I purchased some SS socket grub screws described as M1.6 DIN916 which were to replace some slotted head screws in some photographic equipment

The OD of these new screws measures 1.5mm presumably because the tops of threads are radiused. I measured the old screws at 1.6mm so I now realise they must be M1.7

In these small sizes how does one correctly identify the thread size?

On this bit of equipment (its a lens) the grubscrews hold a knob on to a shaft but weirdly the shaft is exactly 3.270mm diameter. It is a precision item manufactured to metric dimensions and the shaft to bore fit is very close. It seems odd not to use 3mm or 3.5mm (or even 1/8&quot as a non standard diameter would need special reamers etc.

Ian P

Kwil, It was the Maryland site I was using but looking again after you linked to it I realised I had missed the column that shows min and max thread OD.

John, I like your 'be wary of bought screws' advice. However,,,, I dont fancy screwcutting 1.6mm threads in stainless, let alone forming the 0.7mm AF hex recess!!!

Ian P