top of page

Cummins 6BT 5.9L 12v Engine (Design, Problems & Reliability)

Updated: Jan 9

The Cummins 6BT is a 5.9L 12-valve inline six-cylinder turbocharged diesel engine, it produces 160-230 hp and 544-589 nm of torque. It's the most well-known Cummins B series truck engine

  • 1984 is when this engine began to be used in agricultural applications.

  • After 1989 it was used light-medium duty vehicles, and in some heavy duty trucks and buses.

  • The 6BT is also popular for engine swapping some vehicles.

  • Between 1989 and 1998 this engine became popular in Dodge Ram pickup trucks due to its power at low rpm and across the rev range, as well as better fuel efficiency than petrol V8s.

The 5.9L Cummins, also known as the 12-Valve Cummins, was the first member of the Cummins B-Series to be used in a light truck vehicle. It was the first diesel engine to replace V8 gasoline engines in full-size pickup trucks.


Table of Contents:

Cummins 6BT 5.9L 12v Engine

Cummins 6BT Engine Design

The 6BT used Bosch fuel systems, injector, and VE rotary pump and P7100 inline injection pumps. Early versions were supplied with CAV rotary pumps before the Bosch system became the standard.

The 3.9L 4BT engine is a smaller version of the 6BT, the engine's architecture is the same, although the cylinder count is different.

With six cylinders and a displacement of 5.9 litres, the Cummins 6BT engine is more powerful and smoother (due to being an inherently well balanced inline six engine) than the 4BT.

The engine block and cylinder head are made of cast iron. It has an OHV valvetrain with 12 valves and solid tappets (2 valves per cylinder). The engine is frequently referred to as a 12-valve Cummins when it is in this form.

At the front of the engine are timing gears (no timing belts or chains are used). Both a fuel pump and an oil pump use gears instead of the typical belt or chain setup.

A mechanical injection pump and fuel injectors are included in the engine's design. Prior to 1994, although some early models utilised a CAV rotary injection pump, most engines used a Bosch VE44 rotary injection pump.

The Bosch P7100 injection pump is used in engines created after 1994.

Since 1991, all 6BT engines are equipped with a turbocharger and an air-to-air intercooler.

Instead of using a traditional glow plug system to assist with cold engine starting, a grid heater is installed on the intake manifold.

A 24-valve 5.9L ISB Cummins engine with Bosch high-pressure Common Rail fuel injection replaced this 12-valve 6BT engine in 1998.

The turbochargers used in the 6BT were manufactured by Holset:

  • Holset H1C (1989-1993)

  • Holset WH1 (1994)

  • Holset HX35 (1994-1998)

A variety of transmissions were offered with this engine:

  • Chrysler 47RH (4-speed automatic)

  • New Venture NV4500 (5-speed manual)

  • GETRAG G360 (5-speed manual)

6B Engines

The 6B is the naturally aspirated version of the 6BT, it has a power output of 150-173 hp and a compression ratio of 19:1. This version was not used in automotive applications.

6BT Engine Specs



Production Years


Block Material

Cast Iron

Cylinder Head Material

Cast Iron

Fuel Type


Fuel System

CAV rotary injection pump, most engines used a Bosch VE44 rotary injection pump (pre-1994), Bosch P7100 injection pump (1994+)


Inline six-cylinder


OHV 2 valves per cylinder


4.02 inch (102 mm)