Abstract: The present invention pertains to releasable locking mechanism for a hollow circular member such as a fill pipe for an oil tank and cup-shaped closure cap of non-magnetic material which is connectable to the hollow circular member, in combination with a locking member of ferrous material supported by said cap for axial movement vertically relative to the cap and hollow circular member but non-rotatable relative to the cap, one end of the locking member having a configuration that is engageable with the hollow circular member in non-rotatable relationship to prevent rotation of the cap with respect to the hollow circular member when the locking member is in the latter position, the locking member being adapted to be raised from said locking engagement with the hollow circular member by a magnet which may be mounted within a wrench for use with the cap to remove the same from the hollow circular member.