The inner workings of the long-lived BMW M10 engine.
Inside BMW’s long-serving M10 engine which served in the 316 and also as 1100 bhp F1 powerplant.
T he M10’s debut at the 1961 Frankfurt motor show was in the all-new BMW 1500 ‘Neue Klasse’ (New Class) saloon intended to plug the widening hole in BMW sales and save it from bankruptcy. The 1500 was a completely new and very modern car – neat styling, front disc brakes, all independent suspension with rear semi trailing arms and under the bonnet, the all-new alloy-head SOHC engine.
Named M10, this engine took nothing from previous BMW engines apart from the rigorous quality. The iron block was immensely strong – just how strong we’ll see later – but it had large 82mm bores and the five crank main bearings set high up into the block for extra rigidity. The crank was forged steel with two-inch big ends and decent sized counterweights. Stubby forged steel connecting rods with studs and nuts for the big ends caps linked onto forged alloy pistons.
The oil pump was bolted to the front base of the block and was run from a separate chain from a sprocket on the nose of the crank. A pressed steel sump was used. The cylinder head was alloy with hemispherical combustion chambers and large valves, 39mm inlets and 35mm exhausts.
Because of the operating angle, BMW needed either two camshafts or a single cam with rockers either side and the last option was chosen. Long rocker shafts were fitted into the head either side of the central camshaft with cast alloy rockers that had steel tips where they met the cam lobe and steel eccentric adjuster ends that operated on the valves. These were secured by a 10mm nut and steel bolt and could be slackened, the eccentric turned until the correct valve clearance obtained and retightened. The cam was driven by a duplex timing chain from the crank with a simple channel rail on the inlet side and a tensioned sprocket on the exhaust side.
Cam lubrication was by a steel tube with suitable oil holes that squirted oil on the area between two cam lobes (it gets onto both lobes) and the oil supply came from two oil galleries in the head and a pair of banjo bolts that took the oil supply and directed it into the steel tube. A cast alloy cam cover with studs and nuts was used.
Carburation was by a single Solex 36 PDSI single choke carb and the pump was bolted to the side of the cylinder head with a pushrod running from an eccentric cam lobe to operate it. The distributor was fitted at the back of the head and ran from a gear on the camshaft. The cooling system was slightly unconventional because whilst the radiator and water pump were pretty normal, the thermostat was a remote unit that linked the hoses from the top and bottom of the radiator and the water pump.
Bore and stroke were 82mm by 71mm to give a capacity of 1499cc and power was rated at 80bhp – a far cry from the 50-odd bhp of a single carb BMC 1500 B-Series.
Apart from a few instances of connecting rod breakage of the first batch of 1500s, the M10 was a very tough and reliable
engine suited to high speed operation on the Autobahn and it was well received when it went into production in February 1962. The first change was a capacity increase to 1800cc in September 1963. This extra capacity was achieved by an increase in both the bore size to 84mm and also stroke to 80 mm to give 1773cc. Power was 90bhp with a single 38 PDSI single choke Solex carb.
In 1964, to reduce manufacturing costs, the 1500 was replaced by the 1600, and its 1573cc engine was arrived at by combining the 1800 block with the 1500 crankshaft. By this time, the tensioner sprocket for the timing chain had been replaced by a nylon coated blade that proved to be very reliable. 1964 also saw the launch of two important new models, the 1800 Ti and 1800 TiSa. The Ti was a logical upgrade of the standard 1800, a car begging for more power to exploit its good handling.
As well as the suspension and tyre upgrades, the engine was tweaked with bigger valves (42 mm inlet, and 38mm exhausts on later versions), a new cam profile as well as a higher compression and the most visible difference, a pair of sidedraught 40PHH Solex carbs.
With these mods, power shot up to 110bhp but more was to come from the TiSa. The 1800Ti was a competitor for the Alfa Romeo Giulia as well as the Lotus Cortina but needed some parts homologating to be able to compete in racing. The compression was raised to 10.5:1, the Solex carbs replaced by 45DCOE Webers to give 130bhp, impressive stuff for 1964.
In 1965, the BMW 2000C Coupe arrived and with it the biggest stretch of the M10 to 1990cc. This was achieved by increasing the bore size to 89mm and retaining the 80mm stroke of the 1800. Two versions were available – the 2000C
(mostly as an automatic) had a single 40 PDSI Solex to give 100 bhp on a 8.5:1 compression, and as a 2000CS (manual only), 120bhp with twin Solex 40 PHH carbs from the 1800Ti and a 9.3:1 compression - all 2000 engines had the bigger 42mm and 38mm inlet and exhaust valves. In 1966. these two 2000 engines were used in the 2000 saloon (single carb) and 2000 Ti (twin carb).
In 1968, the popular 1800 engine was revived with a capacity change from 1773cc to 1766cc. That happened because BMW needed to reduce costs and they did this by combining the 2000’s 89 mm bore with the original 71mm crank stroke from the 1500 and 1600. The smaller two-door BMWs arrived in 1966 starting with the 1600 and joined by a 2 litre version in 1968 called the 2002 – this was joined by a 120bhp Ti version at the same time.
1969 saw a new 130bhp fuel injection version of the 2000 engine. Fitted to the 2002Tii from 1971 and the 1969 2000Tii (the 2000 Coupés had been discontinued in 1968) the new engine used mechanical Kügelfischer injection requiring a new head casting, and the compression ratio was 10:1.
