Studs in imperial dimensions

To ensure that bolted connections in soft materials such as aluminum or heat-stressed materials such as cylinder heads always hold tight, and that the material is not stressed by frequent loosening, people in the design department like to use studs: a bolt whose one end remains permanently bolted into the base material and on whose other end sits the nut with which the bolted components are ultimately fastened.

Like all the other bolts on vintag, classic and even fairly young Harley motorcycles, the stud bolts on the engine, carburetor, transmission, etc. have their own special dimensions, which are based on inch sizes. Both ends have a thread, sometimes the same, often with a different pitch. The coarse thread is usually the one that remains bolted into the material, for example in the gearbox or engine housing. The nut that holds the cover or exhaust runs on the fine thread.

Fitting bolts are special stud bolts

With engine housings, it is important that both halves align exactly to each other. The cylinder bores and the lugs of the engine mount must not be offset from each other. Normal stud bolts always have a certain amount of play in a bore without a thread so that the parts do not have to be fitted with a hammer and the parts can still be aligned with each other if necessary. To align components precisely, modern designers use hardened dowel pins that sit in exactly opposite holes. The effort involved in drilling these holes is quite high and the parts cannot be easily detached from each other.

Since the days when the very first Harleys were still young, there has been another tried and tested trick: two of the holes for the stud bolts that hold the housing together are drilled with particular precision. During assembly, stud bolts are then inserted into these holes whose shank (the part without the thread) corresponds exactly to the diameter of the holes. This works best if the bolt is slightly larger than the threads at the end, as otherwise the threads of the stud bolts could damage the hole when they are pushed through. Such dowel pins can be fitted with small hammer blows. Nothing will happen, because the clever boltdriver will of course turn a suitable nut three or four turns onto the end of the bolt so that it never hits the bolt itself.