12 American Clock Keys 2 5 Pronged Winding Sz 2,4,6,8,10. Two Size 4 Clock Keys. This is a new set of 2 size 4 clock keys. You receive: One size 4 key with a 1.75 mm hole and a 3.25 mm hole. One size 4 key with a 1.95 mm hole and a 3.25 mm hole 5 Sided Clock Key. This is a new 5 sided clock key. It is used to wind clock mainsprings. The sizes are: Size 2, 2.75 mm. Size 4, 3.25 mm. Size 6, 3.75 mm. Size 8, 4.25 mm. Size 10, 4.75 mm 5 Sided Watch Key. This is a new 5 sided watch key. It is used to wind watch mainsprings. The sizes are: Size 2, 1.06 mm. Size 4, 1.15 mm. Size 6, 1.20 mm. Size 8, 1.67 mm. Size 10, 1.75 mm.