In 1972, the New Class was replaced by the new E12 5 Series with 520 and 520i models. The M10 engine was revised with new ‘triple hemi’ cylinder head chamber and port designs from the six cylinder M30 engine, new pistons and new carburettors. The 520 used twin Stromberg 175 CDET carbs whilst the 520i used Kügelfischer injection – the 2002 went to the new E12 units but the carb version retained the single Solex.
The next change arrived in 1975 when the 2002 was replaced by the new E21 3Series. Again, the head and piston designs were revised as was the fuelling. The 316 and 320 (1573cc and 1990cc) used the twin choke Solex 32/32DIDTA carb introduced on the 2002 in 1972 and the 320i used Bosch K Jetronic injection. These new units were also fitted to the BMW 518 (1766cc) and 520 / 520i models from September 1975.
In late 1977, the 2 litre M10 was discontinued and replaced by an all new 1990 cc straight six, the M60/M20 unit and
these new sixes were used in the 320 and 520 models with the injection versions discontinued for the time being. The 1600 and 1800 versions continued until September 1980 when a couple of important changes were made. The Duplex timing chain was replaced by a Simplex (single row) chain, the gear on the back of the cam was changed so that the distributor now ran the other way and carburation was changed again with a new Pierburg twin choke 2B4 carb phased in to replace the old Solexes.
Electronic ignition as also fitted. A 1.8 injection with K Jetronic arrived for the E21 318i and E28 518i models (not for the UK yet) and these engines were used in the original E30 316 (1766cc) and 318i models. In September 1983, the 316 and 518 received the Solex Pierburg 2BE ECU controlled carb and the injection models received Bosch LE Jetronic fuel injection. Power outputs were 90 bhp and 105 bhp respectively.
By this time, the M10 was over 20 years old and a replacement was already in the design stage. In September 1987, the facelifted (plastic bumper) 318i went over to the all new M40 engine, a unit sharing design features with the equally new V12 engine and which was lighter, more powerful and much more economical.
Only two cars now used the M10 by then: the carbed 316 and the injected 518i E28. Carb and injection units used different heads with port differences that make them non interchangeable. 518i production ended in December 1987 and the 316 finally ceased production in July 1988 to be replaced by a 1600 M40, and that was the end of the M10 engine after over 25 years in production. In a way, it carried on a bit longer because the 16-valve S14 unit in the original E30 M3 was based on an M10 type block, and capacities ranged from 2.0 in the 320iS through to 2.3 and finally 2.5 litres. THE 2002 TURBO The 2002 Turbo was not, as is often claimed, the first turbocharged road car - General Motors had that years before. But the 2002 Turbo was the first European turbo road car and it went into production in July 1973. Bassed on a 2002 Tii unit, the turbo had a
very low 6.9:1 compression, a KKK turbocharger and Kügelfisher injection but with a slide throttle rather than a standard butterfly type. Power was rated at 170bhp at a high 5800rpm with 180 nm of torque. Production ended in 1974 after only 1672 cars were built.
In the mid eighties, BMW built a number of M12 engines for Formula 1. These were turbocharged 1500 cc units and in 1983, Brabham using the M12 gave 800bhp in qualifying and 650 bhp for racing – by 1985 the qualifying bhp had risen to 1100 and the engine took the 1983 World Championship in a Brabham for Nelson Piquet. What has this got to do with the M10 engine? The M12 was based on an M10 block, highly modified but secondhand high mileage ones at that due to casting stresses being long gone. STRENGTHS AND WEAKNESSES Given 6000-mile oil changes, the M10 will do 100,000 miles and we know of a 1987 518i that did over 210,000 and was still running well. The forged steel crank was gradually phased out on under 2-litre engines during the 1970s, not that it made a lot of difference. Valve guide wear was quite common leading to blue smoke and top end clatter but again, regular oil changes helped prevent much of that.
Cam wear was caused again by poor maintenance - you need to ensure the oil spray bar holes are clear ands the banjos are tight - the spray bar can be removed from its saddles and the holes enlarged with a small drill, whilst BMW sell pre-threadlocked banjos with a very slightly different thread
pitch to prevent any possibility of them working loose. Do not threadlock them yourself as it will congeal in the centre of the banjo and starve the cam of oil. Be aware the spray bar only fits one way – fit it the wrong way around and it will only lubricate four of the eight lobes. Cam sprockets on the post 1980 engines with the single row chain can wear badly, but they’re easily replaced. Cylinder head corrosion can be a problem if the antifreeze is not replaced on time. Heads can be welded and skimmed but if you want to replace it, make sure the head is replaced with the correct type – 121, E12 or E21 – the markings are cast on the side.
Bottom end problems are rare, but an oil light that takes a long time to go out can be a worn oil pump or crank bearings, but can also be a failed ‘O’ ring on the pipe that supplies oil from the pump to the front of the block – you need to remove the front timing covers to access it. Regular tappet adjustment is important and you may find you need overside eccentrics if the rocker pads are worn. Snapped exhaust manifold studs are quite common and a pain to drill out and replace.
Old Pierburg carbs are generally trouble and are best replaced buy either a new Weber or a good condition old DIDTA Solex from an E21. K Jetronic and LE Jetronic are both good and reliable – K Jet needs setting up properly and the air intake bowl cleaning out and the plate resetting if needed, and LE Jet can have trouble with the seven pin relay for the fuel pump and injectors – worth carrying a spare in the car.