BMW M20B25

The 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

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.

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.

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

Vehicle applications

Author: Sergei Wers
About: Co-founder and CEO of a multi-brand auto repair shop (engine repair, chassis repair, body repair and painting) for over 15 years. I have been developing internet sites with technical specifications for more than seven years.