M42

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The BMW M42 is a 4-cylinder 16-valve engine, fitted to the 318iS as well as early E36 models.

M42 engine
M42 engine

Contents

History

The M42 is the last chain-driven engine fitted to the E30, and the first 16-valve unit. It came in one capacity, 1796cc, and was fitted to one model, the 318iS. It featured a number of innovations that made it a far more modern engine than any other fitted to the E30, including a twin-valve throttle body, coil pack ignition and bi-concave combustion chambers. This made the 318iS both an economical and performance model, with good power and torque characteristics.

The engine also saw service in the E36, which added a serpentine belt front pulley system and a different intake manifold. It was finally superseded in that model by the M44 in January 1996.

The engine produces 136BHP at 6000RPM and 127 ft lbs of torque at 4600RPM

Versions

Models
EngineDisplacementPowerTorqueRedlineYear
M42B181.8 L (1796 cc/110 in³)101 kW (136 hp) @ 6000175 N·m (129 ft·lbf) @ 450063001989-96
100 kW (134 hp) @ 5500172 N·m (127 ft·lbf) @ 4500 63001989-96

E36 Differences

As the M42 engine was also fitted to the E36, it's important to know what the differences are when looking at a replacement engine.

If you have bought an E36 lump, you will need to swap the following components to E30-specific parts to fit your car:

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 M42 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 using twenty 10mm nuts. This sump is the same as the M40 sump.

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)

In the event of a failing or failed pump, replacement is quite a lengthy process due to its location. It is required to remove the valve cover, upper timing cover, lower timing cover, harmonic balancer, the aircon bracket (if applicable), thermostat housing, water pump, and then remove the timing chain assembly including guides and sprockets. So basically the lower timing case which bolts up to the block will be empty. Then this case must be removed with care, since the cylinder head fits on top of the case.

Then after having taken the lower timing case off, you must remove a cover on the back of the case and this will reveal your oil pump. At this point it is highly recommended to change the timing chain, guides and ancillary sprockets at the same time due to the complexity of the operation.

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

The M42 Block is a development of the earlier M40 block, meaning it is possible to use an M40 block to repair an M42 engine. There is also a forwards compatibility with the M44 engine from the E36.

However, almost every internal component in this blocks is different, meaning the crankshaft and bearings are specific for that engine.

The block itself is a pearlitic cast iron unit, with a central bore of 84mm and a bore spacing of 91mm. The cylinders have a maximum ovality of: 0.01mm/0.0004" with an identical maximum taper. Compression pressure is 10-11bar (143-156psi), with a compression ratio of 10:1.

The bore also features integrated oil jets (or "squirters") in the big end to provide lubrication and cooling to the pistons.

It is possible to rebore the M42 block for a higher displacement. The absolute maximum is reboring to 87mm; combined with a stroke of 88mm will produce an M42 with 2.1l displacement.

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.

The standard crank has a stroke of 81mm. However, it is possible to use cranks from other BMW engines to increase the stroke, as long as the pistons are changed to match. The available cranks are:

  • M44 - 83.5mm stroke
  • M47 - 88mm stroke

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. These are all held in place with bolts, which became stretch bolts in 1992 which cannot be reused.

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, centred by a pilot bearing. 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 M42 engine.

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 crankshaft has an endplay tolerance of 0.0031-0.0064". If your engine has more than this, consider replacing the pilot bearing.

Piston Rods

The piston rods connect the pistons to the crank. At the top or 'small end', the piston is held in place with a gudgeon pin, while at the bottom or 'big end', rod caps bolt around the cap.

M42 rods are 140mm, 5mm longer than their M40 equivalents. However, they are significantly lighter at 609g and are forged. All rods should be within 4g of each other without bearing shells. Rods are also interchangeable with M50 rods.

Pistons

One size of piston was used for M42 engines, featuring a small dish to provide 20% of the combustion area, as well as notches for valve interference.

M42 Pistons
Engine M42B18 10:1
Year 1989-95
Ø: 84
KH: 31.650
ÜH: 0
VT: 1.650
MT: 4.500
GL: 51.150
BO: 22.0 x 54
Stroke: 81
Conrod Length: 140
Volume 1796
Deck 212.15
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

The standard piston has a diameter of 83.980mm. Three overbore sizes are also available: 84.060mm, 84.230mm and 84.480mm. The clearance is set between 0.0004-0.0016" The piston rings have the following gaps: Top & bottom compression rings = 0.0080-0.0160" ; oil ring = 0.0080-0.0180"

M50B25 pistons are compatible with the M42 engine, and can be used to increase compression if needed.

Head

The 16V head is a chilled-cast alloy, related to the original M1. Heads are interchangeable between E30 and E36 engines. This is convenient for sourcing spares, as the M42 head is prone to warping and cracking, particularly from the exhaust valves to the waterways. Cylinders 2 and 3 are most common.

