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Welding, Bonding & Fastener engineering FAQ

Fastener Selection

How many threads should protrude beyond the nut?
Posted: 17 Apr 07 (Edited 11 Dec 13)

People often question how many threads should, or are allowed to protrude beyond a nut. Inspectors especially seem to love counting threads.

While it is not a "standard", per se, the Naval Ships' Technical Manual, Chapter 075 Fasteners lists some good, common-sense rules for thread protrusion.

Quote (Naval Ships' Technical Manual)

075-7.5 LENGTH OF THREAD PROTRUSION

075-7.5.1 MINIMUM THREAD PROTRUSION.

A minimum thread protrusion length is given to ensure that
all the threads are engaged. This is required to ensure a full-strength fastener. Equipment component drawings
should specify the type, length, and size of bolt, stud, bolt-stud, etc. to be used to obtain the proper thread protrusion.
If not specified on the drawings, threaded fasteners of commercially stocked lengths should be used.
Threaded fasteners, when installed and tightened, should protrude a distance of at least one thread beyond the top
of the nut or plastic insert. Excessive protrusion should be avoided, particularly when necessary clearances,
accessibility, and safety are important. Thread protrusion is considered excessive if it could cause damage to
machinery or harm to personnel. Where practicable, the number of threads protruding should not exceed five. In
no case should thread protrusion exceed ten threads. In the case of a stud, excessive thread protrusion may indicate
that the stud has not been properly driven in the blind hole. In self-locking nuts where the distance from the
top of the nut to the locking element (plastic insert) is equal to or greater than the chamfer, the bolt or stud end
may be flush with the top of the nut. For existing or reused fasteners where the thread protrusion exceeds 10
threads, verification should be made that the proper length fastener was installed. For existing installations utilizing
standard nuts, acceptable minimum thread protrusion would be where the male thread, below any
unthreaded chamfer or crown, is flush with the top of the nut. Washers should not be added to reduce protrusion
except as specifically required by equipment component drawings or technical manuals.

075-7.5.2 MAXIMUM THREAD PROTRUSION.

There is no maximum thread protrusion from the standpoint
of the function of the fastener. Obviously, too much is a waste of material and weight. There is also a safety
problem from long, sharp, threaded rods sticking out from equipment, and protruding threads can get in the way
of other parts of the equipment. A good rule to follow is to always use the shortest standard length fastener that
gives a minimum one thread protrusion. Small and/or short bolts come in 1/8- or 1/4-inch increments. As the fastener
gets larger and longer the increments change to 1/2 inch. Up to a five-thread protrusion from a fastener is
certainly reasonable, you shouldn’t take a hacksaw to it for that; but if you have ten or more threads, you could
probably have done a better job of selecting the length. There is no requirement to reduce the length of protrusion
of existing fasteners before reinstalling them. There is also no reason you cannot cut off excess protrusion
if it is obviously too long or in the way. However, you cannot use washers solely to reduce thread protrusion.

If you must quote an actual standard, you may wish to look at DIN 78 "Protrusions of bolt ends".

CoryPad was kind enough to provide the following summary:

Quote (Summary of DIN 78 (thanks to CoryPad))

The content of DIN 78 is summarized by 3 equations and 4 images. The equations are:

total length = clamp length + protrusion

protrusion for standard parts = nut height + 2P (P is thread pitch, so "2P" effectively is 2 threads)

protrusion for lock nuts = nut height + 3P

The images are:

standard screw + standard nut

standard screw + lock nut

stud + standard nut

stud + lock nut

The images are dimensioned with total length, clamp length, and protrusion.

Or

SAE Aerospace Recommended practice ARP700, High Strength Elevated Temperature Bolting Practice.

Quote (ARP700)

For proper engagement of nut and bolt, the bolt should extend a minimum equal to 1.5 thread pitches including the thread chamfer, see ARP1515.

SAE ARP1515 is Threaded Fasteners Protrusion Through Self-Locking Nuts. Its scope is stated as "To establish a recommended practice for the minimum protrusion of external threaded fasteners through one piece all metal self-locking nuts".

If you are working with process piping to ASME B31.1 and are looking at a flanged joint then "negative 1" is acceptable.

Quote (ASME B31.1 Paragraph 335.2.3)

Bolts should extend completely through their nuts. Any which fail to do so are considered acceptably engaged if the lack of complete engagement is not more than one thread.

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