If you want help fitting one of these beasts to your car, learn more about the M20 Engine Swap.
- 1 History
- 2 Versions
- 3 Components
- 3.1 Sump
- 3.2 Block
- 3.3 Head
- 3.4 Ancilliaries
- 3.5 Oil
- 3.6 Cooling
- 4 Common Problems
- 5 Servicing
- 6 Common Upgrades
- 7 Useful websites
- 8 See Also
The M20 was introduced in the 1977 BMW 520/6 and 320/6. With displacements ranging from 2.0 to 2.7 litres, it was the "little brother" to the larger BMW M30 engine, and was then known as the M60. It was intended to replace the larger displacement 4-cylinder motors and was born out of BMW's conviction that a small six had more development potential than a large four (i.e. 2 litres+)
Powering the E21 and E30 3-Series, as well as E12, E28 and E34 5 Series cars, it was produced for nearly two decades, with the last examples powering the E30 325i Touring built until sometime in 1993. By that time, the newer twin-cam M50 engines with 4 valves per cylinder had already been used in the E36 and E34 for a couple of years.
As the BMW M21, it became a diesel engine that was also available with a turbocharger.
|M20B20||2.0 L (1990 cc/121 in³)||92 kW (123 hp) @ 5800||165 N·m (121 ft·lbf) @ 4000||1981|
|92 kW (123 hp) @ 5800||170 N·m (125 ft·lbf) @ 4000||1981|
|95 kW (127 hp) @ 6000||174 N·m (128 ft·lbf) @ 4000||6200||1985|
|95 kW (129 hp) @ 6000||164 N·m (120 ft·lbf) @ 4300||6200||1986|
|M20B23||2.3 L (2316 cc/141 in³)||102 kW (143 hp) @ 5300||205 N·m (151 ft·lbf) @ 4000||6500||1982|
|110 kW (139-150 hp) @ 6000||205 N·m (151 ft·lbf) @ 4000||6500||1983|
|M20B25||2.5 L (2494 cc/152 in³)||126 kW (168 hp) @ 5800||226 N·m (166 ft·lbf) @ 4000||6700||1985|
|120 kW (170 hp) @ 5800||215 N·m (158 ft·lbf) @ 4000||6700||1985|
|125 kW (168 hp) @ 5800||222 N·m (163 ft·lbf) @ 4300||6700||1987|
|M20B27||2.7 L (2693 cc/164 in³)||92 kW (121 hp) @ 4250||240 N·m (177 ft·lbf) @ 3250||4500||1983|
|95 kW (127 hp) @ 4250||240 N·m (177 ft·lbf) @ 3250||4500||1986|
|90 kW (120 hp) @ 4250||230 N·m (169 ft·lbf) @ 3250||4500||1985|
|95 kW (127 hp) @ 4800||230 N·m (169 ft·lbf) @ 3200||4500||1986|
The engine in all E30s is a three-part design, featuring a main Block with a Sump at the bottom and a Head at the top.
Almost all M20 sumps are interchangeable between engines, except for that fitted to the 325iX.
Removing the sump on an M20 in situ is one of the most irksome chores on these engines. You can lift the engine out to get better access, but this requires dismantling a large portion of the engine ancillaries. However, you can attempt the following method:
Drive the car up onto ramps, and chock the rear wheels. Now remove the top radiator bracket, and unbolt both ends of the each rubber engine mounting block. Jack up the engine evenly using blocks of wood under each mounting arm, and remove the rubber engine mounting blocks when they become free. Lower the arms back onto small pieces of wood cut from standard 100mm fence posts placed sideways where the rubber blocks had been, making sure that the radiator moves up and down with the engine. Remove the steering column universal joint clamp bolt at the rack to allow it to slide off the rack. Disconnect one steering ball joint at the wheel hub (this may be difficult with the car on ramps as the wheel has to swivel). Unbolt the two steering rack retaining bolts, prise down one of the flanges slightly and lower the rack to hang loosely. Unbolt the earth strap and oil level sender from the nearside upper sump, under the alternator. Remove the sump bolts - including those hidden by the engine to gearbox shroud. Finally, rotate the front of the sump diagonally forward and down to allow its shallow rear section to clear the oil pump pick-up. Refitting, like always, is the reverse of removal.
