A flange bolt is one of two components that is used to hold the flanges together.  They are subjected to a predefined torque which is designed to stretch the bolt out a little bit.  The reason for this is that there needs to be a certain amount of force to hold the flanges together when the internal pressure is high.  There  is a very specific procedure that is used when torquing the bolts. 

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• A stud is threaded either on both ends or the full length of the stud.
• Studs both have American National Course Threads.
• Length of stud bolts do not include the height of the point.
• Length of stud bolts when used with lap joint flanges will depend upon the thickness of the lap joint stub end.
• ANSI 150 Lb. and 300 Lb. weld neck flange has a 1/16 inch raised face.
• ANSI 400 Lb. to 2500 Lb. weld neck flange has a 1/4 inch raised face.