The bolts installed in tapped holes in API studded flange faces should always have double end, or tap end stud, configuration. The studs should have a short thread on one end, with only enough threads to assure a thread engagement with the tapped hole equal to one diameter of the stud. This short thread should have an end of thread run-out on the stud body of 45° to 60° to facilitate a jam between the end of the thread and the top of the tapped hole.

Note: When installing tap end studs into tapped holes, always apply generous lubrication to the tap end (short) thread of the stud, do not apply too much lubricant in the tapped hole, as lubricant trapped below the stud in the tapped hole may prevent full thread engagement. 1

Jamming the stud in the hole, in the manner described, may cause visible distortion to the first thread at the top of the hole. This jamming will tend to form a seal at the top of the tapped hole, between the stud and the equipment body, preventing the escape of lubricant used in the hole and help to prevent the entrance of water. This jamming, at the top of the hole, will provide a gripping phenomenon to retain the stud in the hole when removing nuts during field use.

Caution: Don’t overdo it when jamming the stud in the tapped hole, moderate jamming will work OK. Do not attempt to use all thread studs in tapped holes, and do not engage the long thread end of tap end studs into the tapped holes. Jamming the stud threaded ends into the bottom of the holes may cause galling at the bottom of the holes, and removal of the stud, if possible, may also remove the threads from the tapped hole. Avoid products like Loc-Tite. 1

Tap end studs properly installed, jammed enough to avoid un-intended back-out from the tapped hole, may prove somewhat difficult to remove from the tapped holes. The act of jamming the stud at the time of installation may cause distortion in the top thread of the tapped hole, this distortion will vary with the extent of jamming force used to secure the stud. Severe jamming may cause first thread galling in the tapped hole. If galling has occurred in the first thread of the hole, the galled portion of the tapped hole may seriously damage the threads on the stud during removal, making reuse of the stud difficult or impossible. In such cases removal of tap end studs previously jammed in tapped holes may require careful re-tapping of the holes by hand so as to accept replacement studs.

1Words in bold italic indicate they have subjective meaning and persons using this information must use experience to improve the reliability of their judgement when the meaning of these words can have impact on performance.

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