Essays : The 500-Series – Leyland loses its head!

The 500 Series diesel cost Leyland a packet – and not just in development costs.

Mike Humble tells the story of this ‘interesting’ engine

Fixed head asset

The 500 in horizontal form as in this early National, was at best, tolerated!
The 500 in horizontal form as in this early National, was at best, tolerated!

CONTRARY to modern day belief, Leyland Truck and Bus was once a formidable force on a truly global scale with British trucks and buses operating in all four corners of the world. At the helm was Donald Stokes, a man reputed as being a true statesman, gentleman and entrepreneur whose father was the General Manager of Plymouth Corporation Transport, buses and engines were in his blood.

Today, we think of Leyland or British Leyland as a joke with misty eyed fondness, but before the formation of BLMC this Lancastrian based empire ruled the roads both here and overseas with an enviable reputation for engineering excellence and reliability. As the ’60s progressed a dark cloud was on the horizon, at that time it was no bigger than a child’s soapy bubble, but foreign competition was building momentum and it was eventually to shatter the British Automotive companies – motor bikes included.

Leyland Motors as it was then known, was a shrewd operator and almost immediately after the formation of BL set upon making a range of diesel engines that were modern, powerful and efficient. At this time,Leyland’s biggest truck engine was the 11.6-litre `680` unit which could develop 220bhp but not without issues of failing cylinder head gaskets. Leyland advertised a position of engineering director with a brief to design and develop a new range of engines to meet and beat the competition.

The vacant post was filled by Dr Albert Fogg, an engineer formerly with MIRA. Dr Fogg was well versed in engine design but his train of thought was very much outside the box, so rather than work on a way of improving tolerances and existing designs (gaskets aside the 680 was a respected engine) Fogg set upon isolating the problem by eliminating the head gasket altogether. With his team of engineers, Fogg developed a high speed diesel engine that broke the mould in design terms.

A prototype engine of 700 cubic inches (12-litre) with a fixed head including an over head camshaft, cross flow manifolding and turbo started to be tested. With the engine having no separate cylinder head, maximum cooling to the piston crowns and combustion area was allowed. The engine was very slim compared to the Gardner and Leyland 680 units owing to the cylinder block not having to accommodate a camshaft, tappets or pushrods.

Because of this revolutionary design having far less moving parts, the engine operated at higher speeds, some 500 rpm more than the normal. Super slim fuel injectors developed by Ambac especially for Leyland offered a more controlled and higher pressure fuel rate into the cylinders. Light pressure turbocharging gave a useful amount of low down torque theoretically enabling good fuel economy and the small number of prototype engines were proving to be quite promising. A fly in the ointment came in the form of Leyland marketing men wishing to have an engine gearbox combination that weighed less than 1000kg.

This critical and fatal change of mindset caused massive upheaval in the design dept as the marketing men ordered the engine boffins to scale down the size of the engine and have running prototypes running as soon as possible. The fixed head power unit quickly became scaled down from 700 to 500 cubic inches and the National bus was to be the first recipient of this engine. Fogg, upset with the constant changes, quit his post and left British Leyland, leaving the design team leaderless, soldiering on with an unproven design that was prove a disaster.

Leyland Bison 6x4 Tipper With 500 Turbo
Leyland Bison 6×4 Tipper With 500 Turbo

The 500-series was hastily developed and put into service with the design team pleas with management begging for further proving falling on deaf ears. Senior BL management seemed to quite happy with making the customers do the development work for them, a trend that continued throughout most of the ’70s. Offering power from 200 to 260bhp, the 500 certainly offered superb performance, especially at the higher power ratings. The distinctive whining clattering sound played a different tune from the growling 680 or AEC 760 engines.

A brave face was put on by the service staff as the engine quickly became reknown for being troublesome and expensive to repair. The idea of having a fixed cylinder head was a brave one, but procedures such as getting to the valves for example required specialist tools and the separation of the cylinder block. The National bus which also received this problematic engine fared slightly better with lower rated engines.

