strong tension springs 58 60 extend respectively between a pin 62 in the bore 54 and a pin 64 secured between the lugs 28 and 30 and between a pin 66 in the bore 56 and a pin 68 secured between the lugs 32 and 34 as can be seen from fig5 these springs 58 60 act to urge the neck ring arms 40 42 into their closed condition in a central bore 70 of the block 2 is mounted a piston 72 comprising a head 74 and a piston rod 76 the bore 70 is closed by a bushing 78 having a central bore 79 through which the rod 76 passes the rod 76 comprises a tapered end portion 80 which engages cam faces 82 84 on the supporting member 24 26 air under pressure is supplied to the bore 70 on both sides of the piston head 74 through bores not shown in the block 2 it can be seen that when compressed air is supplied to the left hand portion of the bore viewing fig4 and 5 the piston 72 is forced outwardly and the tapered end portion 80 bearing against the cam faces 82 and 84 forces the supporting members 24 26 and thus neck rings arms 40 42 apart from their closed condition into an open condition against the.