First manufactured in 1971, and still in production 46 years later to meet a continuous demand for a rugged, simple-to-operate and reliable centre lathe, the M300 is a very compact machine yet, with a centre height of 167 mm (69/16") and a capacity between centres of either 635 mm 1000 mm (25" or 40"), offers a useful turning capacity. A T300 training version  was also listed (though is no longer made); oddly, this was produced in two distinct versions: one based on the M250 with the centre height increased and the other on the ordinary M300. Both were listed with, as standard, an all-metric screwcutting gearbox or screwcutting by changewheels and often (but not always) with the M250-based version fitted with a motor rated at 0.9 kW, compared to the 1.6 kW specified for the M300 type. The "T" model is easily recognised by distinctive badges and, on the M250 model, by a leadscrew and powershaft safety cover arranged to slide on a rail held on brackets bolted to the headstock and tailstock ends on the bed.
Induction hardened as standard, the bed can be ordered either straight or with a detachable gap (in both lengths) able, in the latter case, to accept a piece of material up to 480 mm (19") in diameter and 115 mm (4.5") thick. Most models have 12 spindle speeds arranged in geometrical progression (in a ratio of 1.46) from 40 to 2500 rpm driven (on the English market) by a fan-cooled 2.2 kW (3 h.p.) 3-phase 1500 rpm motor; however, a number of lathes are known to have left the factory fitted with a 1-phase 1.1 kW (1.5 h.p.) 230 volt motor; this very much less powerful unit made a considerable differences to the spindle speeds: the bottom dropped from to 20 r.p.m. and the top to a far less useful 1250 r.p.m. (the full range being: 20, 28, 42, 68, 90, 130, 185, 270, 400, 600, 850 and 1250 r.p.m.) 
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The spindle, which runs in Gamet Super-precision opposed-thrust roller bearings, has a generous 38 mm (1.5") bore and is equipped with a hardened and ground No. 4 D-1 Camlock nose and a No. 5 Morse-taper socket. The headstock gears, all induction-hardened, run in a simple splash oil bath and are moved by two rather short levers on the front face of the headstock; as a safety measure the levers have to be pushed in before they can be turned and their finish, which is far too smooth, can make them difficult to operate with oily hands. Early series lathes were fitted with a headstock gear-selector drum in "Delrin" which distorted, preventing movement of the selector forks. The fault was quickly recognised by the makers and a replacement drum (part: 303-039) made available in metal.  Screwcutting is provided through a well-supported, ball-thrust-equipped 28 mm (1.125") diameter leadscrew, of 6 mm pitch or 4 t.p.i., that is engaged only for threading (a thread-dial indicator is part of the standard equipment) with a separate power-shaft driving the sliding and surfacing feeds. Surprisingly, for a lathe often found in training workshops, the leadscrew is not protected by a cover as on the M250, but there are torque-limiting clutches on both the leadscrew and powershaft - though earlier models were protected by a limiter on the powershaft with just a mild-steel shear-pin through the splined shaft of the top changewheel stud to prevent over-load damage to the leadscrew. Both systems are adequate enough to militate the damage that might otherwise be caused by the over-ambitious cuts often set up by apprentices or - if one dare one say it - the over-enthusiastic amateur.
In comparison with earlier Harrison screwcutting gearboxes the "Universal" unit fitted to the M300 is a considerable improvement and capable of reliably transmitting much great torque. Formed from a one-piece casting it is completely enclosed and holds induction-hardened gears driven by changewheels that have included, over the years, one or more non-metallic intermediates to promote quite running. Controlled by three levers and an 8-position rotary dial the box will generate both English and metric threads: 35 Inch pitches from 2 to 56 t.p.i and 39 metric pitches from 0.2 to 14 mm. By the use of additional changewheels 18 Module pitches from 0.3 to 3.5 MOD and 18 Dimetral pitches from 8 to 56 DP can also be obtained. The range of power sliding feeds varies from 0.03 mm to 1 mm per revolution of the spindle in metric mode and from 0.001" to 0.040" in English; the power cross feed rate is arranged to be half the sliding rate. The box runs in an oil bath (a level window is fitted on the front face) with the lower gears distributing the oil by splash.