The M20 oil pump is a gear-driven unit that takes its power from the crank via an intermediate drive shaft. M20 oil pumps are usually very reliable, and only need to be replaced if you have smashed your sump somehow. To remove and replace, however, means removing the sump, which is a very involved task.
The sump is sealed to the block by a gasket, which can often develop weeping leaks. Because of the nature of the M20 sump, the engine needs to be lifted to provide access to the sump, and any pressure put on the sump (by a jack underneath, for example) is likely to damage the gasket.
There are many choices of gasket available which have their pros and cons. The original cork gaskets, while cheap, are known to crack and split in a short time, which is why the material is no longer used in modern engines. Paper gaskets offer a better compression, but should be greased on both sides prior to application. Since grease and oil don't mix well, this will require a subsequent oil change in the near future. The third option is to use a silicone sealant sparingly on the face of the sump, which will provide a strong and sturdy seal for years. Loctite 518, used sparingly, requires a 2-hour curing time but will seal the sump solid.
The M20 block is a cast iron design with six cylinders bored out to make the various engine capacities from 2.0 to 2.7 litres. The block can be identified by checking the number stamped on the intake side of the block, on a machined flat area just above the sump seal. Type number will start 20 ,25 or 27.
In design, all M20 engine blocks are the same. In practice, two blocks exist; the 80mm bore block which forms the heart of the 320i and 323i, and the 84mm bore block which makes the 325i and 325e. There is no reason why an 80mm block could not be bored out to 84mm.
Bear in mind that although the M20 engine was also fitted to the E34, engines from that car have the dipstick located in the sump. However, an aperture exists on the block, ready to be tapped, and the sump fitted to E30s bolts straight on to relocate the dipstick to its normal position if you wish. But an E34 sump will fit an E30.
While the block determines the maximum capacity for the engine, the crankshaft determines the stroke of the engine, and therefore the actual displacement. Mounted to the bottom of the block, the crank holds the bottom of the piston rods and converts their up/down motion into rotary force known as torque. It does this by holding the rods on a series of lobes or 'throws' which extend away from the centre of the crank by a certain distance.
It is this distance that differentiates each of the crankshafts fitted to M20 engines. The sizes are:
It is also possible to fit the crank from the M21 engine, which has the same stroke as the 325e crank (81mm). However, the M21 crank is heavier at 24.5kg compared to the 325e crank's 23kg, due to it being a forged steel rather than cast iron unit.
The main shaft of the crank is held in place with 'caps', inside which are simple bearings called 'shells', made of steel-backed white metal. The caps and shells fit around the shaft at fixed points called 'journals'. From the journals, arms known as 'webs' extend outwards to hold the 'throws', while the throws themselves connect to the piston rod or con rod.
Both ends of the crank shaft protrude through the block. The front end carries a number of pulleys to control engine timing and to power engine ancillaries, while the rear holds a wheel to bolt to the flywheel. To prevent oil leaking through the block, an oil seal is fitted at either end of the crank inside each end main bearing. These oil seals rarely fail on the M20 engine.
M20 cranks have seven main bearings and six piston rod bearings. The main bearing bolts can be re-used, but the piston rod bolts ('big end' bolts) are stretch bolts and must be replaced.
Before fitting a used crank shaft, check it for cracks. This can be done by lifting the crank off the ground and striking each one of the 12 counter weights with a large wrench (19 mm or larger). It should ring like a bell. If it doesn’t - check the rod journal nearest the dead counter weight and look for a crack. A dead ring indicates a junk crankshaft.
The crank bolt is a 22mm bolt that holds the crank in place. This bolt is best slackened with the engine still in the car:
Remove the radiator and the lower splash panel to give access. Remove either fuse 11 (fuel pump) or the H/T lead to the coil. Place a suitable lump of 4x2 timber on the floor, where the splash panel was. 22mm socket and the biggest breaker bar that will fit in the bay, with its end resting on the block of timber. FLICK the ign switch, until the starter has freed the bolt.
