Abstract: A boss for mounting on a shaft comprising a discontinuous ring member, with a link bridging the discontinuity, a pivotable lever, and an eccentric drive connection between the lever and the link, whereby pivoting of the lever produces relative movement between the link and at least one side of the discontinuity to alter the size thereof to facilitate clamping and unclamping of the boss on the shaft. In one embodiment a second eccentric drive connection is provided between the said other end of the link and the ring member. The second eccentric drive connection is also adjustable to allow for presetting the tension in the link. In another embodiment the effective length of the link is adjustable to allow for presetting the tension in the link. An overcenter mechanism retains the lever in its locking position.