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

Rocker Cover

The top of the rocker cover has a vapour/liquid separator which reduces oil consumption of the engine and bi-concave combustion chambers.

When removing and replacing the rocker cover, it is worth replacing the rocker cover gasket.

Cams

Being a 16V engine, the M42 head features two camshafts; an Intake and an Exhaust. These cams operate directly on the valves.

Inlet Exhaust
Duration 252 252
Lift 10.2 10.2
Lobe height 1.8755-1.8803"

It is possible to make adjustments to the camshaft timing in order to shift the power band higher or lower. Learn more about adjusting M42 cam timing.

When setting the engine to top dead centre you will be required to lock the crank to the block by means of inserting a pin into a hole. This hole is between the underside of the starter motor and a web in the block. There should be a plastic plug in it, with a hole in it to pull it out. Insert a 8mm drill bit in though the hole, and turn the crank until it slides farther in, into the flywheel. Don't forget to remove it afterwards!

Valves

Valves came with stems in two sizes; 7mm for earlier engines, or 6mm after September 1992. All valves are double-sprung.

Inlet Exhaust
Seat angle 45 degrees 45 degrees
Stem to guide clearance 0.020" 0.020"
Stem diameter 6.975mm 6.975mm
Head diameter 33mm 30.5mm
Seat diameter 31.6mm 29.1mm
Seat width 1.4-1.9mm 1.4-1.9mm
Valve clearance automatic

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.

Various head gasket thicknesses are available Thicker gaskets will lower the compression of your engine, which may be useful if you are considering forced induction.

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.

Timing Chain

The timing chain is used to drive the cams from the crankshaft. M42 the chains are rated for 100k miles, but owners believe that chains are indestructible and don't change them. This leads to cases of chain failure and when they break they will destroy the engine due to the interference nature of the valve train.

Other causes of failure are due to known weak points in the chain gear. The weakest is the steel chain guide, which can fracture and break. An indication of a worn guide is a noticeable increase in chain rattle; the engine will sound significantly louder. Guides can be replaced with an upgraded dealer part made of nylon/plastic. You will need one new 45mm long bolt in conjunction with the new guide.

Breather Hose

To balance the pressure inside the head, a rubber hose runs from the top of the rocker cover to the throttle body. This hose is normally placed near the oil filler cap, but was moved to the rear of the head on later models.

Intake Manifold

The inlet manifold is a two pieces design, comprising of an upper and lower manifold. The design is aimed at improving the low down torque of the engine.

If fitting an M42 to your car from an E36, it's not essential to replace the intake manifold; however, there are differences between the E30 and E36 variants. The E30 version is designed for better top-end performance, while the E36 unit yields better torque at lower revs.

Throttle Body

Unlike other throttle bodies on E30 engines, the M42 throttle body has two butterfly valves, a 35mm and 54mm valve. When you are lightly pressing the accelerator pedal, only the smaller valve opens but when you further depress your pedal the larger valve also opens. This enables the M42 engine to use less petrol when the car is on journeys which do not require constant change in speed.

Ancilliaries

Flywheel

There is a lot of debate regarding M42 flywheels. Some engines were fitted with a dual-mass flywheel while others retained a solid single-mass wheel, which is considered to be more desirable. Dual-mass wheels are more commonly found in the US, while the UK market seems to have retained the single-mass wheel.

A common upgrade is to replace the flywheel with an M20 flywheel. A variety of M20 flywheels exist, with the aircon-equipped models being up to 50% lighter than the equivalent M42 wheel. Be aware that the M20 wheel will require an M20 starter motor.

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.

The M42 starter is specific to the flywheel of that engine; it is interchangeable with M40 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.

Torques

When working on any engine, it is important to tighten up all bolts only to their required tightness and no further. Use the following table as a guide:

Nm ft-lbs
Big ends - Step 1 23 17
Big ends - Step 2 +70 degrees
Camshaft sprocket 13-17 10-12
Camshaft bearing cap bolts 13-17 10-12
Crankshaft damper 300-320 217-231
Cylinder head Step 1 30-35 22-25
Cylinder head Step 2 +90-95 degrees
Cylinder head Step 3 +90-95 degrees
Exhaust manifold 22-24 17-18
Flywheel 82-94
Intake manifold 13-17 10-12
Main bearings Step 1 20-25 14-18
Main bearings Step 2 +47-53 degree turn
Viscous fan coupling (LH threads) 40-50 29-36
Timing chain tensioner: 23-27 17-19

Servicing

Main article: Basic M42 Servicing

Every engine needs its fluids and filters changed regularly. And while the M42 is chain-driven, it helps to replace the chain tensioner occasionally to prevent the chain slipping and killing the engine. Learn more about changing the chain tensioner.

