Hence, I went back and bought the 17mm hole saw. I would have bought a M12 1.25 pitch bolt with it, but the hardware-shop had only 1.5 pitch. But as I had to "loose" most of the thread anyway (to fit it thru the hole saw, the short section of pitch 1.5 did the trick as well. I then stiffed a bit of aluminium pipe (a cut off from an old TV antenna) over the bolt - it fitted perfectly and the outer diameter of the alu-pipe was 12.5mm - perfect. Note all the aluminium bits on the ground! Make sure