M40

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The BMW M40 is a 4-cylinder engine, fitted to the 316i and 318i after 1987. It replaced the M10 as the standard 4-cylinder engine in 1987.

M40B18 engine with M42 radiator
M40B18 engine with M42 radiator

Contents

History

The M40 engine was released in 1987, and marked BMW's next generation of four-cylinder engines since the M10. It featured a belt-driven 8-valve OHC design, with an aluminium head on a cast iron block, and was produced in two capacities - The M40B16 of 1.6 litres, and the M40B18 of 1.8 litres. The only internal difference of these two versions was the crankshaft which modified the stroke and therefore overall displacement of the engine.

The E30 was equipped with both versions, powering the 316i and 318i respectively, and continued to use those engines until the end of production in 1994. Both were available with or without a catalyzer.

The M40B16 was also fitted to the E36. The M40B18 was also fitted to the E34 from 1988 to 1994.

The M40 formed the basis of all BMW's four-cylinder engines over the next decade. While its direct successor was the M43, it also spawned the 16-valve M42 used in the 318iS, which shares many compatible components. Considering the success of these subsequent engines, it is unfortunate that the M40 itself has proved to be the worst of the E30 engines, with many documented problems and little scope for improvement.

Versions

Models
EngineDisplacementPowerTorqueRedlineYear
M40B161.6 L (1596 cc/97 in³)75 kW (101 hp) @ 5500143 N·m (105 ft·lbf) @ 425062001988
73 kW (98 hp) @ 5500141 N·m (104 ft·lbf) @ 4250 62001988
M40B181.8 L (1796 cc/109 in³)85 kW (114 hp) @ 5500165 N·m (122 ft·lbf) @ 425062001987
83 kW (111 hp) @ 5500162 N·m (119 ft·lbf) @ 425062001987

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 M40 Sump is a two part design with an upper and lower sump. The upper part bolts directly to the block, sealed with a gasket, and the lower sump then bolts to the upper.

Leaks in this area are normally due to upper bolts coming loose, despite their torque rating. Because some of these bolts are only accessible with the lower sump removed, the two parts have to be disassembled to cure the leak. When removing the lower sump, don't be surprised to find a bolt sitting in the oil that has come out completely from the upper sump.

Oil Pump

The oil pump sits behind the timing case cover, and is directly driven by the crankshaft. It is a gerotor pump (one cog inside another). It is supplied with oil via a pick-up tube into the sump.

In the event of a failing or failed pump, replacement is quite a lengthy process due to its location. It is required to remove al components from the front of the engine, including the timing chain, in order to remove the front end cover of the engine. Then this cover must be removed with care, since the cylinder head fits on top of the case.

Sump Gasket

The sump gasket seals the upper sump to the block. Its design features an internal portion that seals off the oil pickup pipe to the oil pump from the rest of the sump. Unfortunately, due to the problem of sump bolts, it is possible for the gasket to slip and allow the oil pump to suck in air instead of oil; potentially causing oil starvation and engine damage.

Block

A single block design was used for all M40 engines, and they are completely interchangeable between models. All blocks use an 84mm centre bore.

Bear in mind that although the M40 engine was also fitted to the E34 and E36. Engines from those cars 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; an E36 will not.

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 the two crankshafts fitted to M40 engines. The sizes are:

M40 crankshafts are compatible with the M42 engine, although they have a slightly different "nose" which may require adapting to accept the M42 pulley/sprocket.

Piston Rods

All M40 engines use the same 135mm piston rods.

Pistons

The two M40 engines share the same piston design. They differ in their stroke due to their different crankshafts.

M40B16 8.2:1 M40B18 8.8:1
87-94
Ø: 84
KH: 36.15
MT: 2.75
GL: 63.5
BO: 22.0 x 54
Stroke: 72 81
Conrod Length: 140 140
Volume 1596cc 1766cc
Piston Shape:
Measurements (mm) Image:dimensions_mahle.jpg
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

Timing Belt

The M40 uses a belt to drive the valvetrain (unlike the M10 and M42 engines). Therefore, a pulley is mounted at the end of the crankshaft to power the belt, which drives a corresponding pulley on the camshaft. It is recommended to replace this belt and its tensioner every 30,000 miles or three years. Some people recommend 25,000 or even 20,000 miles, just to be safe.

