[JT] The Rover IOE Engine

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Rover’s IOE engine family was central to its products for very nearly 30 years, powering the cars between 1948 and 1967 and powering Land Rovers between 1948 and 1977 with a four-year break between 1958 and 1962. The engines were always very highly regarded, although they were certainly long in the tooth by the time they were eventually retired. There were also multiple variants, and these make the story a very complicated one. For that reason, I’ve divided it into two parts, and the second one will appear later.

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ROVER’S IOE ENGINES

Origins and prototypes
It’s fairly well known that the Rover IOE (inlet over exhaust) engines began with an experimental V6 design drawn up by chief engine designer Jack Swaine in the late 1930s. Chief Engineer Maurice Wilks decided against going ahead with the V6 but he did like its wedge-shaped combustion chamber, which he thought would suit a new generation of in-line engines. However, he asked Swaine not to start on such a design until he was able to discuss it with Robert Boyle when the latter returned to Rover in October 1938 after a stint with Morris Engines.

The upshot of this was that the engines team set to work on those engines in late 1938 or 1939. The wedge-shaped combustion chamber depended on a sloping joint between the block and the cylinder head, and not surprisingly the engines quickly became known as Sloping Head types at Rover. All the prototype and development engines bore numbers with an identifying SH prefix – which of course stood for that Sloping Head.

The first prototypes made were four-cylinder types, both with a 105mm stroke. A 65.5mm bore gave 1415cc (which would have become the nominal 10hp engine) and a 69.5mm bore gave 1595cc (the nominal 12hp). Both were ready before the Second World War began in September 1939, and Jack Swaine told me that they were “fitted into cars and used throughout the war by senior management.” One of those cars, which also had an experimental chassis, lost its prototype engine and became the Rover single-seater racer in the late 1940s.

It looks as if work on the six-cylinder versions of the design were delayed until peace returned in 1945. There were two sizes of this, too. One was a 14hp and the other a 16hp, and they were intended to replace the pre-war engines of those sizes. Examples of both went into P3 prototypes, probably in 1947. Both probably had the same 105mm stroke as the four-cylinder engines; the 16 had 2103cc with a 65.2mm bore, but we don’t know the dimensions of the 14 for sure.

There was another variant of the Sloping Head engine drawn up in the period immediately after the war, too. Both bore and stroke were scaled down to achieve a 699cc four-cylinder that was intended as an 8hp for the M Model. That engine had a bore of 57mm and a stroke of 68.5mm, and its output was 28bhp at 5000rpm. The M Model, a small two-seater car inspired by the Fiat Topolino and created in anticipation of a demand for economy cars in the early post-war years, was built in prototype form only, and was cancelled in favour of the Land Rover. Six prototypes were built, so there must have been at least six engines, but there are no known survivors.

The first production engines, 1948-1950
The Board of Trade was calling for maximum standardisation in the manufacturing industry, so instead of replacing all four existing engine types directly (that is, 10hp and 12hp four-cylinders, and 14hp and 16hp sixes), Rover introduced just one four-cylinder engine and one six-cylinder type. These were the ones developed as 12hp and 16hp sizes. However, they were not called that in production. The old RAC taxation system was abandoned in favour of a flat rate after 1947, and so Rover decided to name the two new P3 models after their approximate power output ratings. The four-cylinder therefore became a 60 and the six-cylinder became a 75.

The other part of the Rover strategy was to use the 12hp (60) engine in the proposed Land Rover. Design engineer Gordon Bashford used to say that the very first prototype (the legendary Centre-Steer) had a 10hp engine, and many people have assumed that this must have been a 10hp OHV engine from the Rover Ten that was then in production. I think this is highly unlikely.

