From Wikipedia, the free encyclopedia
Jump to: navigation, search
BMW M42B18 Motor.JPG
Manufacturer BMW
Production 1989–1996
Combustion chamber
Configuration DOHC Straight-4
Displacement 1796 cc
Predecessor BMW M40
Successor BMW M44

The BMW M42 is a straight-4 DOHC piston engine produced from 1989 to 1996.[1] Following BMW's typical construction techniques, the motor incorporates a durable cast-iron block and aluminum head. Weight-saving measures include aluminum chain cases, oil sump, motor mount arms, accessory mounts and a cartridge-style oil filter housing. Closely related to the single cam M40, the DOHC 16 valve head and chain-driven cams provided a notable increase in power. It was used in North American versions of the E36 318i instead of the M40. The M42 also incorporated other performance features such as a forged steel crankshaft and welded tubular stainless steel exhaust manifold instead of the more typical cast-iron items. BMW also fitted hydraulic motor mounts to decrease the inline four's inherent noise, vibration and harshness (NVH) compared to the smoother inline sixes already in production at that time.

The M42 went through several minor revisions during production. All versions featured a low-maintenance timing chain with a self-adjusting hydraulic chain tensioner and hydraulic valve tappets. The M42 also incorporated the Bosch Motronic M1.7 DME, eliminating a distributor in favor of fully electronic ignition timing. The DME was configured to fire four individual remotely mounted coils. Furthermore, in markets that required emissions controls, the DME also incorporated an upstream O2 sensor and three-way catalyst.

Early models of the M42 were subject to several recalls. The most common recall was due to frequent failures with a profile gasket sealing the lower cam chain case to the underside of the cylinder head.[2] This gasket seals the primary coolant passage within the timing chain case. A significant failure would thus discharge pressurized steam and hot coolant into the timing chain case. In many cases this coolant rapidly contaminates the motor oil in the sump, causing main bearing failure. Most failures were noted with poorly maintained motors, especially where coolant specifications and mixtures were not observed. BMW updated the profile gasket material and instituted a program to repair motors under warranty. In extreme cases, the aluminum mating surfaces in the head and chain case would corrode.[3]

The earliest versions of the M42 also developed problems with the cam chain drive. The hydraulic tensioner, chain guides, idler wheel and rear lower chain case have all been updated to resolve problems with mechanical wear. Most of these changes were implemented in the M42's closely related successor, the M44. These parts can all be retrofitted to the M42 to improve durability.

When installed in the BMW E30, a two-piece oil pan with a removable front sump was fitted to the M42. This design has two notable problems. The first is a vulnerability to road hazards, as the sump sits fairly low and is not protected by a subframe or cross-member. The aluminum pan will crack instead of deform on significant impacts. Steel skid plates have been developed to resolve this problem and are highly recommended in cases where the front suspension has been modified to improve handling.[4] The second problem is far more significant - the upper oil pan casting incorporates the oil pump's supply passage, and is sealed to the crankcase oil filter housing with a paper gasket. Thermal cycles and engine vibration tend to loosen the six 10mm upper pan mounting bolts inside the motor.[5] Routinely, a loose 10mm bolt or two can be found rattling around high-mileage M42 sumps. If not caught in time, the paper gasket sealing the oil pickup passage can be forced into the crankcase by oil pressure, potentially causing a major loss of oil pressure and/or aeration of the motor oil. Thus it is strongly recommended that any new owner of an early M42 remove the lower pan to inspect these bolts. A permanent solution is to remove & clean the six internal 10mm bolts, apply a thread-locking sealant to the bolts, and re-assemble.

In the final versions of the M42, BMW engineered a dual length intake manifold (called "DISA" by BMW)[6]



The M42B18 has a displacement of 1,796 cc, which is achieved through a bore of 84 mm and a stroke of 81 mm.[6] The engine uses Bosch Motronic 1.7 fuel injection. Versions equipped with a catalytic converter produce 100 kW and 172 Nm.[1] and meets with Euro2 norms.

Engine Displacement Power Torque Redline Year
M42B18 1,796 cc 103 kW (140 PS; 138 hp) @ 6,000 175 N·m (129 lb·ft) @ 4,500 6,400 +-30 1989-1996


  • 1989-1991 E30 318i (only North American models)
  • 1989–1991 E30 318iS
  • 1992–1996 E36 318i/318ti (only North American and South African models)
  • 1992–1996 E36 318iS
  • 1996 Z3[7]


A racing version of the M42 Engine, called the S42 is known. It was used in BMW's 320 4-door touring car, participating in the German racing series called STW (Super Tourenwagen Cup). It differed by having individual throttle bodies (ITB) for each cylinder and its capacity was enlarged to 1999 cc. It had eight fuel injectors instead of the standard four. Valve cover and airbox were entirely made of carbon fiber. Lubrication was provided by a dry sump system. Compression ratio was increased and a lightweight head installed.

Horsepower evolution steps of the S42 Engine:

Year 1995 1996 1997
300 hp (224 kW) 308 hp (230 kW) 315 hp (235 kW)

In its final evolution, this engine develops 158 HP/l.

See also[edit]


External links[edit]