A wrong turning that led British Leyland to a dead end

We reprint an article that was first published ten years ago and illustrates the risks of using a mix of metric and Imperial measures.

In a recent comment, Charlie P asks:

“…. Which areas of UK business, commerce, education, engineering, manufacturing, science or technology do you think do not currently use metric as their primary system of measurement and would struggle in dealings with China?”

Very few, if any. But that is not the point. The successful metric changeover in much of the UK economy put the PM in a strong position on her sales trip to China in January, but the reluctance of successive British Governments over the past forty years to complete the process has left wasteful muddle and confusion at home: metric in sport but Imperial on road traffic signs, metric in the supermarket but Imperial for body weight and height, metric in construction but Imperial at estate agents, and so on and on.

We reprint below an article that first appeared in UKMA News in May 2007. It illustrates the risks involved in using a hybrid system of measures, and is by Michael Knowles BSc. It has not been edited and is longer and rather more technical than is normal for this site. Views expressed are those of the author.


Saturday 28th April 2007 was the final day of service of the last substantial UK fleet of Leyland National buses, based near Walsall in the West Midlands. This vehicle is an important milestone in the history of the UK motor and engineering industry, when it tried to produce a metric bus.

The Leyland National bus is a contemporary of the Anglo-French Concorde supersonic airliner, and of greater economic significance. The prototype first ran in 1969 and the production vehicle appeared at the London Commercial Motor Show in September 1970. The buses were produced in a new, purpose-built factory at Lillyhall in the Lake District of northwest England. It sourced diesel engines, gearboxes and axles manufactured in various factories of the Truck and Bus Division of the British Leyland Motor Corporation Ltd, as well as wheels, air-brake equipment, power steering-rack units and other parts made by the UK component supply industry. The steel construction and safety cage made it one of the safest vehicles on the market, compared with conventional British buses having a steel chassis, and coachwork of sheet aluminium and plastic.

Low-profile 11.00 x 22.5 tubeless tyres were another important innovation, fitted on welded well- based rims: the first application of such tyres on a commercial vehicle in Western Europe. The design effort invested in the bus made it the safest and most expensive ever built in the United Kingdom. The original design was offered in two lengths of 10.3 and 11.3 metres to suit a wide range of operating conditions from small towns and villages with tight corners to large cities, in which roads were wider, and maximum passenger capacity was the predominating requirement.

A comprehensive research programme into the regulations of the world’s hard-road markets led to a vehicle of high international potential. It was especially intended for volume sales in the European Common Market, of which membership was confidently expected by both sides of government. After the 1970 election, Edward Heath’s team started to negotiate the Entry Terms with the Six from 23rd June. They were agreed in outline one year later on 23rd June 1971, when customs tariffs on cross-Channel trade in commercial vehicles were at the relatively high level of 22 per cent. Membership of the EEC would reduce this in fifths to zero on 1st July 1977.

The design team of the Leyland National had two particularly interesting challenges before them in the period 1965-70 covering its design and that of the Leyland 500 series engine which was to propel it.  The first was to specify the entire vehicle with International Organization for Standardization (ISO) metric threads, as used on the engine. The marketing reason for this was so that the vehicle could be maintained with metric spanners. For general engineering purposes, there are only three systems of screw threads in the world, namely Whitworth, Unified and Metric. The first two are defined in inches and the third in millimetres. The policy of specifying ISO Metric to replace the other two was recommended at a conference held by the British Standards Institution in November 1965, and the Leyland National Bus was the pioneer vehicle on which it was first attempted in Britain.

The other challenge was to design a driveline for the short rear overhang of the 10.3 metre bus, which also applied to the longer version, as the same power-pack was common to both. This was a major headache for Leyland. Their solution was on view in the maintenance depot of Chase Bus Services. Leyland produced a special single-reduction spiral-bevel driving axle with a pair of helical gears in a casing on its nose. This raised the level of the input, so that it could be driven from the rear and to one side through a slotted tunnel in a special hot-pressed steel axle housing.

The vehicles running the all-day service from Walsall (on 28 April 2007) bore registration numbers of 1978 and 1979. They were 10.3 metre versions with Leyland 510 turbocharged engines. Both were fully loaded, with standee passengers, to about 16 tonnes gross weight. The 8-litre engines had a soft tick over and smooth take-up through their oil-charged fluid couplings; their electro-hydraulic gear change was faultless when timed correctly by the experienced drivers; the rack-and-pinion power steering was car-like and precise; the body-shell was rigid and the air suspension good.

