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Subject: Engagement of Studs in Tapped Holes


How to identify the length of bolt that is threaded inside a component like 3-1/8'' 5M studded tee?


API Spec 6A requires that Tap End Studs have a minimum of thread engagement equal to the nominal diameter of the stud. In your case, 3-1/8" 5M studded flanges use 1-1/8" diameter studs, these studs should have approximately 1-1/4" of measured threaded end that screws into the tapped hole. This 1-1/4" of measured length includes the chamfered end and is measured to the last scratch made by the runout of the thread on the body of the stud, plus 1/8", minus 0."  

The information above assumes the equipment is assembled with what are called Tap End Studs, which have full diameter bodies and threads cut on each end, as specified in the document linked below. Should the equipment have All Thread Studs installed, they have probably been tightened against the bottom of the tapped holes, and the depth of the tapped holes varies with the practice of the manufacturer. WOODCO USA strongly recommends against the use of All Thread Studs in tapped holes.   

To get complete information on the dimensions of studs used in API flange connections, see  

Should you need any studs, nuts, ring gaskets, or complete pieces of equipment, please Contact Us.

Other Q & A's related to Engagement of Studs in Tapped Holes include:


Tap End Studs, Cut Threads or Rolled Threads


Tap End Studs for DSA's


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