Interference fit studs are used where the stud needs to be restrained in its tapped hole against loosening in service or when the nut is removed. The tap-end thread is a special coarse thread called Class 5 or NC5 (National Coarse thread, class 5).
A coarse thread nut will not screw onto a Class 5 thread. But will thread onto the other end as shown in the photograph above.
The Class 5 threadform has extra space at the thread root to allow for plastic flow of material into this space. This eliminates seizing, galling and abnormally high and erratic driving torques.
Interference fit threads are not intended for use where regular removal for component maintenance is required.
Driving torque charts are used to determine if there is the proper amount of interference (typically 50% greater than the torque required to break loose a properly tightened nut.
Shouldering, which is the practice of driving the stud until the thread runout engages with the top threads of the hole , causes upward bulging of the material at the top of the hole. One should not tighten the stud into the shoulder.
The picture to the left is an example of shouldering on a Lycoming aircraft cylinder exhaust port.
"...the assembled external thread shall not bottom nor shall the unfinished threads engage" ASME/ANSI B1.12-1987 page 12.
The minimum torques are intended to be sufficient to insure that externally threaded members will not loosen in service; the maximum torques establish a ceiling below which seizing, galling or torsional failure of the externally threaded component is reduced.
Exhaust port on Continental aircraft cylinder (shown left) showing countersink. Compare this with the FAA/PMA "equilivant" cylinder that is not countersunk (shown below).
Attention to details is important on troublesome exhaust ports. It appears to me that the FAA/PMA cylinder is made cheaply and does not meet the quality standards of the Continental cylinder.
"holes should be countersunk" is a direct quote from ASME/ANSI BL.12-1987 standard for interference fit threads.
Another example of a countersunk. Holes should be countersunk to a diameter greater than the major diameter in order to facilitate starting of the externally threaded product and to prevent raising a lip around the hole after driving due to insufficient cavity space at major diameter.
It may be necessary to use studs providing more interference by using oversize studs. For example, Aircraft Continental and Lycoming exhaust port studs are available in plus .003, .007, .012 oversize
Bottoming, which is engagement of the stud threads with the imperfect threads at the bottom of a tapped hole, causes the stud to develop mechanical or hydrostatic pressures. Deliberate bottoming to "lock the stud in place" adds to the tensile stress and weakens the stud.
The minimum length of engagement is 1.25 of stud diameter for copper and ferrous alloys and 2.5 of stud diameter for nonferrous materials.
One should not tightening the stud against the bottom of the hole to make it tight is it adds stress to the stud.
The following recommendations are from the Navy Ship's Technical Manual
For non-high temperature applications (less than 300 degrees F.), Class 5 interference fit studs may be replaced by studs installed using anerobic-locking compounds. (editor comment: Locktite 222 or 271)*
Remove existing studs. Retap holes, While Class 3 threads are preferred, Class 2 threads are acceptable.
Clean the hole thoroughly by repeated flushing and scrubbing with a general purpose liquid detergent and a soft wire brush. Blow out all liquid, and dry with oil-free compressed air or a clean cloth.
If studs are in good condition, use thread due to clean up the studs to a Class 3 (preferred) or Class 2 (acceptable) dimensions. If studs are damaged, obtain new studs. Scrub the studs with the same detergent used to clean the tapped holes and dry with oil free compressed air.
Apply primer (activator) in accordance with manufacturer's instructions.
Apply anaerobic locking compound to both male and female threads. Apply enough compound so that the gap between the male and female threads will be completely filled.
Install the studs. No lubricant is to be used as the anaerobic compound will act as a lubricant.
After curing is completed, ensure the studs are properly bonded by applying an inspection torque.
This chart is not intended to cover the use of externally threaded products made of stainless steel, silicone bronze, brass, or similar materials.
0.2500-20 12 3
0.3125-18 19 6
0.3750-16 35 10
0.4375-14 45 15
0.5000-13 75 20
0.5625-13 75 20
0.6250-11 120 37
0.7500-10 190 60
0.8750-9 250 90
1.0000-8 400 125
1.1250-7 470 155
Nominal Size Max lb-ft Min lb-ft
Torque at 1.25D engagement.
A 5/16-18 stud (0.3125-18) should take between 19 and 6 foot pounds of driving torque when engaged .320 inch.
Unless otherwise specified, studs that must have their set end locked may be locked with an anaerobic threadlocking compound instead of a class 5 fit, provided that a class 2 to 3 thread fit is achieved and the requirements for cleaning, priming, temperature, and curing are met.
The purpose of locking the set end of a stud is usually to enable the self-locking nut on the opposite end to be to be installed without rotation of the stud in its tapped hole. To this end, use MIL-S-46163 Type II, (Loctite 222) Grade N, as its breakaway torque is greater than that of a reusable self-locking nut. Where greater locking torque strength or permanent locking is required, use NSN 8030-01-171-7628 (Loctite 272).
Notice the reduced shank diameter on these studs. This is called a "wasted" stud. Reduced shanks are used to even-out the stress and to provide more stud stretch. Stretch is necessary as it provides the spring-back force that keeps the parts firmly clamped together.
Picture to the left shows what happens when you don't countersink the stud hole. Red arrow points to metal cracking. Will get worse when tension is applied to the stud.
