M20

The M20 is an straight-6 piston engine of BMW, whose 12-valve, belt-driven SOHC design in three parts, featuring a main Block with a Sump at the bottom and a Head at the top.

M20B25 Engine with battery-in-boot

History

The M20 was introduced in the 1977 BMW 520/6 and 320/6. With displacements ranging from 2.0 to 2.7 liters, 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.

Versions

Models

Engine	Displacement	Power	Torque	Redline	Year
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

Components

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.

Sump

The lowest part of the M20 engine is the Sump, a steel dish that provides an oil bath for the crankshaft, as well as feeding the oil pump. Oil is drained through a single bolt,

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.

Oil Pump

M20 Oil pump and sump gasket

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,

Sump Gasket

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.

Block

Pistons

M20 Pistons

	M20B25 8.8:1	M20B25 9.4:1	M20B25 9.7:1	M20B27 8.5:1	M20B27 9.0:1	M20B27 10.2:1	M20B27 11.0:1
	87-90 (all with cat)	87-88 (??)	85-89 (Euro i)	87-88 (USA super ETA)	82-87 (USA)	85-87 (Euro e)	83-85 (Euro e)
Ø:	84	84	84	84	84	84	84
KH:	34.2	34.2	34.2	36.2	35.7	35.7	35.7
ÜH:	4.2	4.2	4.43	3.6			0.65
VT:	-0.4/-1.5	-0.4/-1.5	-2.12/-2.22		-1.2	-0.9	-1.2
MT:	-6.2	-5.1		1.1	-3.5	-2
GL:	63.63	63.43	73.63	64.81	77.7	77.7	78.35
BO:	22.0 x 54	22.0 x 54	22.0 x 54	22.0 x 54	22.0 x 54	22.0 x 54	22.0 x 54
Stroke:	75	75	75	81	81	81	81
Conrod Length:	135	135	135	130	130	130	130
Volume	2493	2493	2493	2692	2692	2692	2692
Deck	206.7	206.7	206.7	206.7	206.2	206.2	206.2
Piston Shape:	100px	100px	100px	100px	100px	100px	100px

Head

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 next version was #1277731 aka the 731. This head was the same as the 200 but featured larger intake ports. 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.

Rocker Cover

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.

Breather Hose

To balance the pressure inside the head, a rubber hose runs from the top of the rocker cover to the throttle body.

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 is picked up by the oil pump and

Oil Cooler

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.

Cooling

Main article: Cooling

Common Problems

Not Starting

For all non-start situations, learn more about troucbleshooting your engine.

Servicing

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.

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.

Common Upgrades

Stroker Engines

The M20 block is capable of much larger capacities and power than BMW made use of. If you'd like to rebuild your M20 engine into a larger capacity, learn more about the most common stroker permutations.

The most common option is a 2.8 stroker, which can be built using standard components. Learn more about building a 2.8..

One of the Zoners, Daimlerman, took the time to document his own construction. Learn more about Daimlerman's 2.7

Engine Management

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 converting to 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.

See Also

Engines M21 M30