Drummond headstock castings and their bearings went though several changes in the first years of production: on the earliest model (shown above) the headstock was decidedly flimsy, with cast-in bracing beneath the bearings and the latter each pinched by a bolt that passed transversely beneath - the same arrangement can be seen in the pictures of a rare circa 1900 4-inch Drummond shown here. This design, almost certainly copied from a design used on some earlier Pittler B Pattern lathes, did not last long and by 1903/4 the headstock casting had been strengthened and the bearings changed to a type with a taper on their outside face that incorporated a screwed ring and serrated nut by which means they could be pulled into the headstock casting - and so closed down to set the running clearance on the 3/4" x 8 t.p.i. spindle.

1903/4 to circa 1905/6 Headstock - an expensive-to-manufacture arrangement whereby the bearings were drawn into the headstock casting by large castellated nuts screwed onto the ends of the bearings themselves - a method that Drummond were to return to in 1921. 

1905/6 to 1912 headstock  - a much cheaper, cruder design with the bearing drawn into the headstock by two screws passing through the casting and pulling on a large washer. Note the wide, rounded pulley on the top speed position - the only variant so far discovered to have such a feature and as found on some smaller models of Pittler.
This design of headstock was notorious for allowing chatter to develop.

1912 to 1921 The same design of bearing assembly as above, but with the headstock braced by an overhead bar in an attempt to reduce "chatter". Details of this lathe and its headstock can be seen here.

In 1921, on the introduction of the new M-Type, a complete (though ultimately unsuccessful) alteration was made to the headstock bearings. Increased in size, they reverted to a form of adjustment that dated back to an earlier design where a large ring, screwed onto the outer end of the bearing, pulled it into the tapered headstock housing and provided a means of setting the clearance. Although the arrangement appeared, externally, to be the same as used on the next and all subsequent versions, it was different in a number of important details and had certain drawbacks. While the type that superseded it could (very cleverly) be adjusted for clearance (by the screwed ring) and then locked solidly into the headstock by means of a tapered key being forced into a tapered slot cut into the top of the bearings by the screw-in oiler tube, this model had a parallel key made in lead. Tightening the large ring drew the bearing into the housing (in the usual way) and so closed down the slot until it met the key, compressing it slightly as tightening proceeded. However, if the key was too narrow it allowed the bearing to be over-tightened to the extent that it locked solidly onto the spindle - but if too wide failed to give the right clearance and led to a sloppy fit. Perhaps keys were provided of slightly varying width - or, more likely, the original could have been compressed in a vice to change its dimensions either way until a compromise was reached However, even if the setting achieved was perfect and the spindle rotated without play, the bearing was merely held into its housing and not expanded out tightly against the surround walls. The result was a tendency for cuts to chatter and only a change to the very clever "expander" design, four years later (and described below), finally solved the problem. 

The much stronger M Type headstock as used from 1924 until the late 1940s with improved draw-in spindle bearings. The design included an expander wedge that, pushed by the hollow oiler bolt, was forced into a tapered slot cut into the top of the bearing. As a result the bearing was forced outwards to make firm contact with its housing so improving the rigidity of the whole structure (though the adjustment of the setting is critical). In addition a quick-release bull wheel mechanism was fitted together with a 1" x 12 t.p.i. nose thread backed by a 11/8" register. In the picture above the two cast-iron covers normally fitted over the backgears have been removed for clarity.

1921 to late 1940s M Type headstock with the an extra changewheel stud mounted in the front clevis - an arrangement that allowed left-hand threads to be generated.

The end of the spindle carried a simple screw-on sleeve to set the end float (the locking screw had a small copper washer beneath it to protect the thread) and another push-on sleeve, retained by a grub screw engaging against a small flat on the shaft, that carried a stud and hole to mate with the first gear in the changewheel train.