For a start, to use such an engine would have been nonsensical when there were prototypes of the new Sloping Head engine already in existence. Secondly, pictures of the Centre-Steer show the exhaust system running along the left-hand side of the chassis, which it would with a Sloping Head engine; on the OHV engines, the exhaust emerged on the opposite side. It’s therefore very likely that the Centre-Steer’s engine was one of the 10hp Sloping Head prototypes. The switch to the 12hp type for production may have been a result of the decision not to put the 10hp engine into production, or it may have been made because the 10hp type had insufficient power; we simply don’t know.

For the Land Rover, the 1595cc 12hp or Rover 60 engine was modified with different carburation and a lower compression ratio to tolerate poor-quality petrol. Yet it was still basically the same engine. So from early 1948, there were three versions of the new Sloping Head engine in production. These were the 1595cc four-cylinder, with 52bhp for the Land Rover and 60bhp for the Rover 60, and the 2103cc six-cylinder, with 75bhp for the Rover 75.

There were early plans for both of these engines to go into the P4 saloons that would replace the P3 types in 1949, but the P4 prototypes turned out to be heavier than anticipated. The 2103cc six was therefore given twin carburettors to give the new model a decent turn of speed, and the idea of using the 1595cc four-cylinder was abandoned altogether. As a result, the P4 came only as a six-cylinder 75 model when first introduced.

Of interest is that the earliest four-cylinder production engines had two detachable side-plates on the inlet side of the cylinder block, allowing access for maintenance. On the P3 car engines, these were made from aluminium, but on Land Rover engines they were steel. The cylinder block was subsequently redesigned with core plugs instead of the side-plates, and from autumn 1949 these began to appear on Land Rovers.

 

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Enlarged capacities from 1950
Almost as soon as the Sloping Head engines had been committed to production, Rover began looking at how to develop more powerful versions for later. Work started in earnest during 1950, using the simplest solution of enlarging the cylinder bores.

For the four-cylinder, an enlargement to 77.8mm gave a nominal 2 litres of swept volume (actually 1997cc), and prototype engines were tried successfully in a batch of 50 Land Rovers. The 2-litre engine therefore went forward to production in autumn 1951 for the 1952-model Land Rovers.

Using the same 77.8mm bore for the six-cylinder engine would have created a 3-litre size and taken Rover into the low-volume luxury car class. However, the company wanted to achieve the higher sales volumes available to cars with engines half a litre smaller. They therefore developed the six-cylinder engine with a smaller bore of 73.025mm, which gave a nominal 2.6 litres (actually 2638cc).

Thirty of these six-cylinder engines went into a test batch of Rover 75 saloons during 1950, and one car suffered from cylinder bore scuffing during its evaluation period. The problem was diagnosed as inadequate cooling caused by the siamese-bore design (the water passages were arranged to cool the cylinder bores in pairs, not individually). Repositioning the bores within the block to give water around each cylinder was the only realistic solution.

The engine was therefore redesigned with more widely spaced bores (still of course with the original sloping head design) and in this guise became known as the Spread Bore type; prototypes were numbered with an SB prefix. It entered production in 1953, and with that same 2638cc swept volume it became the engine for the new 1954-model Rover 90 saloon. The mid-range 75 meanwhile retained its 2103cc siamese-bore engine, and the new 60 went into production with a Spread Bore version of the 2-litre engine four-cylinder engine from the Land Rover.

The further spread of Spread Bores
These changes left Rover with a rather untidy engine line-up for 1954. There were two different versions of the 2-litre four-cylinder engine in production, one with the old siamese-bore design for the Land Rover and one with the new spread-bore design for the 60 saloon. There were also both siamese-bore (75, 2103cc) and spread-bore (90, 2638cc) six-cylinder engines.

This had come about because Rover had to stagger major production changes, and rationalisation followed in autumn 1954. At that stage, Land Rovers took on the latest spread-bore four-cylinder engine, their version of the engine being detuned, with a lower compression ratio and different carburation from the car type in the Rover 60. The siamese-bore 2103cc six-cylinder in the Rover 75 also ended production, and was replaced by a spread-bore type. This was readily created by using the existing cylinder block with a new short-stroke crankshaft, and so from autumn 1954 the 75 saloon had a new engine of 2230cc.

