hole 202 b serves as a clearance hole for an adjusting screw 253 next an adjustment coupling 210 will be described with reference to fig1 to 20 as shown in fig1 the adjustment coupling 210 is composed of an adjusting screw 253 abutting the lower end portion 205 d of the core rod 205 a bolt member also referred to as cap bolt 251 and a washer spacer collar 252 for fastening the adjusting screw 253 and the lower end portion 205 d of the core rod 205 to each other and a lock nut 255 to be threadedly engaged with the outer peripheral surface of the adjusting screw 253 until it abuts the other end portion of the retainer collar 250 as shown in fig1 a the retainer collar 250 is formed as a cylinder the side surface of which exhibits two flat faces formed by cutting due to this two face cutting the retainer collar 250 makes no axial rotation with in the large diameter hole 202 c further the retainer collar 250 is equipped with a through hole in alignment with the center axis a threaded portion 250 a is formed in the inner peripheral surface of the through hole the adjusting screw 253 is formed as a hollow cylinder having a threaded portion 253 a on its outer peripheral surface further the adjusting screw 253 which is formed as a hollow cylinder has an inner hexagonal wrench hole 254 in the inner peripheral surface at one end thereof the adjusting screw 253 is threadedly passed through the through hole of the retainer collar 250.