Largest of all the conventional lathes ever produced by Colchester, the Magnum was a beautifully built machine and sold in two forms: the conventional gap-bed,  Model 1250 and the special-purpose Type LB800. With a centre height of 317 mm (12.5") and 1.5, 2.25, 3 or 4 metres capacity between centres, the 1250 was equipped with a bed 480 mm (18.89") wide with its V and flat ways induction hardened and ground. Its front and back walls were braced by enormous cross ribs and a detachable gap piece fitted as standard - this allowing the 1250 to swing a job 36 inches (900) mm in diameter and up to 8 inches (218 mm) deep on the faceplate - the corresponding figures for the LB800 being the same depth but 41 inches (1040 mm) in diameter. Two or more direct-to-floor heavy cast-iron plinths were used for support, these being joined by chip and coolant trays. Fitted with either a 16 or 22 kW (20 or 30 h.p.) main motor that drove direct to the headstock input pulley using five V-belts, jobs up to 1.75 tonnes in weight could be machined, the 24 spindle speeds being arranged in  four overlapping groups of six with a range from 6.3 to 1250 r.p.m. An ammeter was provided to help prevent overloading of the lathe when tackling very heavy jobs.
A version  intended for large-capacity through-bore and heavy engineering work, the Magnum LB800 was identical in specification to the 1250 save for the following: a centre height raised to 15 inches (380 mm); the spindle bore increased to 611/16" (170 mm) and - replacing the standard Camlock on the 1250 in order to reduce chuck overhang- an 11-inch A2 U.S.A. nose. Spindle speeds were reduced to twenty in number and arranged, as on the 1250, in four overlapping ranges of 6 speeds, the range spanning 10 to 800 r.p.m.  As the top speed of both models was high for the size of lathe, the makers warned against using ordinary "grey iron" chucks that might burst.
On all versions, and capable of being adjusted for alignment, the headstock held induction hardened Reishauer-ground gears driven through multi-plate, hydraulically-operated forward/reverse clutches. Fitted as part of the standard equipment was a powerful, fail-safe, automatically applied electromagnetic brake - this having a release push button that lit up blue when operated. As was usual for Colchester lathes, the spindle ran in high-precision, pre-loaded bearings made by Gamet, a company in the same parent 600 Group. At the front of the spindle the bearing was a double-row type and at the rear a single, the assembly giving a roundness accuracy of better than 2 microns (0.0025 mm). Headstock lubrication was by an electrically driven pump that supplied filtered Tellus hydraulic oil at 160/180 lbs.sq.in to the clutches with a low pressure supply directed to the spindle bearings and into a perforated gallery from where, by oil mist and splash, the gears and their shaft bearings were lubricated. With a large oil tank to absorb heat (its dip stick was easily accessed by removing the changewheel cover) the oil supply had the effect of cooling the clutches. To check that the oil supply was running, an indicator window was provided on the front face of the headstock and, for safety, as the pump motor was electrically interlocked with the main motor starter switch, should a hydraulic malfunction occur the machine refused to start.
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Totally enclosed, the screwcutting and feeds gearbox held hardened and ground gears with all shafts running in anti-friction bearings. 70 metric pitches could be generated from 0.2 to 112 mm; 88 imperial from 1/4 to 112 t.p.i.; 37 module from 0.3 to 24 MOD and 41 diametral from 1 to 60 DP. Direct changes of pitch were selected by three levers and a single knob on the face of the gearbox but with the range of pitches - fine, medium and coarse - by a lever on the face of the headstock marked, respectively, L, M and H. The H position was for generating very long leads suitable for oil grooves and similar jobs and was intended, of course, for use only at slow spindle speeds, these being marked in red on the spindle-speed selector dial. The maker's handbook has essential data about the limitations of the spindle-speed to feed-rate settings.
Feed to the sealed, splash-lubricated apron for screwcutting was by a leadscrew 45 mm (1.75") in diameter, with a pitch of either 12 mm or 2 t.p.i. together with the usual separate power shaft - on long-bed versions both being supported by adjustable, intermediate hangers. Power sliding and surfacing feeds were driven through a constant-mesh wormwheel and spur gear assembly and Vee-tooth clutches, these being adjustable and fitted with an overload trip that could be set to work against adjustable stops in either the sliding or surfacing direction. Built into the apron was a rapid-traverse motor that drove the sliding feed at 4000 mm (60 inches) per minute and the cross feed at half that rate - engagement being by pressing a button built into the top of the joystick lever.
Of minimalist appearance, the apron's mechanical controls consisted of just a (safety disengage-able) handwheel for manual movement of the carriage, a lever to engage the screwcutting clasp nuts and a directional joy-stick lever that acted to both engage and set the direction of feeds. In addition, on the face at bottom left, was a manually operated plunger pump that directed lubricant to the bed and cross-slide and, to its right, a fluted knob to set the loading at which the power feeds tripped out. For main motor start, stop and reverse, a line of push buttons and warning lights was provided on the apron's upper right-hand face.
Using tapered gib strips, the cross and top slides were fitted with micrometer dials reading either to 0.001" or 0.02 mm. The dial on the cross feed was set to indicated the reduction in workpiece diameter while that on the top slide showed the tool travel. As fitted to all later Colchester lathes, the cross-feed nut was split and, by means of a central wedge, could be adjusted to remove backlash. For safety, because a rapid power cross-feed was fitted, the cross-feed handwheel was spring loaded outwards into a disengaged position and had to be pushed inwards to engage a dog clutch; however, a latch was provided to lock it in position - though if the operator forgot to disengage this before using rapid feed the danger remained of a spinning wheel catching loose clothing.
Able to be set over for the turning of slight tapers, the tailstock had a 6 Morse taper barrel with a travel of 190 mm (7.5 inches). For heavy drilling operations an eye bolt was provided at the front, this being coupled by a clevis pin to the carriage (the pin was stored in a parking position). The maximum feed force was limited, of course, to that available from the saddle feed mechanism i.e. 1000 Kg f (2200 lbs f).
Supplied as standard with each new Magnum was a 380 mm 3-jaw chuck, a simple slotted toolpost, a driving plate, headstock spindle reducing bush to 6 Morse taper, a full-length rear splash guard, chuck guard, thread-dial indicator and a set of spanners, a handbook and an accuracy chart..