NBC (National Bus Company) fleet engineers hated them and yearned for the service and forget nature of engines like the Gardner 6LX. Some municipal bus companies tried the National with firms from Cardiff, Nottingham, Northampton and Plymouth getting pretty good service from the 500. Experience proved that fleets with high standards in the workshops got the best experience from the 500. After worrying levels of warranty claims, Leyland set up an internal squad to get to the root cause of why the 500 was such a disaster. Quality control proved to be shocking with stories of gaps up to 9 thousandths of an inch in the engine block join would cause monumental oil leaks and blow holes in the castings contributing to many water leaks.

Manufacturing tolerances were in fact tightened up as time progressed, but the main reason for the failure of the 500 series boiled down to poor development and in some cases, sub standard parts. Confidence both in house and with customers waned as the problems of BL continued to pull the entire group ever downwards.

They were well known for being smoky, partly due to badly calibrated fuel pumps and in part thanks to the turbocharger being slow to boost. Overfueling was a common headache and for many years a 500 piston with a melted crown adorned my mantle piece. Such a shame, for this engine truly was a wonderful piece of design and technology, but as was the case with so many other BL ideas, badly managed, badly developed and simply left to soldier on. Bus companies who had suffered with the 500 quite often repowered the vehicles with other engines and Leyland towards the late ’70s seemed to admit defeat by offering a de-rated engine with 170bhp laughingly called the long life unit.

UK hauliers fed up with Leyland, bought into other marques offering Cummins or Rolls Royce engines while up in Scotland, the SBG (Scottish Bus Group) refused to buy any further Nationals. When the National series 2 was introduced in 1979, Leyland showed another sign of no confidence by offering a Gardner 6LXB engine and the Trojan 680 was developed into the L11 & TL11, the 500 died a quiet death. Customer loyalty was damaged beyond repair thanks to the 500 unit.

Around the time of the cancellation of the 500 series engine, some senior managers and engineering staff were taking stock of the turbulent situation in a meeting at Leyland’s HQ Lancaster House. Questions were raised about how and why this engine was designed in the first place and general comments about the long departed Mr Fogg discussed. One technical manager mentioned about the fixed head idea being used on a Bentley car many years prior, a colleague then barked, ‘…and aye, it never bloody worked for Bentley either’.

So was this design a total failure? Well on a personal level, the engine was certainly a feat of amazing design for the time, designed in a period where Leyand were a supreme force on a global scale. In terms of the National bus, the high tech factory and running gear saw many rival engineers visit the Workington bus plant and Leyand’s engine plant.

From a customer point of view, this marked a very important era whereby operators and owners felt that Leyland were forcing this untried and unproven technology upon them without consultation. Too many internal forces were pulling the company apart and where time should have spent on courting the customer and developing a product which worked, British Leyland lost their way, struggling in a world bogged down with debt and industrial strife. Managers and directors became fire fighters instead of industrial figureheads and as the quality of the product sank ever further, that small dark cloud I mentioned earlier became a storm as European competition set up stall and exploited the weaknesses of BL.

The Leyland 500 series could give decent results, but it was a poor solution to a problem that could have been, and in fact was, easily cured. It never really became a good engine, the standards of reliability was truly awful and it was only reknown for its spirited performance and clouds of black smoke.

Almost a decade ticked by with an engine that not even Leyland had any real confidence with. The reputation with the National for example, caused a flurry of activity with disheartened fleet engineers throwing away the 500 and replacing with either DAF, Volvo or Gardner engines. Oddly enough, operators such as Chase Bus in Staffordshire found the 500 to give excellent service, but as mentioned before, operators with exacting levels of quality often gained the best results.

The Leyland 500 In Horizontal Format. Note How Slim The Block Is.
The Leyland 500 In Horizontal Format. Note How Slim The Block Is.

