Fig. 9.
Fig. 10.
Other methods may be approved, in writing, if found by test and experience to have especial merit.
b. These stems must be screwed into the seats on a straight, tightly fitting thread, and the lower end then well headed over into a countersink. The valve guard and nut must be of composition.
In [Fig. 9] the upper part of the stem is slabbed off on two opposite sides and fits a corresponding hole in the guard.
The guard, therefore, cannot turn. The outside of the special nut is fitted on a taper to the inside of the guard, and the nut tapped out to fit the 5⁄8 U. S. thread on the stem.
The action of the valve, whether with the spring or without, tends to drive these taper fits together, producing a frictional lock similar to that of a friction clutch; and although the nut may be loose on the thread, it cannot possibly work off.
It will be apparent that the taper fit on the nut must be so made as to always bear on the taper fit in the guard, and not bottom in the guard.