Fitted on either the left or right of the apron (to the customer's choice) the carriage traverse handwheel can be disengaged when using power feeds. The double-wall apron carries a supply of lubricant in its base (with a level window let into the headstock-end face) that can be distributed to the bed and cross slide by a simple hand-operated plunger pump. Feeds are engaged by a traditional push-pull plunger and engaged and disengaged a very light-action, flick-up-and-down lever the operation of which is not affected by the how hard the lathe is working.
Fitted with feed screws carrying twin ball-thrust races, that for the cross slide on early models had a nut closed by a wedge (in common with many conventional lathes made by the 600 Group) to allow the elimination of backlash - though when the assembly is badly worn this arrangement can disguise the imminent failure of the threads. Later machines were modified and the nut made solid--the change also affecting the design of the cross-feed screw, The satin-chrome finish micrometer dials are sensibly large with especially clear graduations with that on the cross slide arranged as a "direct-reading" type to shows the amount taken off the diameter rather than the radius of a workpiece. Whilst the top slide has a traditional screw-adjusted gib-strip, the full-length cross slide is fitted with a tapered type and the cross and top slides, as well as the saddle, are equipped with locking screws.
Carefully thought out, the specification of the heavily constructed, set-over tailstock was very user-friendly -  with especially long and easy-to-use locking handles fitted with gently tapered and consequently comfortable handgrips. The 42-mm diameter quill, marked with inch, metric (and sometimes both graduations) is fitted with a ball thrust bearing, a micrometer graduated collar and accepts a No. 3 Morse taper of the tang type with, usefully, a drift slot provided to aid its removal. As a final touch, tailstocks on later versions of the M300 carry felt bed wipers to help reduce the inevitable wear to the front section of the sole plate.
Electrical stop, start and reverse of the spindle is by a "third-rod" system controlled by a lever attached to, and moving with, the apron. Both the electrical isolator and coolant pump switch are mounted on the left-hand face of the stand - the latter a unit that is well made and supplied with a chip tray, splash-back and locking tool cupboard as standard. Although safety equipment has varied during a long production run most machines have a useful "power-on" light, a rather small headstock-mounted, mushroom-headed emergency stop button, a long, foot-operated (and very powerful) spindle-brake and "motor-off" bar and electrical interlocking of the changewheel guard. The electrical system is built into the left-hand end of the stand and accessible thorough a door equipped with an interlocked isolating switch that can, for additional security, be padlocked in the off position.
Although of an excellent mechanical specification equipment provided as standard with the M300 has generally been sparse: a single toolpost, drive plate, a 5 to 3 Morse adaptor bush for the headstock spindle (often missing on used machines), 2 No. 3 Morse centres, spanners, keys, oil gun, final inspection chard and a handbook and parts list. An M300 weighs about 583 Kg (1288 lbs) in short-bed form and 685 Kg (1512 lbs) as a long-bed model..

Harrison M300
saddle and compound slide rest detail. Centralised lubrication is fitted, activated by the button on the bottom right hand side of the apron. The lathe shown is fitted with the optional dual English and Metric micrometer dials. As is not uncommon, the photographer has put the toolpost in the one position to be avoided when taking the first cut …..
The cross and top slides are identical to those used on the later Colchester 1800, 2000 and 2500 series Student and Master lathes

The 38 mm (1.5") bore 5 Morse-taper spindle was fitted with a hardened and ground No. 4 D-1 Camlock nose and ran on Gamet Super-precision bearings; the headstock was lubricated by oil splash. The speed-control levers had a finish that was far too smooth making them difficult to operate with oily hands.

Gears in the screwcutting and feeds' gearbox were all induction hardened and ground with the threading and feed ranges selected by three levers and an 8-position rotary dial. A total of 39 metric, 35 English, 18 Module and 18 Diametrical pitches could be achieved by moving the control levers and either rearranging the standard changewheel set - or employing the extra non-metallic (hence quiet-running) gears that were supplied with the machine in order for it to achieve its full threading range.  The two large cylinders on the end of the shafts protruding through the side of the apron housed the leadscrew and powershaft overload clutches.