Technical Specification:

Leyland designed and produced in line six cylinder high speed turbo diesel. Built at the Leyland Spurrier plant Lancs.
8.2-litre (500 cui)
170-260bhp in horizontal or vertical format
Overhead camshaft with bucket & shim tappets with constanst mesh timing gears & C.A.V/Ambac fueling system
Crossflow combustion with Holset light pressure turbocharger
Cylinder head casting integral with cylinder block and seperate block/crankcasing
Compressor and fuel pump drive take from the nodal point of the camshaft

Mike Humble


  1. Harold Musgrove went on record and said his biggest regret at BL was “been remotely associated with the Fixed Head engine”. I’ve never actually known what he was talking about until now. Sadly it proves the old maxim that ‘there is nothing new under the sun’. I’m thinking ‘damp’ K-series engines here……

  2. Is massive block/head join strength really worth making your engine singularly difficult to work on without the fingers of a gynaecologist?

  3. Nice to read a bit of info on the 500. The national is a bus I’ve wanted to drive for a long time, more character in its front wheel bearings than the modern low floor crud I have to put up with!

  4. A friend of mine used to own RRM148M. Nice motor. I used to have a National 2 with a Gardner lump, which was a hell of a lot bigger, and heavier, and a lot more reliable. Smoked just as much though.

  5. Fascinating write-up. A sad story with the same ingredients as cars like the Rover SD1 (Great idea, insufficient development, poor quality of construction etc). Good to read about the commercial vehicle side of the business.

  6. I think the fixed head idea is a good one. What is the reason for most head removals?, to fix a leaking head gasket.

    How often does the head or valvegear itself fail? Pretty rarely I suggest.


    • Unless you have badly calibrated fuel pumps delivering too much fuel, which will burn out the exhaust valves.

  7. The one theme throughout a lot of these stories is the lack of pre-production QA.

    Perhaps if they took the mantra of “test, test, test, test!” and ironed out issues before they hit the road, the company would have been stronger (and indeed, still with us today!)

    • The problem was profitability, BMC and later British Leyland like Roots with the Imp, did not generate enough cash to fund the testing needed, National, Allegro, Maxi, Princess, SD1 and TR7 customers suffered as a result.

  8. Fascinating story of the Leyland 500 diesel – the “headless wonder”. I believe that it also had a close relative, a horizontally opposed flat 12 that was used in tanks. Its reputation for unreliability was equally terrible, surely not a good thing in a tank! Anybody know anything more about these, or am I talking rubbish?
    Cheers, David

    • The tank engine was a scavenged two stroke opposed-piston design, like a Junkers or a Napier-Deltic. The pistons faced each other in the cylinder, and breathing was by ports in the centre, not valves, so it had nothing really in common with the 500 in this article.

  9. I`ll answer these seperately.

    Chris: Valve gear failed fairly frequently especially in the highest rated vertical truck units, they were partial to dropping valves. To repair them, the crankcase had to be split to gain access.

    David: The tank engine was the L60 and I think it was fitted to the Cheiftain. A horrible engine so my father told me, designed to run on any grade of fuel and was also a two stroke I think!

  10. Super good article.500 could become a good engine,but TYPICALLY English end op with poor quality.In my company we have bus with TL 11 engine that has driven over 1 million km without problem.

  11. What happened to Albert Fogg,had he not been the Research Director at Austin Morris. Or am I seeing things.

  12. Splendid write up Mike!

    Do you have an exploded view of this engine? Have always been curious to see this what this characterful lump looked like! Nationals always give me memories of that rear fan behind the mesh window being spun round on the early ones, Why did they later blank it from view? I always remember the high pitched scream from the Mk1s, The mk2s had a very Deep growl and for me lost some of the Character, Northern Transport were huge customers of these, later replaced by the Lynx, (They still have a few mk2s re powered by Volvo.. now used for training…I think some are for sale?), Wonder why Mr Fogg didnt use Hydraulic Tappets instead of those fiddly shims? And “Dropping Valves?” did the Valves burn out or bounce off the Pistons? Or did they literally fall out..?

    • The LN2 had it’s radiator positioned at the front rather than rear, negating the need of a fan. The LN2 had a more bulbous front to accommodate the rad. The LN2 had the 680, TL11 or Garnder’s 6HLXB/6HLXCT engines as apposed the 510 on the Mk1.

