Engines : O-Series

The O-Series was designed to act as the company’s mid-range engine mainstay for the late 1970s and beyond.

If at first you don’t succeed…

The formation of Austin-Morris from the ashes of BMC in 1969 allowed BLMC’s product planners to focus on the demands of the middle market, without worrying too much about the more specialised demands of Rover-Triumph. One thing was clear – lots of new car development may have taken place under the old regime, but none of it was very close to seeing the light of day following the Leyland takeover. Designing new cars was one thing, but what about engines? After all, the A- and B-Series engines were both well over ten years old, by this time and yet, were proving difficult to replace.

In the case of the B-Series, it was not as if it were a bad engine; in fact, in twin carburettor form it proved more than adequate of propelling the Morris Marina and MGB with some degree of verve. The B-Series engine’s troubles lay its inability to pass the upcoming US emission laws, proposed for 1975, and given that the MGB was sold in such huge numbers over there, it was essential for BLMC to develop a “clean” engine.

It was in 1971, that the need was formalised, and a project to develop a B-Series replacement was instigated. At the time, it was Geoffrey Johnson’s responsibility, as Chief Design and Development Engineer, to oversee this plan. Reporting directly to Harry Webster, Johnson (who himself was probably still in some way finding his way around the Longbridge “system” as a new recruit to the company) wrote to his boss, laying out what the best plan of attack would be. He stated that an OHC version of the B-Series, sporting an aluminium cylinder head and displacing 2-litres would deliver more than enough power and torque to outgun the B-Series, even in “anti-smog” guise.

Engine development was carried out on an earlier 2-litre B-Series prototype, produced by BMC engine-man Stan Johnson. This engine dated back to 1964-65, but was not considered a clever enough update of the original B-Series engine to warrant serious consideration. This particular unit displaced 1998cc and featured siamesed cylinder bores and off-set conrods in order to use the existing 1.2-/1.5-litre cylinder block.

According to Lyndsay Porter’s AUTOCAR O-Series article, it was in the summer of 1972, that this plan was dealt something of a shattering blow.

“In the summer of 1972 a bombshell report landed on Webster’s desk. The B-OHC engine had been developed to the point where the block had been revised to take advantage of the absence of pushrods, although the camshaft was retained as a jackshaft to drive the distributor and oil pump. The problem was that the engine could not be built!”

Essentially, B-Series tooling was past its prime. Porter again: “The plattens which guided the cutting tools into position had been re-drilled so many times that they were like ‘paper doyleys’ in the words of one Longbridge engineer. ‘If it hadn’t been for the good grace of the blokes on the shopfloor, who would pack the machines into line daily using shims or even bits of cigarette paper, the whole thing would have been packed up years before,’ another engineer has told me.

So that was it – BLMC’s ambitions for the Austin Marina allied with continuing MGB success in the USA meant that increased output was required – and that would require an entirely new facility in order to meet demand.

However, this set-back did not completely arrest B-OHC’s development, even though it should have. Throughout 1972, work continued, and projected (gross) power figures were reported back to management: 112-115bhp for B-OHC, compared with 106bhp for the 2-litre B-OHV and 96bhp for the existing standard MGB unit. By September 1972, the design for the prototype engines was pretty much finalised, and although anticipated production volumes were still fluctuating, it was still to be produced in serious numbers, given that it was planned to be used in the MGB, Marina (ADO73 facelift version) and the ADO71. As can be seen from this list of cars, B-OHC would need to be capable of being used in in-line as well as transverse installations – as well as being compatible with the ADO71’s T-I-S gearbox, as well as the Marina/MGB’s end-on unit.

Because of the tremendous need to get the new engine into production at the earliest opportunity, Geoffrey Johnson was required to order new tooling for the engine’s production line, even though it was yet to be formally signed off! This led to all sorts of problems with the final development phase of the engine – after all, it severely compromised the design process… Porter’s article relates that Geoffrey Johnson really was hamstrung: (We needed to put a breather modification in place), “…but it couldn’t be done because there was already a particularly crucial piece of transfer line machinery where we wanted to put it. Getting round that problem was the biggest difficulty we had with the whole engine.”

By the autumn of 1972, the O-Series name was formally applied to the B-OHC, and it marked the point in time when the final link with the original engine was removed (…”the deletion of the “camshaft”/jackshaft so that the whole B-Series tappet chest could be pared away”). The B-Series’ design had acted as a starting point, but no parts were shared – and this is demonstrated by the cylinder block’s light weight (due to the walls being thinned), meaning that the strength of block could be braced in to the exact tolerance required. This weight management would underpin the O-Series’ transformation into the M-Series during the 1980s.

