The 13-inch Harrison was not merely a 12-inch with a greater capacity, it was an almost completely different design, albeit one that was styled in the familiar "round" fashion of the 1950s and so closely resembled both the smaller and larger machines in the maker's range. With a 3 hp single speed motor two ranges of nine speeds were available from the clutched and braked all-geared headstock: 35 to 750 rpm or 59 to 1250 rpm; with the optional two-speed 2.5/5 hp motor 18 speeds were provided of 41 to 1750 rpm.
The spindle was bored to pass a 1.625 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; it was fitted with an American type L0 nose - thankfully without the option of a threaded fitting. The main double gear that slid along the spindle was cut from a single forging and slid on seven splines - the complete assembly being dynamically balanced. The gears were all hobbed from the solid, shaved, induction hardened and honed to produce the correct tooth form.
The 8.5" (216 mm) wide and 8" (203 mm) deep induction-hardened bed was supplied in one length, that admitted 40 inches between centres, and either with or without a gap. With the gap piece removed a piece of material 20.5" in diameter and 4.5" thick could be machined on the faceplate.
The screwcutting gearbox was, unlike those on all smaller Harrison lathes, completely enclosed and provided with oil-bath lubrication. It offered a choice of 36 threads and feeds from 4 to 60 tpi and it was possible to specify, at the time of ordering, a built-in metric-conversion cluster that enabled a range of 11 metric pitches to be generated by the movement of a single lever that emerged horizontally from the inside face of the changewheel cover. The power feeds ranged from 0.0017" to 0.025" ((0.043 mm to 0.63 mm) sliding and from 0.001" to 0.015" (0.025 mm to 0.38 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 L5 and L5A, it appears that a similar device to protect the leadscrew was not listed. 
The top slide could be turned through 360 degrees and 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;  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. To protect the cross-feed screw a thin sheet-metal insert (that is frequently damaged or missing on used machines) was set above it at the front with a robust, cast-aluminium cover over the rear.
The doubled-walled, completely-enclosed apron was much deeper than on the smaller lathes and held an oil supply in its base from where it was distributed by splash; it employed the usual kind of push/pull button to select the power sliding and surfacing feeds, with the engagement lever positioned in the middle of the casting rather than hanging down beneath. A thread-dial indicator was fitted as part of the standard equipment.
The tailstock barrel was fitted with a No. 3 Morse taper, and self-ejected the centre as it was drawn fully back from its 4.5 inches of travel.
The stand was fabricated from 3/16" thick steel plate 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 dished plate hinged along its lower edge and pivoted on two bosses welded low down on the back of the stand behind the headstock. The plate was arranged to swing backwards and could be lowered to the floor behind the lathe so completely exposing the motor. Inside the plate were two pairs of motor rails, arranged at ninety degrees, that allowed not only easy adjustment of the belt tension but also, because the motor could be shuffled around on the rails until it lined up exactly with the headstock pulley, for an easy change to be made to one of a different size or make..