Since its introduction some twelve years ago, the Dennis "Lancet" chassis has under-gone progressive improvement without any major departures from the original layout. As in the case of other leading British passenger vehicle types, the Lancet III evidences a close attention on the part of its designers to the all-important subject of chassis maintenance and passenger comfort.
Cast in copper alloy iron, the cylinder block and crankcase has walls extending slightly below the crankshaft centre line, adequate support for the seven-bearing crankshaft being afforded by transverse webbings. A cored tunnel near the top of the casting on the off side accommodates the camshaft and the cylinder water jackets surround the bores almost for the full length of the piston stroke.
Machined from Brivadium iron alloy, the cylinder liners are flanged at the top and have double packing rings for the bottom joint. There is a housing for the dynamo, a facing for the water pump, an oil filter connection assembly and a support for the injection pump. At its forward end, the crankcase is faced to receive the fan mounting and forward support of the unit the casting being machined straight across at the rear for the timing case and flywheel bell-housing. A cast aluminium sump of 4-gallons capacity closes the bottom of the crankcase.
Covering three cylinder bores each, the detachable heads have vertically disposed valves operated by inclined push rods and rockers. The crankshaft, machined from a 58-ton tensile nickel chrome molybdenum steel forging, has 4-inch diameter journals hardened by the Tocco process. Heavy main bearing caps of "H " section, cast in copper iron alloy, are secured by large diameter studs.
The "I "section connecting rods are machined from nickel chrome molybdenum steel forgings and have diagonally split big-ends to permit withdrawal of the rod and piston assemblies through the tops of the cylinder bores. The webs are drilled through for lubrication of the gudgeon pin bearings. Silicon aluminium alloy is used for the piston castings which are heat treated and anodized. There are struts above the gudgeon pin bosses, and also horizontal webs. The compression ratio is 15 to 1 and the piston crowns are machined to form toroidal cavities, the swirl being augmented by bringing the piston crown close to the underside of the cylinder head.
The four bearing cast iron cam-shaft has harmonic profiles of the constant base circle radius type, and is driven by helical toothed gears from the rear of the crankshaft. This follows standard Dennis practice, its objects being to prevent transmission of crankshaft torsional oscillation to the timing gears and to provide a steady drive for the fuel injection pump. The location of the two auxiliary spindles, one above the other, provides maximum accessibility for the components. The lower spindle actuates the Dennis gear-type exhauster, dynamo and centrifugal water pump, while the upper drives the fuel injection pump and the fuel supply pump. Provision for adjustment of fuel injection timing is provided by slotted bolt holes in the coupling flanges.
The centrifugal circulating pump draws water from the bottom tank of the radiator and forces it through an external pipe to a gallery cast integrally with the cylinder block and extending along the full length of the water jacket. The flow is directed through an internal distribution pipe in each head, supplying cool jets to the areas of the valve-ports and injectors. The fan is belt driven from the front end of the crankshaft, belt tension being regulated by adjustment of the pivoted fan mounting. Driven by spur gearing from the crankshaft pinion, the horizontal gear type pump draws oil from the sump through two detachable strainer gauzes. On the pressure side of the pump, the oil is conveyed through an external filter and cooler unit to a gallery connected with each of the main bearings. Oil forced through the hollow crankshaft supplies the big-ends and gudgeon pin bushes, surplus oil from the sides of the big-ends being thrown up into the cylinder bores. An internal duct supplies oil to the camshaft bearings and valve tappets, the overhead rocker mechanism being fed with oil from the camshaft.
The whole of the fuel injection equipment is situated on the near side of the engine. Fuel drawn from the main tank by an Amal pump is delivered to the C.A.V. 6.5 mm pump through two filters. Standard centrifugal governor mechanism is incorporated with the injection pump unit, the maximum and minimum speeds being set at approximately 300 and 1800 r.p.m. Four-hole in-jectors are employed and set at pressure of 150 atmospheres.
The clutch shaft is carried on two ball bearings, and its hollow centre accommodates the roller bearing for the spigot of the main first motion shaft which is supported by a ball bearing housed in the partition between the two parts of the gear box casing. The splined main shaft is mounted on a roller bearing in the constant mesh gear, and has two ball bearings at its rear end to which the propeller-shaft coupling flange is splined.
At its forward end, the layshaft is carried on a parallel roller bearing, and there is a ground portion on which the overdrive constant mesh wheel revolves when any gears except those for the overdrive are in engagement. To the rear of the central bearing, the layshaft conforms to conventional practice, and chief interest in the design lies in the over-speed drive mechanism and the method of engagement. When the overdrive ratio is selected, the direct drive remains in engagement and, at the same time, the change speed lever causes a toggle lever actuated by a pressure spring to disengage the dog clutch from the dogs of the clutch shaft gear.
This also engages the dogs for the constant mesh gear that normally revolves freely on the layshaft, thus securing it to that shaft. Under these conditions the drive is transmitted from the clutch shaft constant mesh pinion, through the over-speed gear on the layshaft to the normal constant mesh gear, back to the constant mesh pinion and to the main shaft through the ordinary direct drive. The dog clutches used for the fifth speed cannot be engaged or disengaged until the speeds synchronize and there is no torque. Apart from the extra gate position for the over-speed selection, the gear-changing operation is the same as with the conventional form of four-speed gear box.
Layrub flexible couplings are fitted to the forward portion of the two-piece open propeller-shaft, Hardy-Spicer universal joints being used for the rear part. The forward portion incorporates a torsional vibration damper and a self-aligning spherically mounted bearing supports the assembly from a chassis cross member. Unlike the majority of passenger vehicle propeller-shafts, this one is on the longitudinal centre line of the chassis.
Steering is by Dennis worm and nut mechanism in which the steel worm works in a sleeve or nut lined with white metal which moves vertically in the machined part of the steering box. At its lower end this sleeve is bored to receive a steel trunnion carrying a trunnion pin secured by splines to the drop arm spindle.