However, it was not all sweetness and light. Although the engine casting could now be considered an all-new unit, Harry Webster decreed that the B-Series powered Marina’s cast crankshaft could be used in the O-Series, in order to amortise that item’s costs to the company…

Development chronology

The closing months of 1972: It was stated that it was critical to get the O-Series in production by April 1974, so that the US-Spec Marina and MGBs would be ready for the tighter emmissions imposed for the 1975 model year. This plan was changed when it was decided that the MGB would be killed-off in favour of the TR7 – although the O-Series would still be a requirement for US Marinas. Shortly after this, the MGB was given a reprieve…

February 1973: Marketing decided that the MGB would be replaced by the TR7, but it would remain in production as an insurance against the late introduction of the new Triumph.

May 1973: The decision was taken to use the O-Series in the MGB after all – and that it was possible to sell 1000 ‘Bs per week.

Summer 1974: The O-Series was proclaimed as an essential addition to the MGB for the 1977 model year… until the following month, when this was put back until 1978.

Mid-1975: North American Austin Marina cancelled…

If this list looks bad, remember that the company was going through a degree of turmoil at the time, as model plans were revised, or cancelled at an alarming rate. Throughout this period, two things were constant – the MGB’s O-Series conversion had become a running issue within the company, especially when viewed alongside long-term TR7 plans. The second factor was that the O-Series would be used in the front wheel drive Princess and the rear wheel drive Marina. The only question was – when.

O-Series finally launched

The first production car to use the O-Series arrived in 1978 – and that was the Marina. Sadly, Harry Webster’s preferred plan of offering the O-Series in 1.6- and 2-litre form was scuppered by the cost advantages of using 2-litre version’s head casting on the smaller unit, raising the capacity to the unusual size of 1.7-litres. Advantages over the B-Series unit were a lower all-up weight, but in terms of refinement and performance, the motoring press found little to get excited about.

Shortly afterwards, the O-Series Princess 2 appeared – and this was the first showing of the larger 2-litre version. The 1994cc unit produced a less than sparkling 93bhp at 4900rpm in single carburettor form, and did nothing to improve the Princess reputation with buyers for being a slow car.

That is not to say that the O-Series was without its merits; only that it was a bit of a “rough diamond” in the early years, offering little advantage over the outgoing B-Series – certainly in terms of performance and refinement. As stated though, it was a good 20kgs lighter than the B-Series engine, and the block was shorter and therefore easier to package. Also, the aluminium cylinder head allowed for unleaded fuel usage… (it is a shame that the O-Series powered MGB never made it).

It was also usefully reliable.

It also figured in Austin-Rover’s 1980s plan for prosperity.

In 1980, the first rear wheel drive 2-litre O-Series powered car appeared: the Ital 2.0 Automatic. In 1982, a 2-litre twin carb version was installed under the Rover SD1’s sleek bonnet, and then in transverse form in the Ambassador. In 1984, it appeared in the Montego and Maestro with an end-on Honda designed gearbox (and very effective it was, too) – so there was no doubt that the O-Series was an adaptable engine.

Of course, the O-Series story did not finish with the Maestro and Montego – and after an intensive development programme during the mid-1980s, it was transformed into the twin-cam 16V M-Series…

Fundamentally, the O-Series was a good engine, and if nothing else, demonstrates that when asked, BL could get the job done in the most trying of circumstances. Perhaps, it was launched when in need of final development, but given the lack of resources available to BL at the time, it is a miracle that it appeared at all.

Keith Adams


  1. No matter how many times you read this excellent site you cant help questioning What was the logic in….or Why did they…. Here is another example whilst not a bad engine, It certainly was highly adaptable and had a long life. But why develop this when they had the Triumph/Saab unit? which by the time of the O’s development The Triumph unit had 16valves and was as far as I know, already capable of Smog testing.

    Could they have saved a few ££ by just using Triumph engines? Although the Sprint engine had reliability issues but with extra funding they could of cured these problems, These engines lived a long carrier with SAAB (some say were better than the GM cast offs that replaced them).

    Perhaps it was a case of keeping Triumph special and unique which only used its own engines (Stag springs to mind, though the last of the Midgets used the Triumph unit) and keeping the Austin/Morris stuff to themselves (The Marina not using Dolomite suspension etc but did have a Triumph Gearbox, which I think needed a different box housing to accept the B series engine?).

    There was possibly good reason Why they spent Millions transferring the B into the O but on paper developing 1.7 and 2.0 engines when they already had 1850 and 2.0 sounds a bit Mad, But then again they already had the E series 1.5, 1750. and 2200 (E6).

    History would later repeat the process of developing existing size engines to compete with each other, The Triumph/Rover 6 2.3/2.6 when they already had the E6.. and later the S when the O could possibly be reduced to 1.6? Leyland/Austin Rover moved in mysterious ways…

    • Good point with regards the slant 4. If they had gone with that as the corporate 2 litre and the E series for the 1.6 the money saved on O could have been spent on a decent end on transmission/transaxle for FWD cars – adapted from the SD1/TR7 LT77 box perhaps – allowing the noisy, nasty shifting gear in sump boxes to be dumped.

