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Re-launch for BMC's ET

8th March 1968, Page 68
8th March 1968
Page 68
Page 69
Page 68, 8th March 1968 — Re-launch for BMC's ET
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Which of the following most accurately describes the problem?

by A. J. P. Wilding, AMIMechE, MIRTE

BMC was the first of the medium-weight truck producers to bring out tilt-cab models with a mechanical specification in respect ol such things as brakes arid suspension that is in line with present-day requirements. The time was September 1964 and the model the FJ. As so often happens with a completely new design, operators found there were teething toubles and reports suggest that FJ 7and 8-tonners and the 18-ton-gross tractive unit had a hard time.

But the model was not unique on this score and many makes of vehicles have given their operators more worries than they ought in the past few years when operating conditions—higher speeds, more intensive use ol vehicles and so on—have changed so rapidly.

It is one thing being landed with a reliability problem and quite another finding the answers. But BMC engineers are now confiden1 that they have found all the answers on the FJ. Modifications have been made to all components that have given trouble and aftei lengthy and severe testing the latest version5 have been given a clean bill of health.

New that every modification has been effectively approved [MC has decided that the FJ should be "re-launched----the current design, they believe, can be expected to give E radically different account of itself in service from that of the original product.

The major changes The major part of the changes have been tc the engine and cooling system, aimed a overcoming gas-sealing problems. Filling vvith coolant—and sometimes even' topping Uf —were difficult on the original design if trap Ping air in the engine coolant passages was tr be avoided. The filler was remote from ar expansion tank and if the pressure cap in th( latter was off during the filling, air in th( system was inevitable. An intermediati change to a header /expansion tank close t( the thermostat did not provide the completi .answer so the cooling system has been corn pletely revised.

A major alteration is that a vertical-flow radiator is used instead of a cross-flow and a 2-gal filling /header tank is located behind the cab—on a bracket from one of the cab supports. The system is conventional with the feed from the tank into the pipe from the radiator bottom tank to the water pump and the thermostat interposed between the top radiator connection and the water manifold on the cylinder head; there is an air-bleed pipe from the thermostat housing to the header tank.

Head design

To improve water circulation inside the engine, four pairs of water holes in the head face (those directly below the exhaust ports) are now joined. Other changes to the head design have been concerned with stiffening up—with internal ribs—the bottom face to prevent distortion and consequent gasket failures.

Advanced techniques to determine gasket loadings showed an unsatisfactory state of affairs and with the head-casting changes and complementary modifications to the top area of the cylinder block even loading has been accomplished.

This is illustrated in a table of gasket loads obtained by BMC before and after modification and to improve the position further there has also been a change from set bolts to studs for cylinder head holding down. To get the loading exactly right, the gasket has been "tailored" to the head block design. The original copper-asbestos component was not satisfactory so a special Coopers "backbone" gasket is used. With this, there is a central steel member sandwiched between asbestos layers with steel overlap edging at the cylinder bores and other apertures.

The heavier FJs have been dogged with transmission troubles and this has resulted in a change in gearbox. There is no difference in the make or type on the latest design—the unit is still an ENV475 five-speed with synchromesh on the upper four ratios--but a Mk IV version replaces the Mk. III. Main differences are that first gear is constant mesh with an increased pressure angle and wider teeth for greater strength. The reverse idler is non-sliding and is now supported by needle-roller bearings instead of plain bushes and the sliding synchronizer-dog teeth on second, third and fourth are backraked to prevent jump out. Thrust washers and oil sealing and breathing of the box have also been improved and there is a better side-power take-off arrangement.

One benefit coming from the gearbox change is that the "gate" has been simplified. Originally the "first" position was remote from the "second" calling for a wide-U-change. In the latest box, first is opposite reverse with the rest of the gears in progressive sequence —there is a , three-rail gate instead of a four-rail—which allows quicker changes between the two lowest ratios.

To raise the durability of the differential unit in the BMC rear axle used in the 7-tonner, the number of bolts locating the two halves of the differential cage has been increased from 8 to 12 and the diameters have gone up from

to On.; the-crown-wheel bolts have also been

increased in diameter—from in. to Ain.

Hardening of the crown wheel and pinion has been revised. Case depth is increased from 0.03in. to 0.05in. and an improved method of retaining the pinion nose bearing on its spigot has been incorporated together with full bearing kick pad to minimize deflection of the crown wheel under load.

Braking

Braking on the original FJ design was in excess of the standards which have since become legally required. There has therefore been no need to make a change here but because handbrake performance was well in excess of even Code of Practice standards on the 7-tonner (and the 5-tonner), air assistance is not now employed on these two chassis. When the FJ was designed it was not fully known what the future standards would be but with a 5in. longer handbrake lever the unit will hold a laden 7-tonner on a 1 in 4 gradient without air assistance. Main and secondary brake requirements are met with the standard air-operated hydraulic system which has a dual air actuator and tandem master cylinder feeding independent circuits to the front and rear axles.

To prove the reliability of models incorporating all these changes a fleet of 12 to the latest specification were taken from production and put through 24-hour-a-day road testing in the UK. Some of these are now up to the 50,000-mile mark and engines and transmissions are reported as giving no trouble.

Further accelerated proving tests are at present underway and in addition a team of engineers and drivers took vehicles to Finland for testing in an area north of the Arctic Circle. Temperatures well below zero were not expected to show up over-heating problems but this was a market that had its share of engine and transmission trouble. Reliability of the latest design was checked at the same time as other work aimed at improving the stan dard of heating, etc., for markets such as Northern Finland. In spite of some very hard testing no mechanical troubles were experienced in over 7,000 miles of running.

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