Hanaldo (13/09/2015)
How thick a piece of metal are you using? I always think of sheet metal as being like 0.5 to 3mm thick, which isn't really enough for fasteners... Depends what loads the fastener sees, but the rule of thumb is you need at least 4 turns of thread for the fastener to be doing anything.
I wouldn't be using anything less than 10mm.
The last one was a hard 4.5mm piece of alloy for an M4 thread. Seems to have held up OK for the last year although it's not supporting anything massive, just a small header tank.