A number of long, uphill gradients on the commemorative round-trip showed that the road performance was adequate, even with a full load of passengers. The bus industry average of sixteen passengers per ton (1016 kg) was probably exceeded. The range of engine revolutions available for acceleration and hill climbing seemed wide compared with classical slower-revving bus engines. It is a credit to the maintenance staff that the 510 engines were in such good condition, as production of this unit was discontinued at the close of the ‘seventies.

Since the bus was designed forty years ago, it is remarkable that its appearance is so clean and timeless that it could still to be in production today. However, its assembly factory and supply chain have all perished, after the stagnation and decline which blighted industry after 1975, as demand for UK engineering goods tailed off and collapsed through the nineteen-eighties. The Leyland Society, created after the centenary of the original company in 1996, has published in its excellent quarterly magazine Leyland Torque that there was a great sale of machine tools from the works in that period. It is a paradox that the Leyland Group factories, which pioneered metrication in 1965-70, joined the others that stagnated and stalled, because it was not completed.

The Leyland Society has also published an article on the Leyland 500 engine, but its most important feature and the basic clue to the demise of the motor industry was omitted. The Leyland 500 engine was the first product of the British motor and engineering industry to be specified with ISO metric threads, which were introduced into the UK as BS3643 in 1963. They were being issued by ISO when Britain’s first motorway, the Preston Bypass section of the M6 London-Carlisle route, was opened by Prime Minister Harold Macmillan on 5th December 1958.

Motorways and the motor vehicle are intimately associated. Germany was the origin of both the former, and of the metric standards used in European industry. At that time, there were thousands of British Army personnel in workshops maintaining the vehicles of the BAOR. It was common knowledge that, for the free circulation of British motor vehicles and equipment after the war, most UK production would eventually have to be aligned with continental practice. This meant that all UK motor vehicles and machinery would have to be specified for maintenance with metric spanners, and have metric ancillaries like ball and roller bearings, oil seals and fasteners.

The decision by UK government and industry on when and how to do this was delayed until Harold Wilson’s government of October 1964 acted in response to a Parliamentary Question by the MP for Oldbury and Halesowen, not far from Walsall. On 24th May 1965 a Parliamentary Written Answer was published in Hansard, stating that, “British industries would adopt metric units (and standards) within ten years” ending on 31st December 1975. The Wilson industrial metrication programme was the basis on which Edward Heath’s team negotiated the UK Entry Terms to the European Common Market, which allowed industry a buffer period to convert by the end of 1975. In 1976, the author took the export chair at Rolls-Royce Motors Ltd Diesel Division in Shrewsbury, when it was about to launch its CV range of metricated military engines.

The evidence from that forbidden period of British history has been exhibited and discussed at the Bath Royal Literary & Scientific Institution in academic and political freedom, beginning with a commemoration on 3rd-4th October 1998 of the 1898 Congress of Zurich. The Congress was hosted by the VSM (Association of Swiss Machinery Manufacturers) at the ETH (Federal Technical University) in Zurich. The purpose was to agree a common standard for screws, nuts and threaded components throughout the railway industry on the continent of Europe, to end the confusion of a multiplicity of screw threads pervading the railway network, especially since the expansion of international trains such as the Orient Express, and long-distance rail-freight traffic.

Whereas locomotives operated only within their home countries, the rolling stock made long journeys, say from Paris to Istanbul or Copenhagen to the toe of Italy. Maintenance or repairs undertaken away from base would be done using the threaded items and screwing tackle of the workshop involved. Original holes were drilled out and new threads cut, so that even a single vehicle might have several thread standards. It was an increasingly problematic situation, which had reached crisis point. The Congress concluded with consensus to adopt the S.I. (Système International) metric screw threads among all participating nations, which included Austro-Hungary and Russia. The S.I. metric threads were updated in 1958 with the Unified profile, and issued to all signatories to ISO, the official languages of which were/are English, French and Russian.

Metrication is only relevant to inch-based industry. The primary practical reasons for it were to make a British motor vehicle or machine maintainable in a metric workshop, and to facilitate the combination of a British engine with a metric gearbox or vice-versa. The ultimate academic analysis of the Leyland 500 engine is that it should have been designed and manufactured fully to ISO metric standards, just as a Mercedes-Benz, MAN, Renault or FIAT engine of the same era. However, from the start, this ideal aim was compromised and the unit became a mixture of inch and metric standards. It had superficially metric bore and stroke of 118 x 125 mm, although the latter was produced on an inch lathe at Leyland’s machine shops, which are now a supermarket.