So now all the Rover engines had spread bores, and there were just two cylinder blocks being manufactured. The four-cylinder block served the Rover 60 and the Land Rover, with different cylinder heads, carburation, and pistons. The six-cylinder block served the Rover 75 and Rover 90, the main difference being short-throw (75) and long-throw (90) crankshafts.

That neat arrangement was short-lived. Rover now started looking at automatic gearboxes because these were clearly the coming fashion. Automatics in the mid-1950s were not very efficient, and usually absorbed a lot of power to the detriment of performance. Early experiments with one in a Rover 90 must have prompted a search for yet more power from the six-cylinder engine.

The most cost-effective solution was to fit twin carburettors, although neither Maurice Wilks nor Jack Swaine was keen on the idea because they anticipated problems keeping them in tune. Nevertheless, the plan went ahead, and the 2638cc engine appeared in twin-carburettor form with 108bhp in the new 105R saloon in autumn 1956. With its Rover-built automatic transmission, this had acceleration similar to that of the four-cylinder 60, but in the mean time the Rover engineers had recognised that the 108bhp engine would also create a new high-performance model if allied to the standard manual gearbox. So the 105R was accompanied by a
105S that did exactly that.

These were the members of the original Sloping Head family:

1595cc four-cylinder 69.5mm x 105mm, 60bhp, 1948-1949
Land Rover version with 52bhp, 1948-1952
1997cc four-cylinder 77.8mm x 105mm, 52bhp 1952-1954
2103cc six-cylinder 65.2mm x 105mm, 75bhp 1948-1949
Twin-carb version with 80bhp, 1949-1954

These were the members of the Spread Bore family:
1997cc four-cylinder 77.8mm x 105mm, 60bhp, 1953-1959
Land Rover version with 52bhp, 1954-1958
2230cc six-cylinder 73.025mm x 88.9mm with 80bhp, 1954-
1959
2638cc six-cylinder 73.025mm x 105mm, 90bhp, 1953-1959
Twin-carb version with 108bhp, 1956-1959

Next time: the seven-bearing engines.
(The first two pictures show the 799cc engine of the M Model and an end-on view of the P3 60 engine, where the sloping head can be seen very clearly. There are more pictures in the Comments.)

 

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This is the 1595cc engine in Land Rover form for the 80-inch model.



The six-cylinder had a single carburettor and 75bhp for the P3 75 saloon.



This is the 2-litre Spread Bore engine for a mid-1950s Land Rover.



This is a real rarity and may be the only surviving example of the test batch of siamese-bore six-cylinder engines. Picture by Colin Blowers (thanks!). You can see how the cylinder bores are paired.



This is the enlarged capacity (2638cc) Spread Bore six-cylinder for the Rover 90. The air cleaner is a Vokes paper-element type that gave better breathing than the oil-bath types used with some other versions of the IOE engine.



...and finally (for now!) this is a demonstration chassis of the Rover 105 dating from 1959 (the plain 105 replaced the earlier 105S). The engine's twin carburettors and oil-bath air cleaner are very clear here.

 

[Patrick_R]

Bryce.
I’ll be sure to give these full attention when I get home from our Easter trip. These are always of great interest to me.

 

[Patrick_R]

Very interesting read Bryce.
I really knew nothing about this engine.
Thanks for posting.

 

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Here’s the second part of our look at the Rover IOE engine family, which had started life as a design before the Second World War and entered production in 1948. This part of the story is a lot less complicated than the earlier part.

First of all, though, there’s an important correction to my last post. I said there that the final 2.6-litre six-cylinder Land Rover engines were built in 1977. Not so – it was 1980. Read on to see where the new information has come from.