Common Problems

Profile Gasket

The most common problem with the 318iS was the profile gasket failing on the engine. The chemicals in the coolant would deteriorate the gasket until it failed. The repair for this job is extensive (cylinder head, radiator, thermo-fan, crank pulleys and the front timing chain covers are just a few components that had to be removed). The gasket would normally fail around the 50,000 miles mark and wasn't always covered under the warranty.

Part Qty Part Number Part Qty Part Number
Gasket 1 111 217 215 46 Gasket 1 116 117 344 90
Bolt set 1 111 217 219 39 Nut 4 183 017 377 74
Valve cover 1 111 217 218 76 Valve cover 3 111 217 214 75
Gasket 1 111 412 478 49 Gasket 1 115 317 211 72
Gasket 1 111 412 478 37 Gasket 1 116 117 346 84
O-ring 1 121 417 272 20 O-ring 1 115 317 212 18
Seal ring 1 071 199 633 55 Throttle-b 2 135 412 474 00
Gasket 1 183 017 119 69 Gasket set 1 111 417 219 19
Antifreeze 1 821 414 677 04

At this stage, most E30 M42's should have had their profile gasket replaced through by a dealer or by previous failure. It would be worthwhile checking with previous owners on when and who replaced the gasket, although you may have to go back a number of years into the car's history. If you cannot determine if it has been replaced, assume that it has, considering the mileage of cars that are now at least twenty years old.

However, keep in eye out for water in oil symptoms (a grey-brown sludge build up easily evident on the oil filler cap or dipstick), unusual oil pressures, water loss (leaks or otherwise) or water temperatures.

Timing Chain Tensioner

The symptoms of a wearing tensioner is a rattling noise coming from the front of the engine which can easily be heard in the car from around 2000-3500rpm. It is relatively easy to change the chain tensioner. However, you can also re-fit an M44 tensioner. It is recommended that you change the tensioner as soon as possible when the symptoms begin to occur; a worn tensioner will cause the timing chains to stretch resulting in in new cam chain sprockets, timing chains, guides and various gaskets and can cost in the region of £550 (including labour from a specialist).

  • Timing Chain tensioner part (x1): 113 117 431 87
  • Timing Chain tensioner washer part(x1): 071 199 633 55

Tapping/ticking noise

When most people hear a ticking engine they instantly believe that the tappets are worn. This is not always the case, sometimes it can be a crack in the exhaust manifold. Normally the ticking will go away once the car has been driven for a while, so people believe the the oil has reached operating temperature and cured the tapping problem. However, it could be a small hole in the exhaust manifold that is sealed when the manifold gets hot (ie expanding metal covering the hole up!). This is most common on cylinder one.

Fuel smell

Some times you can smell petrol fumes on startup, which can smell a lot worse if you turn on your fans. This can be a small fuel pipe just behind the inlet manifold which can perish or sometimes just needs the jubilee clip tightening. Petrol cannot always been seen (unless the engine is cold) as the petrol instantly evaporates from the heat of the engine. But you should see the leak leaking onto the fuel rail trim and down the fuel inlet and return pipes for the engine.

Lumpy idle

The M42 engine is prone to the air breather pipes perishing. This results in unwillingness to start on cold mornings and a lumpy uneven idle. If you open the bonnet with the engine running you may be able to hear a hissing sound. Then all you need to play is hunt the sound considering there are a lot of breather pipes to the engine. If there is no hissing sound it could be the valve in the idle control valve (ICV) sticking. Simply disconnect the ICV by undoing the jubilee clips (its found between the engine and bulkhead on the passenger side and is a "T" shape) and spray some carburettor cleaner into the valve. This should stop the valve from sticking, or you could replace the ICV completely.

Loose Sump Bolts

Due to the two-part design of M42 and M40 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 M42 sump bolts.

Common Upgrades

Coil On Plug

Coil On Plug (COP) locates a set of M50 coil packs onto the spark plugs as opposed to having the standard packs on the wing and HT leads. Learn more about installing coil on plug.

Throttle Body Heater Delete

Some users have found that removing the throttle body heater does make a noticeable impact on the engine, based on cooler air flowing through the intake. Learn more about removing your M42 throttle body heater.

M20 Flywheel

Not all M42's came with dual mass fly wheels which is a common misconception. A cheaper and more road friendly alternative to an aluminium flywheel is to fit an M20 flywheel and clutch assembly.

It is common that the M20 flywheel will rub on the M42 sump. Some people grind down the parts of the sump where it touches, but it is not really the proper way. The best thing to do is get the back of the flywheel machined down,which would be lightening it too so is worth while doing. The flywheel would then have to be balanced.

Chips

A variety of performance chips are available to perk up the performance of otherwise-standard M42s.

Stroker Engines

The M42 block is capable of much larger capacities and power than BMW made use of. If you'd like to rebuild your M42 engine into a larger capacity, learn more about the most common stroker permutations.

See also