The basic engine was machined to metric standards, but this only applied to the main castings and ISO metric threads issued in Britain as BS3643. Alas, the supplier industry could not offer a metricated fuel-injection pump; the large alternator was an American Leece-Neville device made under licence in Leyland at British United Traction, a division of Leyland Motors. The fluid coupling was inch, the planetary gearbox consisted of a long-established drum of rotating parts and gears in a new, rationalised casing, which should have been produced with ISO metric threads but instead, was made with Unified (inch). There were numerous inch pipes and unions for lubricating oil and compressed air. The rear axle and differential were designed with ISO Metric Coarse threads and Unified Fine or British Standard Fine (BSF). French and German citizens have difficulty in speaking expressions like 9/16” A/F in their own languages.

The gearbox is an ideal example of a product covered in British Standards Institution (BSI) Policy Document No. 6424:1968, published under the stewardship of Mr A W Benn as Minister of Technology. Section 3.6.4 is headed Partial Metric Design and states that a product can be designed with interior dimensions in imperial terms, but incorporate metric hardware items such as fasteners, oil seals, etc.  This was found impracticable, since it is impossible to fit a metric oil seal to an imperial shaft. For a 3” (76.2 mm) outside diameter ground shaft, there was only a choice of 75 or 78 mm seals in the standard range derived from the German standard DIN 3. The BSI never translated DIN 3 into English, and this standard was not included in BSI Handbook No. 18 Metric standards for engineering first published in 1966, nor in the expanded second edition published in 1972 on the eve of Accession.

Finally the axles were another mixture of standards. The wheel fixing studs and nuts were 7/8” BSF, the wheel bearings were inch sizes manufactured by the British Timken Company at Duston, Northampton. The fasteners were ISO Metric coarse with Unified Fine or BSF, because Metric Fine threads were not available in the UK in time, so the vehicle was a technical hybrid, unattractive in any market. The first five years of its production were aimed at the domestic market, where it was allegedly difficult to maintain, but UK operators managed it somehow and production rose to a reasonable level. A good 1975 example belonging to Yorkshire Traction was examined with Leeds Metropolitan University in 2001, with a view to dismantling it and performing an inventory and analysis of the mechanical parts, but UK state funding was refused.

From launch to 1975, the Leyland divisional management should have ordered the redesign of the running gear to metric standards, so that a fully or at least competently metricated version would be available for the 1976 model year. From 1st January, the import tariff on cross-Channel trade was reduced to 4.4 per cent, so a left-hand drive 11.3-metre demonstrator was sent to Paris for evaluation by the Paris Transport Authority, along with a number of other contenders. It is more than likely that the French engineers investigated the screw threads on this vehicle, since they had specified the Wilson gearbox for their standard single-deckers in 1960, and thousands of licence-built metric versions were in service. They probably measured the wheel stud size and pitch and drew a half-shaft, to find that the fixing studs were ½” BSF – and not a metric standard.

The explanation of this lies in an instruction found in the Leyland Engineering Manual No. 211, left with the University of Bath for safe keeping about five years ago. The instruction advised draughtsmen not to design a new part if an existing part already existed. The hubs and half-shafts of the Leyland National Bus were originally from the Truck Cargo 10-ton 6 x 4 Leyland Hippo of 1946. The flanges of the forging were turned to match the hub diameter and bored with clearance holes for eight ½” BSF studs, which bottomed in eight female threaded holes tapped in the hub body. This minimised the inventory, and ensured interchangeability of parts between Leyland vehicles. However, the correct solution for the metricated Leyland National Bus was to bore the same shaft flange with clearance holes for ten M.14 setscrews. The SAE (Society of Automotive Engineers) inch splines on the inner end would remain, but intentionally workshop staff would have found the Leyland National Bus half-shaft non-interchangeable with other models. They would have recognised the part through the ten fixing holes, saying “That’s a metric one for a National Bus” and everyone would have known about it.