  13. I know that one operator on a couple of Nationals, removed the turbocharger and made new manifolds. The buses were a lot slower, but didn’t smoke anywhere near as much.

    Also the vertical 501 series was used by Bristol(BLMC) in the VR series 3 bus chassis. All of the exported RELL chassis to New Zealand were also Leyland 510 powered, and do sound strange.

  14. The 500 was the engineering equivalent of driving up a blind alley and straight into the wall at the end. Head gaskets are easy to fix generally and tend only to be a problem with badly designed alloy castings with insufficient stiffness and / or bolting arrangements. Tolerances are also a key factor but hey, making a flat surface is not rocket science!

    The benefits of a separate cylinder head are that you can replace and regrind valves easily, replace piston rings easily and even inspect water and oil passages for corrosion and blockage more easily.

    If fixed heads were a good idea, all modern engines would have them. As for shim and bucket tappets, another shot in the foot. Gapping them needs a collection of shims and stripping the cam out of the carriers to replace. another idea that is long past its sell by date.

    I can remember never driving too close to a National due to the plume of smoke whenever they accelerated!

  15. Fixed cylinder heads are just a cost saving measure in manufacture and nothing else. Less components to fit and less tolerances to meet.
    They may as well have made it a 2 stroke.

    It’s like deleting the drain tap from radiators because they have the potential to leak, in reality it just shaves a few pence off cost of a radiator.

  16. Great story and one that brings back smoky memories.

    Like the crude, non-tilt Ergomatic cab; WTF were they thinking?

  17. On the tank engine, from 1940-43 a Meadows 12-cyl horizontally-opposed unit of 300bhp was mounted in Covenanter tanks, none of which were cleared as battle-worthy. Water and oil cooling arrangements were clumsy and ineffective and together with other design flaws contributed to the Covenanter’s sorry reputation. It was withdrawn from service without firing a shot in anger but not before 1700 useless vehicles had been produced by the manufacturers (LMS Railway, English Electric and Leylands).

    The Leyland L60 was a 19-litre 6-cyl 2-stroke compression ignition unit producing 585bhp in its prototype form and 720bhp in its ultimate service form after many years of struggle to overcome its deficiencies. It was promoted by an influential faction within the tank design function in response to a 1957 NATO edict on multi-fuel engines, replacing the original powerplant concept of a V8 diesel. The L60 was derived from a Junkers Jumo aero engine, which had independently fathered a high-speed marine diesel unit for air-sea rescue launches and which had then led to the basic design of the Deltic diesel locomotive engine.

  18. I know this is not exactly related to the essay, but why have we missed the stories of the Leyland tractors? Okay, so they probably do not bring on the excitement or fond memories of the car range to most folk.
    But these were probably the best product that Leyland produced through it’s troubled history. Here in New Zealand you can still find many of these amazing machines humbly earning their keep down on the farm as their tinwork slowly rots away.

  19. i drove a new 1974 bison for wimpey they had a hole fleet of them.i loved it went like s/off a shovel happy days.

  20. Oddly enough, the early horizontal installation in Nationals had a design fault in the sump that enabled oil starvation whenever the bus turned right.

  21. I have just bought a 1974 Bison with a fixed head turbo from ebay,  should have her home next week,  there were quite a lot of Bisons with the 500 engine about when I left school and I think if you got a bad one you could throw it away, but if you got a god one it was great.  There was no grey area.  The sound they made was like nothing else.  I can remember a haulier in Inverness who had so much trouble with his Bisons with the 500 he thought about fitting the Leyland 410 to them.  But another haulier I knew had so much trouble with a 410 in his Reiver he threw it away and fitted a 500 to her.

    • I too ran Leyland Reiver tippers as an owner driver and had the same problems with head gasket failure and being a time served diesel fitter could never understand why the fault kept occurring. I eveventully gave in and purchased a Bison with a 510 in and you are right it went like SOAS for about 40000 miles then the flywheel bolts sheared and totally destroyed everything in sight. When the engine was stripped to fit a new crankshaft /flywheel clutch and housing I decided too remove the pistons and discovered that the pistons had all split due to excessive heat build up in the cylinders. Leyland had been advised too intercool these 510 and 410 engines to lower combustion temperatures in the cylinders but they just ignored the advise. They opened the door to Volvo and Scania who always turbo-inter cooled their engines which ran faultlessly for hundreds of thousand miles, what a BOS Leyland were and I am so glad they went bust.

