thread although not clearly shown in fig1 thread 100 has a decreasing axial height as shown in fig3 a metal cap 104 with a rounded end is slipped into the hole so that its rounded end protrudes past the opening of the hole at the lower end 102 of the thread the hole not shown includes an internal recess that engages an end flange 106 of the cap to retain the cap inside the hole and prevent it from being pushed out of the other end when installed as further described below a resilient member such as a plug 108 made of an elastomeric material such as plastic or rubber or a compression spring not shown has a shaft portion 110 that extends into the interior of cap 104 rubber plug 108 includes an enlarged head which engages the surface of the bore into which shank 10 extends to keep it compressed thereby urging the rounded end of the metal cap past the lower end 102 of the thread into a stop hole not shown in fig1 for releasably locking the bolt in place for turning the bolt head 108 is preferably provided with a connection such as a square protrusion 112 or hole shown in fig1 for turning the bolt with a wrench or the like disposed between the upper end of shank 10 and the lower end of enlarged head portion 92 is a resilient member.