If you enjoy reading the story below, please click the “Like” button because this helps me to judge how popular one subject is against another. And, as always, if you want to share it or copy it, please feel free to do so – but don’t forget to acknowledge where you found it. (https://www.facebook.com/james.t.roverphile)

ROVER’S IOE ENGINES, PART 2
Let’s recap on where we got to last time. By the start of the 1957 season in autumn 1956, the Rover petrol engines were all spread-bore, sloping-head types. There were 2-litre (1997cc) four-cylinders in the Rover 60 and the Land Rover; there was a 2230cc six-cylinder in the Rover 75; the Rover 90 had a 2638cc six-cylinder; and the Rover 105R and 105S had a twin-carburettor version of the 2638cc six-cylinder. In this rationalised range, Rover needed to manufacture only two different blocks – one four-cylinder, and one six-cylinder.

The seven-bearing engines, from 1958
By this stage, though, all these engines were on the way out. The new 2.25-litre OHV four-cylinder was waiting in the wings to replace the sloping-head 2-litre in Land Rovers during 1958, and the same engine would power the Rover 80 saloon that replaced the Rover 60 in mid-1959. Mid-1959 would also see the end of the 2638cc six-cylinders, as they gave way to another new version of Jack Swaine’s original sloping-head design.

Rover had been looking at a V6 engine to replace the spread-bore six-cylinder, which by the middle of the 1950s had just about reached the limit of its development. However, work on the V6 was halted, perhaps because of cost, and by 1956 the engines team had been asked to come up with a 3-litre version of the straight-six to power the planned new Rover 3-litre, or P5 saloon, that would take the marque into a higher price bracket.

Jack Swaine gave the job of designing this to his assistant, Norman Bryden, who came up with what Swaine described to me as an “elegant” solution. Bryden retained the 105mm stroke but enlarged the bores to the 77.8mm of the four-cylinder engine, which gave a swept volume of 2995cc. To accommodate the larger bores, he redesigned the block yet again with wider bore spacing, and this gave him room to add a seven-bearing crankshaft in place of the earlier four-bearing type. It was ideal to ensure smooth running and reduce stresses within the engine. The new engine was introduced for the new 3-litre models in autumn 1958 and would always be known within Rover as the 3L7 (3-litre, seven-bearing) type.

The “next stage” development of this engine was to get more power out of it, and to do this Rover redesigned the cylinder head with a bolt-on inlet manifold instead of the original integral type, so giving better control of the inside surfaces of the castings to improve gas flow. With further assistance from independent performance consultant Harry Weslake, they prepared this to enter production in 1962 and, more for publicity than for any other reason, it was always known as the Weslake-head engine.

There was another “next stage”, too, and this time it involved creating a 2.6-litre version of the new seven-bearing engine to replace the old six-cylinders in the P4 range. This was achieved by using a short-stroke (97.025mm) crankshaft in the 3-litre block to give 2625cc, and the first version of that engine was introduced in autumn 1959 in the Rover 100 saloon. Much the same engine was used in the Rover 95 (autumn 1962), and this was accompanied by a more powerful version with the Weslake head in the Rover 110 saloon.

Not very well known is that Rover also used the Weslake-head 2.6-litre engine in some export versions of the P5. In addition, they developed a special 2.4-litre (2490cc) engine with an even shorter stroke for Austria. It didn’t sell; just 25 were made before the model was dropped. (All this is yet another story for another time!)

Land Rover six-cylinders, from 1962
The last, and longest, chapter in the Rover IOE engine story focuses on Land Rovers and begins in 1962. It overlaps with the last years of the engine’s use in Rover cars: the 2.6-litre engine disappeared from Rover saloons in summer 1964, and the 3-litre followed it in summer 1967.
The full story of Rover’s experiments is one more quite involved story that I’ll come back to one day. For the moment, let’s just say that the experiments began when a larger and heavier Land Rover called the 129 was under development from 1959. It didn’t enter production, but it kick-started the idea of a six-cylinder Land Rover and several experimental vehicles were built in the first half of the 1960s.