Do not let anyone stand close to the car, do not attempt this with the car in gear, do not attempt this with another car close by. When you flick the ign switch, the car WILL leap into the air; it's the weight of the car coming back down again that loosens the bolt.
There's no perfect length for rods, but its worth paying attention to the rod ratio; the length of a connecting rod (center to center) divided by the stroke of the crankshaft. Lower rod ratio numbers are typically associated with lower rpm torque motors, while higher rod ratio numbers tend to be high revving high horsepower motors. Shorter rods are great for low-end torque, but generate a lot of sideways force which results in engine wear. Longer rods reduce these sideways forces on crank, making for a smoother engine, while also increasing the 'dwell' of the piston stroke, or time spent at the top and bottom of the cylinder. The advantage of this is increased compression and therefore higher power, offset by the amount of wasted time the piston spends sitting at the bottom of its stroke doing nothing.
The rod lengths for M20 engines are as follows:
- M20B20 - 130mm
- M20B25 - 135mm
- M20B27 - 130mm
- M21D25 - 130mm
The 135mm rod weighs 640g.
M50 rods can also be fitted between M20 cranks and pistons, providing either a longer rod length (up to 140mm) or a lighter rod, allowing the engine to rev that much more freely. The M3 or 328i piston rod fitted to the M50/S50 engines are the same dimensions as M20B25 rods at 135mm, but weigh significantly less (540-575g).
The crank pulley mounts to a hub at the end of the crank and is held in place with six 13mm bolts. The crank pulley is the main driving wheel for the timing belt, and does not need to be removed if changing the belt.
Three different head castings were used over the engine's production run. The earliest was #1264200 aka the 200. These were used in all E21 320/6 and 323i and E12 520/6 engines, and later in the E28 and E30 eta engines (eta = "Power With Economy").
The final version was #1705885 or 885 introduced in the 325i. Ports were further enlarged, valves were larger and the combustion chamber was redesigned to improve flow and thermodynamic efficiency.
Despite the three designs, the only differences are valve head sizes, port shapes and combustion chambers. This means that certain parts such as bearings, oil seals and even camshafts are interchangeable between heads.
The heads themselves are not interchangeable; fitting a 731 head to a 325i will result in the valves hitting the pistons, while fitting an 885 head to a 320i will result in the valves hitting the bore lip. 885 heads are therefore completely incompatible with B20 or B23 blocks, and the 200 head can only be used with the 325e pistons.
However, a 731 head from the 320i or 323i can be used on a 325i (and is essential if building a 2.7), by modifying the engine. The recommended method is by profiling the squish band of the head where the pistons protrude. It is important to remember that the 885 head is designed for profiled m20b25 pistons and the 731 head is designed for pistons that are flat at the squish band and do not protrude from the block, if pistons of a different type are used then combustion chamber shape may be compromised resulting in less power, emission issues and detonation.
When skimming the head, look for dimples in two corners of the head face. These are your depth gauges, and the head can be skimmed until these dimples disappear. Any further skimming compromises the clearance between the piston and head within the squish band; overskimming will likely result in contact, and engine failure.
The M20 is a belt-driven engine (unlike other 6-cylinder motors, which use a chain). This belt needs periodic replacement. Learn more about replacing the M20 timing belt.
The head is held in place with stretch bolts, which MUST be replaced after removing. Check the torque settings for the correct tightening.
Below is the bolt tightening sequence:
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Despite the three styles of Head, the rocker cover is interchangeable across all M20 engines. It is positioned on 8 studs and fixed with M10 nuts. It has one hole for the Oil Cap, and one port for the breather hose.
When removing and replacing the rocker cover, it is worth replacing the rocker cover gasket.
An overhead camshaft drives the intake and exhaust valves through rocker arms. This camshaft is belt-driven on the M20 engine. The cam is held in place using up to 7 bearings. The end of the cam protrudes from the engine to drive the distributor.
Despite the three types of head available for the M20 engine, the camshafts are, to some degree, interchangeable. The cam bearing dimensions are identical for models 325i, 323i and 320i. The 320i cam has about 250 degrees duration whereas the 325i cam is about 258 and has about 1mm more lift at the valve.
