Although boasting only a little more capacity than the 11-inch L5A Model (that was based on the small 9-inch L5) the 12"/13" Harrison was over 22% heavier and came with a full screwcutting gearbox as standard. Although marked as a 12" in the UK, export versions appear to have been labelled as 13" machines, though if these had an increased centre height is not known. This L6 was to be built in Mk. 1, a very rare Mk. 2 and Mk. 3 forms - with, oddly, the Mk. 2 being the most radically changed with a lever-and-dial-operated gearbox and an apron with a face-mounted power-feed selector borrowed from the larger lathes. Whilst the Mk. 3 was a further step towards the modern M-Series lathes it reverted to an older design of carriage, used a traditional "tumbler" gearbox and was fitted with a "third-rod" control system for the spindle start, stop and reverse.
On the Mk. 1 a 2 hp motor provided, via a twin V-belt input drive, 45 to 1000 rpm as standard or, at extra cost (with a two-speed 3/1.5 hp motor) a choice of either 34 to 1500 rpm or (more rarely encountered) 45 to 2000 rpm. The spindle was bored to pass a 1.375 inch diameter bar and ran on the usual Harrison arrangement of opposed Timken pre-loaded taper roller bearings at the front and a single-row ball race at the rear. Fitted with an American type L00 nose as standard, the option was available (should the customer have been so foolish as to order it) for an ordinary screwed nose to be supplied. Drive was through a combined clutch and brake unit - of doubtful long-term reliability and effectiveness it has to be said - a specification that, when new, provided a useful improvement in comparison with the contemporary Colchester Student and Master whose optional (and very expensive) Matrix clutch had no brake and so allowed a fast-rotating job to rotate endlessly when disengaged. The main double gear that slid along the spindle was cut from a single forging and ran on seven splines - the complete assembly being dynamically balanced. All headstock gears were hobbed from the solid, shaved, induction hardened to 450/500 Brinell and honed to produce the correct tooth form; to improve oil-tightness the joint face of the headstock cover was formed on one level, rather than the leak-prone cut-away step used on the L5 models.
As standard the 7.75" (197 mm) wide and 8" (203 mm) deep bed was supplied hardened and without a gap, though the latter specification was offered as an option on both the 20-inch and 40-inch between-centres models.
Open at the front, and without an oil bath, the screwcutting gearbox had to be hand-lubricated by the operator at frequent intervals. It offered a choice of 36 threads and feeds from 4 to 60 t.p.i whilst a set of extra-cost conversion gears (28t, 36t, 48t and 127t) enabled the box to generate 15 metric pitches from 0.5 mm to 7 mm pitch. As an option the box could be fitted with a built-in metric-conversion cluster that enabled 11 metric pitches to be generated by the movement of a single lever. The power feeds ranged from 0.0021" to 0.0327" (0.05 mm to 0.82 mm) sliding and from 0.0012" to 0.0187" (0.03 mm to 0.47 mm) surfacing. The leadscrew was only used for screwcutting and engaged by a simple, hand-operated, sliding dog-clutch at the gearbox end of the shaft; 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 - unlike the smaller lathes, it appears that a similar device to protect the leadscrew was not available. Although not mentioned in the contemporary sales literature, lathes with the built-in metric conversion set, and those with a top speed of 1500 rpm or more, had their changewheels enclosed inside an oil-bath case to promote quiet running.
Mounting a top slide that could be turned through 360 degrees, the cross slide was fitted with a proper taper gib strip - instead of the flat strip with push-screw adjustment often found on smaller industrial lathes machines of this age - and the zeroing micrometer dials, whilst not big enough, were nonetheless of adequate dimensions for younger eyes. A "clog-heel" toolpost was fitted as standard - but both 4-way toolpost, and later a quickset toolholder, were on the options' list and could be easily fitted to the T slotted top slide; the 4-way toolpost had a clamping bar with a rather-too-small white plastic ball on its end - and it is surprising just how many of these palm-hurting devices have survived unmodified when, for a few pence extra, something more comfortable could have been fitted. Unfortunately the cross slide was not, as on the later M Type lathes, of the full-length type; instead a short slide was used that, over time, tended to wear just the middle section of its ways on the saddle. The short slide made it easy to retrofit a taper-turning attachment, but this should have been supplied complete with a short slide to replace a longer one. Thin sheet-metal inserts (that are frequently damaged on used machines) were set above the cross-feed screw at the front and rear to protect it from the damaging effects of swarf; later models retained the tin sheet at the front but were given a more elegant and robust cast-aluminium cover at the rear.
With a push/pull button to select either power sliding or surfacing feed, the doubled-walled apron employed the usual (and very effective) trade-mark Harrison snap-in-and-out engagement lever that hung below the bottom edge. Spring loaded, the lever could be moved instantly and easily from engage to disengage - even under the heaviest of cuts. A thread-dial indicator was fitted as part of the standard equipment.
Fitted with a No. 3 Morse taper, the tailstock barrel had self-eject and 4-inches of travel. The barrel clamp, being a split-bar type, was effective but the bed-clamping arrangement required a good heave on the lever to work properly.
Fabricated from 3/16" thick steel plate, the stand was provided with a single locking storage cupboard (when there was ample room for two) and deep chip tray; the makers recommended that the stand was not bolted down but simply made "stable" on the floor. The motor was mounted on a large rectangular "pan", hinged along its lower edge and pivoted on two bosses welded low down on the back of the stand behind the headstock. The pan was arranged to swing backwards and could be lowered to the floor behind the lathe so completely exposing the motor. Inside the pan were two pairs of motor rails, arranged at ninety degrees, that allowed not only a very easy adjustment of the belt tension but also (because it could be shuffled around on the rails until it lined up exactly with the headstock pulley) for a simple change to another make or size of motor..