is coupled to the exterior of the wellhead for providing an inward force to the lock pin a retainer is mounted to the wellhead by threads while the lock pin is held in the inner position under hydraulic pressure the retainer is rotated inward against a shoulder provided on the lock pin to hold the lock pin in the inner engaged position the hydraulic pressure can then be removed application_number US 4810587 A description referring to fig1 a part of a wellhead 11 is shown the portion shown is commonly called a tubing head but will be referred to herein as a wellhead wellhead 11 is located at the top of a well and supports within it one or more casing hangers not shown and a tubing hanger 13 the casing hanger is supported within a part of the wellhead 11 called a casing head which is located below the tubing head shown the tubing hanger 13 is supported in the bore of the wellhead 11 in an area known as the tubing bowl a string of tubing not shown will be suspended by the tubing hanger 13 a plurality of radial passages 15 only one shown extend through the wellhead 11 the radial passages 15 are evenly spaced around the circumference of the wellhead 11 a lock pin 17 is carried in each radial passage 15 the lock pin 17.