The M20B27 "eta" cam uses 4 out of the 7 cam bearings, the 3 unused bearing housings don't have the oil feed drilled into the head from the rocker shaft. the eta cam is extremely mild and has a lot less lift.
This comes in useful for building your own 2.7 engine, where a 325i cam does indeed make more power than a 320i cam.
Two types of valves are fitted into an M20 head; Intake valves and Exhaust valves. Because the M20 is an interference engine, this means that the valves protrude into the cylinder when opened; for this reason, any problem with the timing belt will automatically mean damage to at least three valves. The valves are adjusted at the rocker arms, and should be set to 0.25mm when cold. Learn more about adjusting your M20 valves.
Between the heads, different sized valves are used. The 320i valves are 40mm intake and 34mm exhaust, while the 325i has 42mm intake and 36mm exhaust.
If you wish to fit larger valves to your 320i head, then obviously new larger valve seats need to be fitted, although the valve throat has plenty of meat to reshape to the new seat and the port can be opened up to 325i sizes and a few mm beyond if you wish.
The valves are opened and closed by rocker arms, which are pressed on by the cam. These rocker arms perform a dual function of cam follower valve adjuster, as one end contains the eccentric and the adjustment nut. The rocker arms pivot around rocker arm shafts.
If you have broken a rocker arm, they can be easily replaced by removing the rocker cover. If, however, you are suffering from repeated broken rocker arms, it is more likely that you have bent a valve and will need to remove the head.
To remove a rocker arm, you must remove the rocker arm shaft. This is possible with the head still in place, but may require lifting of the engine to allow the shaft to exit the engine through the front.
To remove the rocker arm shaft, remove all the adjuster eccentric discs and all the clips locating the rockers. Push any rockers that are loose sideways to clear the valves, turn the engine until the remaining rockers come loose (i.e. on the heel of the cam lobe) and push them sideways as well. Rocker shaft should now slide out easily, as long as you've removed the bar that locks both shafts in place at the front of the engine. If you have already removed the timing belt, then ensure that no pistons are at TDC (put crank at 60 degrees past TDC mark) and turn the camshaft. When you put things back together, set the cam mark at 30 degrees past the mark, put the belt on, turn the engine carefully back to TDC, and check the belt isn't a tooth out.
The Head gasket forms a seal between the Head and the Block, maintaining compression in the cylinder while keeping the Oil and coolant separate. Any failure in the head gasket will cause a drop in engine power, and will also lead to mixing of the oil and coolant, known as mayonnaise.
Various head gasket thicknesses are available Thicker gaskets will lower the compression of your engine.
To diagnose a broken Head gasket, look for a creamy substance underneath the oil filler cap. If present, remove your dipstick and look at the oil. If it resembles milky coffee, then it is very likely your head gasket has failed.
In the event of a failed gasket, it is important to check the condition of the head; more severe damage may have been done, including cracking of the metal which will render the head useless. This is especially common on heads fitted to the 325i.
Learn more about changing the head gasket.
The M20 flywheel is a single mass wheel which keeps the engine spinning long after you take your foot off the throttle. It is also the flat surface to which the clutch mates, ultimately transferring power from the engine to the Drivetrain. The edge of the flywheel is toothed. Known as the ring gear, it is what allows the starter motor to bite onto the engine to start it. The flywheel bolts to a pulley at the end of the crank. Pre-facelift flywheels have different teeth to facelift flywheels, therefore requiring different starter motors.
On early Jetronic engines, the flywheel also has lugs which are used by the crank position sensor to detect engine speed. These earlier engines also have different ring gears, meaning a Jetronic flywheel needs a Jetronic starter motor.
Two weights of flywheel were fitted to M20 engines. The standard flywheel is 8.4kg, while cars fitted with air conditioning have a flywheel 1.3kg lighter.
Lightened flywheels will result in faster acceleration, at the cost of fuel economy when cruising. The absolute lowest weight an M20 flywheel can be lightened to is 5.8kg, although for safety reasons it is recommended to keep the weight over 6kg.
The starter motor is a standard 1.4kW motor with a bendix engagement wheel. It is an all-in-one unit containing motor, solenoid and relay. When activated, the starter gear extends to meet the flywheel and then spins, driving the engine.
