BMW M20B25

BMW M20B25 engineThe BMW M20B25 is a 2.5 liters (2,494 cc, 152.19 cu-in) straight-six, four-stroke cycle natural aspirated gasoline engine from BMW M20-family. The engine was manufactured on Munich Plant (Munich, Germany) since 1985 to 1991. This engine was developed on base of low displacement the M20B20 (2.0 liters).

The BMW M20B25 engine features a cast-iron block and aluminum head with a single overhead camshaft (SOHC) and two valves per cylinder (12 in total). It has an 84.0 mm (3.31 in) cylinder bore and 75.0 mm (2.95 in) piston stroke. It was equipped with Bosch Motronic 1.1/1.3 Adaptive fuel injection.

An early version of this engine was equipped with the Bosch Motronic 1.1 engine management system and manufactured from 1985 to 1986. This engine has a compression ratio of 9.7:1 and a redline of 6,500 rpm. It produced 171 PS (126 kW; 169 HP) at 5,800 rpm of output power and 226 Nm (23.0 kg·m; 166.8 ft·lb) at 4,000 rpm of torque.

In 1987, the engine management was upgraded to Motronic 1.3 and the compression ratio rating was reduced to 9.3:1. This engine produced 171 PS (126 kW; 169 HP) at 5,800 rpm. The M20B25 version with catalytic converters has a compression ratio rating of 8.8:1 and produced 170 PS (125 kW; 168 HP) at 5,800 rpm of maximum horsepower and 222 Nm (22.6 kg·m; 163.8 ft·lb) at 4,300 rpm of torque.

The breakdown of the engine code is as follows:

  • M20 – Engine Family
  • B – Gasoline
  • 25 – 2.5 Liter Displacement

General information

Engine Specifications
Engine codeM20B25
LayoutFour stroke, Inline-6 (Straight-6)
Fuel typeGasoline
Production1985-1991
Displacement2.5 L, 2,494 cc (152.19 cu in)
Fuel systemFuel injection system
Power adderNone
Max. horsepower 170 PS (125 KW; 168 HP) at 5,800 rpm
171 PS (126 KW; 169 HP) at 5,800 rpm
Max. torque 215 Nm (21.9 kg·m; 158.7 ft·lb) at 4,000 rpm
226 Nm (23.0 kg·m; 166.8 ft·lb) at 4,000 rpm
222 Nm (22.6 kg·m; 163.8 ft·lb) at 4,300 rpm
Firing order1-5-3-6-2-4
Dimensions (L x H x W):
Weight175 kg(385 lbs)

Cylinder block

The BMW M20B25 engine has a cast-iron cylinder block, bore is 84.0 mm (3.31 in) and stroke is 75.0 mm (2.95 in). The cylinder block, a monoblock specially cast structure. The pistons are each fitted with two compression rings and a single oil ring.

Cylinder block
Cylinder block alloyCast iron
Compression ratio:9.7:1, 9.3:1, 8.3:1
Cylinder bore:84.0 mm (3.31 in)
Piston stroke:75.0 mm (2.95 in)
Number of piston rings (compression / oil):2 / 1
Number of main bearings:7

Main bearing cap bolts tightening procedure and torque specs:

  • 60 Nm (6.12 kg·m; 44.28 ft·lb)

After securing bearing cap bolts, make sure crankshaft turns smoothly by hand.

Connecting rod bearing bolts

  • Step 1: 5 Nm; 0.51 kg·m; 3.69 ft·lb
  • Step 2: 20 Nm; 2.04 kg·m; 14.76 ft·lb
  • Step 3: Tighten bolts at the angle of 70°

Cylinder head

The cylinder head is made of an aluminum alloy with good cooling efficiency. Cylinder head number is 885. The engine has single overhead camshaft with intake and exhaust duration of 256° The camshaft is driven by a timing belt.

Intake valves are 42.0 mm (1.65 in) in diameter and exhaust valves are 36.0 mm (1.42 in), valve lift are 10.1 mm (0.397 in). The valves are opened and closed by rocker arms. These rocker arms perform a dual function of cam follower valve adjuster, as one end contains the eccentric and the adjustment nut. The rocker arms pivot around rocker arm shafts.

Cylinder head
Valve Arrangement:SOHV, belt drive
Valves:12 (2 valves per cylinder)
Intake/Exhaust duration:256° / 256°
Valve head diameter:INTAKE42.0 mm (1.65 in)
EXHAUST36.0 mm (1.42 in)
Valve lift:INTAKE10.1 mm (0.397 in)
EXHAUST10.1 mm (0.397 in)

Head tightening procedure and torque specs:
Hex-Head

  • Step 1: 40.0 Nm; 4.1 kg·m; 29.5 ft·lb
  • Step 2: 60.0 Nm (6.1 kg·m; 44.3 ft·lb)
  • Step 2: Turn all bolts 25°

Torx Head

  • Step 1: 30.0 Nm (3.1 kg·m; 22.1 ft·lb)
  • Step 2: Turn all bolts 90°
  • Step 2: Turn all another 90°

Maintenance data

Valve clearance
Intake valve0.25 mm (0.010 in)
Exhaust valve0.25 mm (0.010 in)
Compression pressure
Standard10.2 kg/cm2 (9.9 bar, 145 psi) / 350 rpm
Oil system
Recommended engine oil 5W-30, 5W-40, 10W-30, 10W-40, 10W-50, 15W-40, 15W-50
Oil type API
Engine oil capacity (Refill capacity) With filter change 4.25 l
Without filter change 4.0 l
Oil change interval, km (miles)10,000 (6,000)
Ignition system
Spark plugBosch: W8LCR, NGK: ZGR5A
Spark plug gap0.7 mm (0.028 in)

Vehicle applications

ModelYears Produced
BMW 325i (E30)1985–1993
BMW 525i (E34)1989–1990
BMW Z11988–1991
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