L5, L5A, L6
These are gap-bed, geared headstock machines - and by far the most common model of the series. The L5 has a deceptive appearance for, although its weight and bulk are nearly as great as a Colchester student (and it is a very strong machine), its centre height is only 4.5". Both standard and long bed versions of all models were made, offering either 24" or 40" between centres. The bed was fitted with a removable gap which, even on the 4.5" centre-height model, allowed work up to 4.25" thick and 17" in diameter to be turned; as standard the beds were not hardened however, this process was offered as an extra but it is rare to find one so equipped.

Headstock
The geared headstock, always fitted with a clutch and combined friction brake (which makes operation by a single-phase motor much easier and more reliable) had a range of eight spindle speeds. The bore through the spindle of the L5 was a little tight at 0.75", but when the larger L5A lathe was introduced its spindle, with a 1.250" hole, was offered an an option on the smaller machine – but I yet to find one so fitted. Besides the two sizes of screwed spindle the American L00 (L zero zero) taper nose was offered as an option and always featured the larger-bore spindle. The spindle-speed range most commonly found on second-hand machines is 31 to 720 rpm, but various other options were available over the years including:

    21 to 480 rpm
    42 to 960 rpm (16 speeds with 2-speed motor)
    22 to 500 rpm (1 hp motor)
    34 to 750 rpm (1.5 hp motor)
    45 to 1000 rpm
    34 to 1500 rpm (16 speeds with 2-speed 1.5/3 hp motor)
    A few lathes were also made with a "high-speed" headstock, giving 45 to 2000 rpm from a 2-speed motor. As far as can be ascertained, there was no difference in the headstock gears and bearings of lathes running the higher spindle speeds.

Screw-thread Spindle fittings
The small bore spindle was threaded 1.5" x 6 tpi and the large bore spindle 2.25" x 6 tpi

Conversion to Single Phase Electrics
The electric motor is mounted externally, low down on the back of the stand, and fitted to a pair of vertically slotted, adjustable motor rails. This makes motor replacement a relatively simple job, the rails being bolted to a wide, flat surface and easily relocated. I know of several older Harrison lathes where the owners have fitted more powerful motors, together with a larger drive pulley to increase the top speed to a more useful 1500 rpm or so; no problems have been encountered - and lathes appear to run perfectly. Some machines were fitted with motor rails that could be adjusted horizontally as well as vertically in slots and a change of motor on these could hardly be easier.

Selecting the Spindle Speeds
Because the control levers are interlocked, to prevent the engagement of two gears at once (and work in a rather unusual way), it is not always clear to a beginner how the spindle speed should be changed; here's how they work:

    each of the two selector on top of the headstock can be placed in one of three positions
    to select a new speed, either one of the two levers must first be placed in its central, or "neutral", position
    the other lever can now be turned to either its full left, or full right position
    by juggling these positions (one lever in neutral, one engaged) new speeds are selected.

However, don’t expect the speed-selector diagram to help you determine which speed you have selected - it’s masterpiece of convoluted lines and what appear to be randomly placed numbers. Before either lever can be moved it is often necessary to ease the chuck round a little to permit the gears to engage. A third much larger lever, fitted with a spring-loaded safety lock, and located on the headstock front cover, is used to select the high and low speed ranges.
Harrison headstocks are not noted for quiet running, although some machines do seem better in this respect than others. Providing there are no deep rumblings, or obvious vibrations, noise from the headstock has to be considered as just an annoying inconvenience.Non of the speed-change levers on the headstock should be moved until the spindle has stopped turning.

Screwcutting
The L5 was sold with either with a full screwcutting gearbox, in both metric and Imperial versions, or a three-speed "translation box" which, unlike the equivalent Colchester, offered the luxury of three (instead of two) sliding and surfacing speeds for each setting of the changewheels. The leadscrew was only used for screwcutting, and engaged by simple, hand-engaged, sliding dog-clutch at the gearbox end. The power shaft below the leadscrew was provided with a spring loaded, safety over-ride mechanism to prevent damage in the case of a dig-in or other mechanical mayhem. The doubled-walled apron on all model of the L5 employs a push/pull button to select either power sliding or surfacing. The power feed engagement lever, below the centre of the apron, is spring loaded and can be snapped into and out of engagement; a similar design of apron was used across the Harrison range.
The changewheels are made from steel, not cast iron, and as a result can set up quite a ringing noise. Careful adjustment of backlash between mating teeth, and the use of a heavier, "open-gear" lubricant will help to minimise this problem.

HARRISON 11"
Lathes badged "Harrison 11" are just machines produced during the last years of L5A/L6 production; they have a 5.5" centre height but, instead of a screwed thread, were supplied as standard with the safer and more rigid American L00 (L zero zero) fitting.