Two starters were fitted to the M20 engine, for pre- and facelift vehicles, and are not interchangeable; they must be matched to the appropriate flywheel.
The starter is wired directly to the Battery + terminal and the alternator. A smaller connection, fed by a black/yellow wire, is the incoming signal from the ignition switch. On facelift vehicles there is also a black/green wire to load reduction relays.
To test your starter while mounted in the car, bridge pins 11 and 14 of your diagnostic plug. This will bypass any wiring in the cabin of the car, and should make the starter spin freely. If you want to test the car engine, turn the ignition circuits on with your key, and then bridge the same pins. Your engine should start.
It is very rare for M20 starter motors to fail. If you are having problems starting your car, it is recommended to look at troubleshooting your engine before removing the starter motor.
The engine mounts are standard on all M20-equipped E30s; however, if you are taking an engine from an E34, you will need E30 engine mounts.
Main article: Oil
The system is lubricated by a pressurised oil system that oils each moving part of the engine. The oil is circulated from top to bottom and around, and the pressure monitored to ensure that no leaks occur. The driver is notified of this by a warning light of the dash cluster.
Oil Pressure Switch
The oil pressure switch is a basic screw-in plug that drives the Oil Pressure Light on the dash cluster. A single wire runs from the end to the main Loom. If removed, you will see that the switch is little more than a hollow bolt; inside that hole is a diaphragm. When the oil pressure is high enough, the diaphragm flexes upwards and closes a switch, which turns off the Oil Pressure Light. A drop in pressure will therefore break the switch, and make the light glow.
On all models fitted with the M20B25 engine (325i and Convertible) there is an external oil cooler fitted, and mounted under the normal engine radiator. It receives cool air through vents in the front valance. However, with modern semi-synthetic oils, the oil cooler is redundant.
Main article: Cooling
The M20 block is filled with channels for a liquid coolant. These channels are known as the water jacket, and are designed to allow coolant to flow from the the bottom to the top of the engine and out. Pressurised by a water pump, the coolant flows out to the radiator where it is cooled by air flow through the grille. In the case of high engine temperature, extra cooling is provided by a viscous fan mounted behind the radiator.
Coolant flow is regulated by the thermostat, which senses local temperature and opens and closes the supply of coolant accordingly.
The coolant is a mixture of anti-freeze and water, and should be replaced regularly. Coolant is drained at the drain plug.
All coolant components are interchangeable between all M20 engines of the same period. Pre-facelift components are not always interchangeable with their facelift counterparts.
Main article: M20 Water Pumps
The cooling system is pressurised by the water pump, which is belt-driven from the crank pulley.
There are two types of water pump fitted to M20 engines, depending on the age of the engine, and are distinguished by the number of pipe outlets fitted.
Engines need to be at the right temperature to run at optimum conditions, neither too hot or too cold. For that reason a thermostat is fitted to limit the flow of coolant around the system, allowing it to be warmed by the engine. When the temperature is correct, the thermostat opens to allow coolant to flow to the radiator.
Radiators, along with water pumps, are specific to pre- and facelift cars. Later models had a secondary outlet pipe to the expansion tank, and therefore you cannot swap radiators between newer and older vehicles.
The M20 radiator is an aluminium core with plastic side tanks, and is bolted to the front valance of the vehicle. A plastic 'shroud' guides air flow around the rear of the radiator, as well as channelling air blown in by the fan.
Some radiators, (especially ones that have been replaced), have a boss about 2/3 of the way up on the right hand end. This is where the electric fan switch for aircon cars is fitted. there are also spare relay and fuse sockets in the standard fusebox for the purpose.
An external tank was mounted to the inner wing, to allow hot coolant room to expand. Steam was vented from the system by a drainage tube tucked through a hole in the inner wing, while the tank itself was fitted with a cap to allow fresh coolant to be poured into the system. On Facelift cars, this tank is on the passenger wing, while pre-facelift cars have the expansion tank fitted by the battery.
The cooling fan fitted to all M20 engines is driven by the water pump, and uses a viscous clutch to regulate its speed. This is a known weak spot of the engine, with a failing clutch causing the engine temperature to rise dramatically when the car is left to idle.