      • This was seventies British Leyland where common sense didn’t exist and where internal rivalries ruled. Can’t have a Triumph engine in an Austin, old chap, Austin have to use their own engines. Thus you had the ridiculous situation where you had the E, O and Triumph engines, although the end of the Dolomite in 1980 saw the end of the 1850 and 2000 Triumph engines.

  2. Superb torque in 2.0 EFi guise,easily a match for a VW Golf GTi’s Torque-even if not as creamy at top end.

  3. @2 good point Mr Brett, But I always thought Leyland at that time were pretty Skint ? It wasn’t long after it was Bankrupt, So was the O squandering money?, Could the money have been better spent else where? Could they have made the Triumph with more Torque and Fuel Injection? There will be a reason behind the need for two engines, Just another tale of strange decisions.

  4. @1

    There was a stillborn proposal to have a twin-cam version of the 1709cc Saab/Triumph Slant-4 power a Marina and later a Toledo as a rival for the BDA-powered Ford Escort RS1600.

    Also initial prototype versions of the Saab Slant-4 engine were a 1.3 with 55 hp and a 1.5 with 68 hp (later 80 hp), which in hindsight would have probably been more suitable for the Saab 96/95 than the roughly Ford V4 engines.

    Unless Triumph itself had plans to develop a Slant-4 diesel, perhaps the O-Series could have at least lived on in diesel form though regarding the issue of competing engines, things would have been much better off had Leyland and BMC remained separate entities with reshuffled or rationalised brand portfolios.

  5. The O-series did effectively live on in disel form initially as the Perkins Prima unit then as the L series unit. It would have further evolved into the G series but for MG Rover going bankrupt.

  6. Does anyone know whether the O series block and all the rest of it will mate up with the gearbox in sump B series 1.8 sump and clutch etc? Prima conversion could be good if it does.

  7. Some years ago I read an article online about the O series written by, from memory, one of the senior engineers involved with the project. Unfortunately, I’ve never been able to find it again since but I think the following is reasonably accurate.

    When it first went into production, despite having pretty good bhp figures, it wasn’t a particularly driveable engine and got less than glowing reviews from the motoring press.
    His view was basically that maximum torque occurred too high up in the rev range and I think I quote; “people don’t drive the bhp curve, they drive the torque curve”.
    After quite some fighting, he was finally allowed to revise certain things, I guess mainly valve timing but I can’t remember now, to reduce the maximum torque revs and possibly flatten the torque curve too but that’s also lost in the mists of time.
    After the revisions, in his view it became an excellent engine.

  8. The 2 litre version in the Princess made the Princess compete more effectively with 2 litre Cortinas and Cavaliers, as the 1.7 was too small and the 2.2 E6 was too big and thirsty. While no ball of fire in the Princess, it endowed the car with similar performance and economy to a 2 litre Cortina. Also 1.7 versions of the engine in the Marina/ Ital made the cars quieter and more economical than the ageing B series, and the Princess saw an increase in economy, even if the 1.7 that replaced the 1.8 offered no performance benefits in the bigger car.

  9. I also think we need to remember that at this time the whole company was so unionized in a militant way the management walked on eggshells effectively making poor business decisions to placate the unions. For example when we say why not just use the Slant 4 and do away with the B series….well that would not have meant unemployment for staff making the B series and in turn an enormous strike. Just look what the unions did to TR7 & Stag production, they basically closed the brand new factory at Speke for 12 months and that effectively finished Triumph as an attractive brand.

  10. My fathers third Princess after 2 lousy 2200’s was a 2000 ‘O’Series engine equipped car. He drove this one in Holland on LPG for 185000 kilometers and ad it was completely faultless, Think about how great Princess could have been and other BL cars had this engine be introduced earlier.

  11. The early parts of the O-Series story when it was known as B-OHC make one think of the underdeveloped revised C-Series, mainly had the latter not existed or been discontinued early on during development in place of a 6-cylinder B-Series version of the former (the 4-cylinder reaching 2-litres in early-60s). Would it have been possible for a similarly revised 4/6-cylinder B-Series to effectively be in essence a realised B-OHC or O-Series that enters production in the later part of the 1960s (like the revised C-Series in 1967), before the B-Series tooling was completely past its prime as was the case when B-OHC was conceived in 1971-1972 instead of finally reaching production in 1978?

  12. For myself and many other engine designers at Longbridge, the O Series will be remembered for creating a glutenous year or more of weekend overtime. A very welcome meal ticket. I left Longbridge well before the O Series entered production. In fact the very first production O Series engine I ever saw was 25 years later when I found one being used as an anchor on the end of a rusty chain on Eastbourne beach near my home. I actually shed a tear – or was it the wind in my eyes?

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