M40

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.

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, and briefly the E32 in 1992-1993.

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.

Versions

Models

Engine	Displacement	Power	Torque	Redline	Year
M40B16	1.6 L (1596 cc/97 in³)	75 kW (101 hp) @ 5500	143 N·m (105 ft·lbf) @ 4250	6200	1988
		73 kW (98 hp) @ 5500	141 N·m (104 ft·lbf) @ 4250	6200	1988
M40B18	1.8 L (1796 cc/109 in³)	85 kW (114 hp) @ 5500	165 N·m (122 ft·lbf) @ 4250	6200	1987
		83 kW (111 hp) @ 5500	162 N·m (119 ft·lbf) @ 4250	6200	1987

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.

Block

A single block design was used for all M40 engines, and they are completely interchangeable between models.

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:

• 316i - 72mm stroke
• 318i - 81mm stroke

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 9.5:1
	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)
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

Rocker Cover

Despite the three styles of Head, the rocker cover is interchangeable across all M40 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.

Breather Hose

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

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 scatches, grooves or other imperfections, the cam is in bad condition.

The only solution is to replace.

See also

• Engines
• M10
• M42