M42

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

M42 engine

History

In E30 form it's twin cam 16v 4cyl 1796cc engine. It is an aluminium alloy engine which shares a development history with the M1, M5 and M635CSi engines.

The M42 is the last chain-driven engine fitted to the E30, and the first 16-valve unit. 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 It was finally superseded in that model by the M44 in January 1996.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

Engine	Displacement	Power	Torque	Redline	Year
M42B18	1.8 L (1796 cc/110 in³)	101 kW (136 hp) @ 6000	175 N·m (129 ft·lbf) @ 4500	6300	1989-96
		100 kW (134 hp) @ 5500	172 N·m (127 ft·lbf) @ 4500	6300	1989-96

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:

• Sump
• Intake Manifold
• Exhaust Manifold
• Engine loom

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, and are identical to their M40B18 equivalents. However, they are significantly lighter at 609g and are forged. All rods should be within 4g of each other without bearing shells.

Pistons

01 KS PISTON 83,980MM("0") 4 11251721446 01 KS PISTON 84,060MM("00") 4 11251721447 01 KS PISTON 84,230MM(+0,25) 4 11251721448 01 KS PISTON 84,480MM(+0,50) 4 11251721449

Pistons : Clearance: 0.0004-0.0016"

Piston rings : Gap: Top & bottom compression rings = 0.0080-0.0160" ; oil ring = 0.0080-0.0180"

Firing order: 1-3-4-2

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

Head

are prone to warping and cracking

The engine has hydraulic tappets which reduce the maintenance of the engine.

Cylinder head : Height = 140mm/5.512"; minimum height = 139.55mm/5.494"

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.

Camshaft : Duration: inlet 252 exhaust 252 Lift: inlet 10.2 exhaust 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.

Valves

Valves : Seat angle: 45 degrees

Valve stem to guide clearance: 0.020"

Inlet: Solid, stem diameter 6.975mm/0.2756", head diameter 33mm/1.299"

Exhaust: Sodium filled, diameter 6.975mm/0.2756", head diameter 30.5mm/1.201"

Seat diameter: inlet = 31.6mm/1.244", exhaust = 29.1mm/1.146"

Seat width: 1.4-1.9mm/0.055-0.075"

Valve clearance : None, automatically compensated by hydraulic tappets

Breather Hose

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

Intake Manifold

The inlet manifold is a two pieces design aimed at improving the low down torque of the engine.

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.

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

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 tht 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 alot 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 alot 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 completly.

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 freindly alternative to an aluminium flywheel is to fit an M20 flywheel and clutch assembly.

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

• Engines
• M10
• M40