Why studs have a coarse thread at the 'metal end'
When we look at bespoke engine studs, which are generally designed in such a way that engineering is given far more thought than cost, many of them will have a larger diameter and coarser thread at the 'metal end' - the end installed in the casting. Studs for mass production very often carry the same thread at both ends, or have a continuous thread over the whole length of the fastener.
Have you ever wondered why this is so? It is quite often the case that studs are installed into a casting made of a low-density material such as aluminium or magnesium, and it is the relatively low strength of the metal into which the stud is inserted that gives rise to the requirement for a large, coarse thread. If we have a strong stud, and a weak casting, it is easy to imagine the mode of failure if too great a pre-load is applied: the female thread in the casting will 'strip' and be pulled from the casting. This is a common failure, and results from shearing of the casting at the major diameter of the stud.
If we consider the total area in shear, it will be proportional to the major diameter of the stud multiplied by the length of engagement. It also depends on other factors such as the tolerances that have been applied to the female thread, but in all cases the basics should hold true - the shear area increases in proportion to diameter and engaged length.
The rule of thumb concerning stud shear load being proportional to stud major diameter (for a given thread pitch) holds true. However, practical experience will show that the load to cause failure by thread shearing is almost independent of engaged thread length beyond a given length, and this length is surprisingly short.
The reason for this can be found in a large number of engineering textbooks, especially those concerned with threaded fasteners. The distribution of load along the engaged length of a stud (or nut) is very far from being even, especially where the modulus of the casting and stud are equal.
For more than 100 years, engineers have been aware of the fact that the engaged threads closest to the applied tensile load take the vast proportion of the force. Work published in Russia in the early 20th century showed that for a steel nut and bolt with ten engaged threads, 34% of the applied load is taken by the first thread, and 85% on the first four threads.
The load distribution changes little for longer threads, and is not much affected by thread pitch. The shear area can therefore be said to be proportional to pitch rather than engaged length. As the first thread distorts under load, so more load is transferred to the second and subsequent threads, but it is quite common to fail the threads closest to the applied load before any real damage has occurred to the lower threads.
The uneven distribution of load in the engaged thread can be influenced by changing the combination of materials, and using various design features. However, the rule that the load to cause failure increases with the strength of the casting material, the major diameter of the stud thread and the pitch of the stud, is worth committing to memory.
Fig. 1 - This part is typical of many high-quality bespoke studs, having a large-diameter coarse thread on the installed end (Courtesy of Blanc Aero)
Written by Wayne Ward