Learn more about changing the timing belt.

Head

The 8V cylinder head on all M40 engines is a cast aluminium unit, featuring a single direct-action cam to operate the valves. The head is interchangeable between E30 and E36 models, but shares no compatibility with other BMW engines such as the M42, due to that engine's use of a timing chain instead of a belt.

All heads feature hydraulic tappets which reduce the maintenance of the engine.

Rocker Cover

It is positioned on top of the head with 8 collar screws. 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

On the M40, an overhead camshaft drives the intake and exhaust valves via tappets. This camshaft is belt-driven, and held in place using 5 bearings. It is lubricated by an overhead oil spray bar.

The cam is the known weak spot of the M40 engine, and is more susceptible to lobe wear than any other E30 engine, usually caused by a blocked oil spray bar. When the cam is worn, the engine won't rev high.

Cam Pulley

The M40 engine features a vernier-style cam pulley, allowing for a few degrees of adjustment in the timing of the engine.

Valves

Being an 8-valve engine, the M40 head contains one intake and one exhaust valve per cylinder.

Inlet Exhaust
Seat angle 45 degrees 45 degrees
Stem to guide clearance 0.020" 0.020"
Stem diameter 6.975mm 6.975mm
Head diameter 41.9mm 35.9mm
Valve clearance automatic

Tappets

The M40 engine uses hydraulic tappets to control the valves, meaning no adjustment is required to set the gaps.

However, tappets do get worn over time, usually from oil starvation, leading to an unhealthy ticking sound.

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.

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.

Ancilliaries

Flywheel

The M40 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.

For manual engines, the flywheel contains a spigot bearing to centre the input shaft of the gearbox to the tail of the crankshaft. These parts are essential when converting from auto to manual. For automatic cars, the flywheel is little more than a ring gear with a plate to which the torque converter is bolted.

Starter Motor

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.

All M40 starters are interchangeable, and are also compatible with M42 starters.

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 M40 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.

Engine Mounts

The engine mounts are standard on all M40-equipped E30s; however, if you are taking an engine from an E34 or E36, you will need E30 engine mounts.

Servicing

Main article: Basic M40 Servicing

Every engine needs its fluids and filters changed regularly. The M40 engine also needs its timing belt replaced at regular intervals. Learn more about changing the timing belt.

Common Problems

Won't Rev Over 5000

Unfortunately, this is a common sign of engine wear, and indicates a worn out cam shaft. The problem can be diagnosed in two ways. If the engine sound can be described as "sounding like a skeleton wanking in a dustbin" then your cam is very likely worn.

To confirm, remove the rocker cover and baffle plate so that you have access to the cam shaft, and run your finger nail over the cam lobes. If you can feel scratches, grooves or other imperfections, the cam is in a bad condition.

The only solution is to replace. Learn more about replacing an M40 camshaft.

Noisy Tappets

The tappets in an M40 are known for ticking over time. This is usually due to wear from oil starvation. The first thing to do is remove the rocker cover and inspect the camshaft for signs of wear. Each lobe should be smooth; any scratches or ridges indicate a worn cam which makes it uneconomically viable to replace the tappets; you will have to refresh every part of the valvetrain.

If your cam is in good condition, first try an oil change and a new oil spray bar. This will help keep the tappets lubricated. They will continue to tick on cold start but will hopefully quieten down as the engine warms up. If that doesn't solve the issue, consider replacing the tappets, followers and thrust pads with new items; DO NOT use second-hand parts, considering how common this problem is with M40 engines.

Loose Sump Bolts

Due to the two-part design of M40 and M42 sumps, there is a common issue of internal sump bolts working loose. Not only can this cause oil leaks, but there's the threat of oil starvation to critical parts as the sump gaskets themselves work loose and block essential oil channels. To prevent premature engine death, learn more about M40 sump bolts.

Common Upgrades

Unfortunately the M40 is not sympathetic to upgrades. There are no simple "bolt-on" solutions that will give an increase in performance. If you are looking for more power, consider an engine swap to the M42 engine, or investing in forced induction.

The best way to get performance out of an M40 is simply to service it.

See also