Unfortunately the rectifications were not done, so the original monkey-puzzle on wheels was sent to Paris. Then the city of St Etienne ordered ten LHD Leyland Nationals, with 150 kW (200 bhp) engines. They were soon joined by ten more, and Leyland set up a local company to care for them, called Leyland Industriel S.A. The fleet was expanded to 27 vehicles through acquisition of the Paris demonstrator and six vehicles from Dijon wrongly described in a report in Commercial Motor as Leyland Nationals. They were assembled in Belgium by Brossel Frères, as the tariff was still too high to make direct imports of built-up Leyland Nationals economic. It was only possible for St Etienne to maintain these vehicles, because it was the only city in France that retained its tramway after the war. Their American Brill streetcars were built to pre-war US standards, with American National threads, so the older employees could relate to the post-war Unified threads on the Leyland National Buses. When they retired, the younger workshop staff had neither the knowledge nor the patience required, and the Leylands were sold off early to a remote island in the Indian Ocean. Buying British was an expensive mistake for St Etienne, which cost Leyland its market share, killed sales in the Common Market, and closed Lillyhall.” ©.


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7 Responses to A wrong turning that led British Leyland to a dead end

  1. Daniel Jackson says:

    This is the exact situation in the US. Many small companies not large enough to create demand have to produce mixed standard products, a hybrid between metric and USC. All modern, hi-tech componentry is imported and is metric. Even if 80 % of the components are metric, the American company will go out of their way to complete the assembly with USC parts and fasteners. Thus a hybrid.

    In some cases, metric componentry is entered into the company data base with a USC description such that to the company staff it is a USC part. 50 mm x 3 mm copper bus bar for electrical connections is commonly referred to a 2 inch x 1/8 inch, even though it is not that size. Whereas 50 x 3 easily relates to 150 mm^2 wire size and comparable to a 150 mm^2 IEC wire standard, inch call outs don't relate to US wire gauge sizes.

    American products are cursed with the higher costs of fitting rounded metric products with rounded inch products and the constant rework needed on the factory floor to fit parts together and difficulty of parts staying in tolerance. Fasteners is another issue. Except for some major industries, most small American companies unless they are required to by contract with a customer will always spec out USC fasteners, even if it is assembling together two components held together with ISO fasteners. Service is a nightmare as a dual set of wrenches/spanners is required.

    It is no wonder that American technology is lagging in comparison to Europe and Asia. China which was once behind is catching up and is surpassing the US. No amount of flag waving and "Make America Great Again" rhetoric is going to reverse this. The Europeans and The Asians know this and in many ways may be supporting it as a means to prevent the US from ever being able to reverse their decline. The UK could be in the same position. Feed the opposition to metrication, support the movement back to imperial and keep the UK competitor out of the race.

    The world economies may be drawing closer together and even uniting, but some players are obviously not wanted and the metric system can be an effective means to divide the wheat from the tares.

  2. karolynbeatham says:

    Ramsden, thanks for the article post. Really thank you! Great.

  3. Daniel Jackson says:

    Now that Donald Trump is putting tariffs on imported steel, at what point will he put tariffs on metric made products entering the US, or even putting pressure on metric companies, like the auto industry to revert to USC?

    Tariffs and restricted trade practices should be a topic of discussion as to their effects in either promoting the completion of metrication in the resistant countries or to encourage what little progress has been made to be reversed.

  4. Jackthesmilingblack says:

    Interesting how in the US they will use the expression 'International' rather than say 'Imperial' or US Customary (USCS) measurement system.
    Such a joke as only three countries in the world officially use Imperial or USCS measurements: Liberia, Myanmar and the United States.

  5. Daniel Jackson says:


    Myanmar needs to be removed from that list of slackers. They have been effectively metricating since 2013. Liberia is also changing even if slowly. See the metric views article on Myanmar for more information.

  6. Ezra Steinberg says:

    But isn't Myanmar officially transitioning to metric?

    That leaves Liberia (which sounds like it is in its own muddle) and the USA (still trying to restore the 18th century).

  7. Mark Williams says:

    @Daniel Jackson:

    Arghh, the dreaded Pacer railbus (British Rail class 140/ 141/ 142) is also based on these. Probably half imperial, half metric, even to this day—and also failed to sell abroad. Meanwhile, the class 143 & 144 of a similar vintage were based on another bus whose manufacturer, after a few changes of ownership, is still going strong! It is now exporting buses and educating its USA factory workers in metric to bring them up to speed—a very Scottish way of doing things…

    p.s. Some earlier UK motor cars had metric threaded bolts with imperial flats throughout. But that was so you only needed one set of tools 😖!


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