The ends of the housing, as well as the ends of the Thermohousing, are machined so that other components (and sealing subs) can be attached to them. Housings for sub-surface field use, require specialized thread form, finish, and tolerances. These have evolved through years of practical applications. If the threads are designed and machined without the benefit of this specialized knowledge, the probability of a thread 'melt down' (or terminal galling) increases very rapidly. Specially with the use of steels containing large percentages of Nickel and Chrome.