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 and replace, however, means removing the sump, which is a very involved task.

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

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 ...

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.

Crankshaft

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:

• 320/6 - 71mm stroke
• 323i - 76mm stroke
• 325e - 81mm stroke
• 325i - 75mm stroke

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 steel bearings called 'shells'. 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.

Piston Rods

The piston rods connect the pistons to the crank, and greatly determine the power characteristics of the engine. The longer the rod, the more time the piston will spend at TDC

M50 rods are compatible

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:

Measurements (mm)
Definitions	Ø: Diameter of piston KH: Distance from center of wristpin bore to top of piston top ÜH: Height of piston crown over piston top plane VT: Depth of depression in piston crown MT: Depth of depression in piston top GL: Total length of piston BO: Bore (and length) of wristpin

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.

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 head is held in place with stretch bolts, which MUST be replaced after removing.

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 weakens the head too much, rendering it scrap.

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.

Cam

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.

Valves

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.

Rocker Arms

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.

Head Gasket

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.

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.

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.

Oil Leaks

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.

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