To test the fan, the Zone recommended the 'newspaper test'. This involves rolling up a newspaper and poking the fan with it with the engine running. A good clutch will shred the paper, while a broken or failing clutch will let the newspaper stop the fan from spinning, The only solution is a replacement.
A second electrical fan was fitted to vehicles with air conditioning.
All the elements of the cooling system are linked with a series of rubber hoses. The hoses that make up the cooling system are as follows:
- Pump to thermostat hose: 11531287651
- Thermostat to rad hose: 11531708826 or for Jetronic cars 11531278926
- Bottom rad hose: 11531278925
- Header tank to pump hose: 11531289257
- Thermostat to heater: 11531289377
- Throttle body hose: 13541705568
- Block to throttle body hose: 13541719976
- Block to heater hose: 64211380527
- Pump to thermostat hose: 11531287651
- Thermostat to rad hose: 11531722216
- Bottom rad hose: 11531718980
- Thermostat to bottom rad hose: 11531718982
- Thermostat to heater and expansion tank hose: 11531722743
- Throttle body hose: 13541719966
- Block to throttle body hose: 13541719967
- Block to heater hose: 64211380527
To properly drain the coolant from the M20, it is not advisable to simply remove bottom hose. Because of the complex design of the M20 water jacket, there will still be pockets of coolant left in the block and, if the system is being flushed, dirt and grime will still be trapped inside.
Because of this, a bolt is fitted to all M20 blocks to properly drain the water jacket. This bolt is located below the exhaust manifold to the rear of the engine, and is best undone with a 19mm ring spanner.
For all non-start situations, learn more about troubleshooting your engine.
Every engine gets a bit weepy over time, and the M20 is no different. However, the most common cause of black patches on your driveway is the sump gasket, especially at the rear of the engine. Don't mistake this for a broken crank seal, which very rarely fails on the M20. Another rarer cause of an oil leak is the oil filter housing gasket.
The one weak point of the M20 engine is the head; specifically the 885 head fitted to the 325i. This usually happens in situations of overheating, and it is therefore critical to keep the cooling system in optimum condition. Any sign of 'mayonnaise' (a creamy residue) in either the coolant tank, the oil filler cap or under the rocker cover, will indicate a cracked head. Unfortunately, this means the head is scrap and will need replacing.
Main article: Basic M20 Servicing
All engines need their fluids and filters changed at regular intervals, but sometimes more technical things are required to keep things running smoothly.
Learn more about adjusting the valves.
The M20 using a timing belt, not a chain, and this needs periodic replacement. If you've bought a car with an M20 engine that has no record of a belt change, then get it changed immediately. Learn more about replacing the M20 timing belt.
When changing your cambelt, it is recommended that you change your water pump too. Make sure you fit the correct water pump for your engine.
The M20 block is capable of much larger capacities and power than BMW made use of. It is possible to build the engine up to a 2.7 or 2.8 with simple components, and even larger capacities are available.
Learn more about building a 2.7
Learn more about building a 2.8
Learn more about other common stroker permutations.
One of the Zoners, Daimlerman, took the time to document his own construction. Learn more about Daimlerman's 2.7
Upgrading the engine management system will give you better control over the running of the engine. A simple chip can yield better performance or economy for specific situations. Upgrading the entire system to the latest version of Motronic can also deliver results. However, if you want complete control over every aspect of your engine, you should consider third-party engine management options such as Megasquirt.
Big Bore Throttle Bodies (BBTB)
The throttle body is the air flap that directly governs how much air the engine is allowed to suck in, and is controlled by the accelerator. Fitting a bigger throttle body will yield a lot better results than any other bolt-on modification. Learn more about Big Bore Throttle Bodies.
Getting more air in is one aspect of improving performance; getting the waste gas out is equally important. For that reason, a large number of tuning companies, including Alpina and Hartge, built their own exhaust solutions to get the fumes away from the engine as smoothly and quickly as possible.
In the last twenty years, a number of aftermarket companies have built their own systems, copying or improving the original tuning companies' products. BTB is the current best supplier of performance manifolds, with their BTB3 model being the most desired. Check out the full list of exhaust manifolds.