      • It was arrogance, pure and simple. Their way or the highway. Leyland in the 1970s was like a footballer who had been great but failed to recognise it had got old and flabby and just ended up looking ridiculous. Just a crying shame they took so many other names down with them. I’m biased, but I truly believe Bristol would have been a world-beater now if left to its own devices instead of the constant meddling from the mid 1960s until its closure in 1983.

  22. The L60 final programme was called Sundance. Part of the problem with that motor was the big cloud of smoke generated when starting up – gave position away.

    The reason there was no radiator aperture on later Nationals was the rad was at the front on 680 and Gardner engined ones (Cooling was a problem for export markets with the 500s)

  23. Read an article about the 500,

    6 engines were given to the apprentices / test shop at Leyland they were told to run them to destruction.

    all the engines survived ! running perfectly

    only later the guys found out that the 6 engines had been ” blueprinted ” not off the shop floor,

    Conclusion being sloppy manufacturing processes, as mentioned above,

  24. Ok , i have a Bristol Hess with the 510 . Yes it is smokey ( as picked up by VTNZ ) . Is there a fix or is it replacement ? If the latter , what with and how difficult to exchange ?

  25. We had a fleet of Bison 6wh truck mixers at C & G Concrete Ltd. Our non turbo Fixed Head 500s were adequate.

    On truck mixer application they were left running all day. Start up was followed by loading under the plant with the engine at only medium revs, just pulling 60hp for the mixer. On an average day, 5 loads were delivered locally. At end of day, mixers were washed down to remove concrete from inside the barrel and outside. This used low revs and little power. So there was little thermal stress (hot/cold cycles) to cause internal damage to the complex one piece block.

    We put Allison MT transmissions into all our Bisons from the start. Driver abuse and clutch issues were eliminated. With super singles on the back, site performance and delicate control were brilliant, both forward and reverse.

    Gardner LW and LX, AEC 760 and Leyland 600/680 Power Plus engines had been developed over many years to be reliable. The 500 was light and powerful, but grossly under developed and manufactured badly. It was a strategic disaster.

    Meanwhile DAF continued step by step development of their licence built Leyland 680s to produce very reliable 350hp engines. You know what happened next.

    Leyland should have fully developed their established 400 and 680 engines, instead of betting the farm on the Fixed Head 500 and AEC V8 ego trips. UK industry in general nearly went down the plughole in similar style to Leyland, but luckily Maggie Thatcher arrived in the nick of time.

  26. Thames Water had a large fleet of Bisons back in the day and I worked on them extensively. Agree about cab tilt (First disconnect steering shaft then undo cab rear sprung loaded bolts to tilt) all achieved with an air ratchet to save time. 510 Engine was used to haul sludge around using a manual gearbox. We had at least 8 Bisons. We had a decent set of driverse (one bad one) correct servicing. I fully rebuilt one and can still remember how technically advanced it was. Gear driven overhead cam (gears mounted at rear to de-stress crank) Bucket shim adjustment (no side load on valve, little wear on valve guides and valve seats) every time I checked valve clearance they where correct, I agree a real time eater to set correctly but once done they where done. Centrifugal and standard oil filter, these removed large amounts of dirt from the oil, we used to line the inside of the spinning element in the centrifugal filter with gasket paper saves a huge amount of time trying to scrape out sludge. No drive belts ancillaries gear driven. Removing whole block to rebuild anything to do with valves, pistons etc was a pain Extensive use of RTV sealant and few gaskets. Very advanced engine in its day shame it was not sorted and put together correctly. TL 11 engine used to pull head studs and then head gasket failure. We used to have guy come in drill out tap bigger insert plug, drill and tap to standard stud. These where not unfixable problems just bad management. Good job the rest of country was not suffering from the same problem Ha Ha.