The idea gained wings in 1962, when it became clear that the new Forward Control Land Rover was badly underpowered with its 2.25-litre four-cylinder engine. The Rover engineers knew that the torque of the 3-litre six was enough to break half-shafts and other components in short order (probably mainly when multiplied through the transfer box in low range), but the seven-bearing 2.6-litre in detuned form was just right for the job. So that engine became an option for the Forward Controls, and remained so until they went out of production in 1971.

That in turn led to the idea of a six-cylinder normal-control (or “bonneted”) Land Rover, and two 2.6-litre types eventually entered production. First, in summer 1966, was a special NADA (North American Dollar Area) model with the 123bhp Weslake-head engine from the P4 110 saloon. The Americans wanted the performance, but in 1967 the rest of the world got the second type, which was detuned to a rather miserable 81bhp and didn’t have the Weslake head, but did offer a smoother experience than the four-cylinder. That engine was also made standard in the 109 One Ton model of 1969 – although of course when the British armed forces wanted some One Tons with four-cylinder engines, a batch was made specially for them.

The six-cylinder Land Rover with its low-compression 81bhp engine remained in production for another 13 years, although the high-compression NADA type lasted for one year only. Rover – by then, strictly speaking, Land Rover Ltd – built the last examples of the low-compression type in 1980 for Series III models.

I’m grateful to Brain Rawson for copying me a document from his collection that was drawn up to commemorate the production of the last IOE engine. Although its account of the engine’s history contains several errors, I have no doubt that it’s genuine. Signed and dated by the manager of the Tyseley factory where the engines were assembled, it congratulates the nine members of the team who had been building the IOE engines since the first ones were made in 1948. The date is 15 August 1980, which means that the various members of that engine family were in production for 32 years.

These were the members of the 3L7 family:
2995cc “3-litre” 77.8mm x 105mm, 115bhp, 1958-1967
with Weslake head and 134bhp from 1962
2625cc 77.8mm x 97.025mm, 104bhp, 1959-1964
with Weslake head and 123bhp from 1962
Land Rover version with 81bhp, 1967-1980
Land Rover Weslake head 123bhp version, 1966-1967
2490cc Unknown bore x 105mm, 109bhp, 1962-1963

(The first two pictures show an early 3-litre engine in a 1961 P5 Saloon, with the original oil-bath air cleaner; and a version of the engine in a 129-inch Land Rover prototype, in this case with big twin canister-type air cleaners in the background. More pictures in the Comments.)

 

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This one is a Weslake-head 2.6-litre in a Rover 110. The twin-trumpet paper-filter air cleaner was characteristic of the Weslake-head engines.



The 2.6-litre (non-Weslake) engine was used in the Forward Control Land Rovers. As you can see here, access for servicing wasn't altogether straightforward. This is a later IIB 110-inch model, with the "overhead" gearchange.


Serious rarity here, although it doesn't look particularly special.... this is the 2.4-litre Weslake-head engine in a Mk IA P5 Saloon built for Austria. (Thanks for the picture to the Mayr-Harting family.)


Reading through this again, I can see that I’ve introduced some unnecessary confusion here. Rover would not have changed the block for the 2.4-litre engine; they would have changed the crankshaft. So the stroke wasn’t 105mm as I suggested in the table. The bore must have remained at 77.8mm and the stroke would have been shorter than that on the 2.6 engines. That would have impacted on torque, and in a heavy car like the P5 you needed all the torque you could get. If the acceleration was badly affected, that would help to explain why they couldn’t find customers for the 2.4-litre P5.

Last one for now: this is the 2.6-litre in a normal-control Land Rover 109. The early ones like this had a single SU HD6 carburettor, which was a compromise while supplies of the Zenith 175 CD-2S built up.

 

[sean sherry]

In a practical sense ... I went round Australia in 1957 in a Land Rover with the entirely trouble free 4 cyl Engine... Ran like a Top after I reduced the size of the Main Jet in the Zenith Carb. Would start in the Melbourne Winter without using the Choke. Only incident was when the screws holding down the base plate of the Lucas Distributer came loose causing an intermittent miss fire.
An excellent Engine.