN6921")

there`re for sure even better matches that could be used.
.... this just as an idea

G.

PS: my original piston is not magnetic and according to the colour it`s out of brass, this just as a sidenote