  27. Valves DID used to require regular attention, the exhaust valves would burn out, possible as a result of the over fuelling from the poorly calibrated injector pump.

  28. Hello,

    Fascinating read about these old beasts.

    I did my apprenticeship as a Diesel Mechanic at NZMC in Auckland and we were the Leyland Truck and Bus dealers for NZ.

    The 500/510 engine provided me many hours of overtime and trips into the country to vehicle owners with major problems.

    Some were re powered with DD 6-71’s and others with anything the Owners could afford.

    A bad belmish on the once proud brand of British Leyland and all it had achieved and stood for in my opinion.

  29. What is the displacement of the 510 engine?
    I have a Leyland national and the log book says it 3000cc, is that right?

    • If Its a National series 1 and the original 500 / 510 power unit is still fitted, its a cats whisker under 8.2 litres.

      National 2’s were:

      Gardner = 10.45
      Leyland 680 / L11 / TL11 = 11.6

      • The 680 was not 11.6 litres but 11.1 litres (677 cubic inches). The L11/TL11 was 11.1 litres at 670 cubic inches. 11.6 litreswas the capacity of the DAF engine based on the 680.

  30. had about ten 500 bisons on rmcsw spent days changing faulty o rings on blocks fitted high level header tanks to cab new big end and main shells and oil centrifuge filters we had adowty hyd pump on ns of engine and when drive sheared pto pump would blo w front pump apart happy days though good friends remembered 1965 to2015

  31. I was told by a Leyland design office chap that the original concept of the fixed head was for mainly bus engine use. The big idea was that the 500 would be a very high mileage engine requiring little servicing and be highly reliable!!! When it had achieved this it would removed and rebuilt in dedicated workshops. Another fine mess!

  32. One particular problem with high revving engines like the 500 for bus work is the amount of gear changing. With bus stops often a minute apart, you have to change gear two or three times before slowing again. The 510 in the National needed a second gear start in standard set up, with first being very low. Taking it anywhere near full revs would necessitate a long pause to give a smooth change, but when making 150-200 changes per hour there can be a tendency to rush it, causing strain on the running gear and giving a jerky change.

  33. In the 70’s I was a fresh faced fuel injection mechanic and used to dread any injection equipment related to the 500 series. The Ambac injectors were of poor quality and the injection pumps, mainly Sigma, were a poor man’s answer to Bosch’s quality products.

    • I see the mention here of Ambac injectors and fuel pump, and Bosch’s superior quality. I understand that Bosch took over CAV – does anybody know when? After serving my apprenticeship at Leyland, and a few years at sea as an Engineer Officer, I spent 2 years in the CAV Proving Department before leaving for NZ in 1963. CAV had about 8 of we proving engineers running trials of experimental pumps and injectors all over the UK. That seemed to me to be sensible – trials to ensure reliability, both for the CAV compnentes and the non-standard engine parts. I had no contact with CAV after 1963, but would love to make contact with somebody who knows the history of that excellent company. And who were Ambac?

  34. No wonder hauliers began to move over to far superior lorries from Scania and Volvo, which were setting the standard for reliability and driver comfort, when the problems with the 500 emerged. It could have been a brilliant engine, but the bean counters at British Leyland insisting the drivetrain had to be below 1000 kg wrecked it.

  35. The l60 multifuel as used in the chieftain was indeed based on the junkers jumo which was liscence built by Napier and called the Culvern.Awful bit of 2 stroke junk built by Leyland who built lots of good junk and when they went brought a good company AEC with them.However the 680 was reliable and one of their best.Worked for a quarry owner in the 70s who ran a mixed fleet of AEC,Leyland and Ford and when Hino entered the market here in Ireland about 75 he upgraded and never looked back.Dont know if the unions were the demise of BL or was it their poor products?

  36. All BL products lacked proper development. I worked on the cars and the trucks some good ideas but not developed properly

    • Yes it was common for the first few years of customers to double up as product developers.

      Quite a few BL products eventually came good, but by that time too many prospective buyer had gone elsewhere.

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