An apparatus for releasably holding a tubular toolholder shank, comprising: a tool support member having a forward facing surface and a bore intersecting the forward facing surface and adapted to receive the tubular toolholder shank therein, the bore extending rearward from the forward facing surface along a longitudinal axis; a canister disposed within the bore of the tool support member, the canister including a longitudinal passageway and at least two circumferentially spaced apertures formed therein; a lock rod at least partially disposed within the longitudinal passageway of the canister and movable within the tool support member in a rearward and forward reciprocating motion for pulling the tubular toolholder shank rearward within the bore into a locked position and for releasing the tubular toolholder shank from the bore to an unlocked position; and at least two locking elements disposed within the spaced apertures of the canister and engaging the lock rod so that movement of the lock rod causes the locking elements to extend radially outward from the canister to engage the tubular toolholder shank, wherein the lock rod includes a front portion having a hollow interior with an inside surface defining a relief cavity extending rearward from a forward end of the front portion of the lock rod, the lock rod has a maximum outer diameter (D max) defined at the forward end and the relief cavity has a maximum diameter (d max) defined at the forward end, characterised in that a ratio of the maximum diameter (d max) of the relief cavity to the maximum outer diameter (D max) of the front portion of the lock rod is greater than 1 to 4, and wherein the inside surface of the hollow interior of the front portion of the lock rod further defines a coolant channel formed rearward of the relief cavity. a tool support member having a forward facing surface and a bore intersecting the forward facing surface and adapted to receive the tubular toolholder shank therein, the bore extending rearward from the forward facing surface along a longitudinal axis; a canister disposed within the bore of the tool support member, the canister including a longitudinal passageway and at least two circumferentially spaced apertures formed therein; a lock rod at least partially disposed within the longitudinal passageway of the canister and movable within the tool support member in a rearward and forward reciprocating motion for pulling the tubular toolholder shank rearward within the bore into a locked position and for releasing the tubular toolholder shank from the bore to an unlocked position; and at least two locking elements disposed within the spaced apertures of the canister and engaging the lock rod so that movement of the lock rod causes the locking elements to extend radially outward from the canister to engage the tubular toolholder shank, wherein the lock rod includes a front portion having a hollow interior with an inside surface defining a relief cavity extending rearward from a forward end of the front portion of the lock rod, the lock rod has a maximum outer diameter (D max) defined at the forward end and the relief cavity has a maximum diameter (d max) defined at the forward end, characterised in that a ratio of the maximum diameter (d max) of the relief cavity to the maximum outer diameter (D max) of the front portion of the lock rod is greater than 1 to 4, and wherein the inside surface of the hollow interior of the front portion of the lock rod further defines a coolant channel formed rearward of the relief cavity.