What they are
The 6BT is the mechanical 5.9 inline six, close cousin of the early Ram 12-valve. The 4BT is essentially the same design with two fewer cylinders, a 3.9 liter four used in delivery vans and industrial work. Both are known for simple mechanical fueling.
4BT or 6BT?
- The 4BT is shorter and lighter and fits smaller trucks, but is known for vibration and noise.
- The 6BT is smoother but long and heavy; swap shops point it at three-quarter-ton and larger chassis.
- Either is heavier than most gas engines it replaces, so springs, brakes and steering may need work.
What a swap usually involves
- Engine mounts and a transmission adapter.
- Cooling: radiator, intercooler and plumbing.
- Oil pan and front-axle clearance.
- Fuel supply and return.
- Wiring, gauges and throttle linkage.
- Driveshaft lengths and gearing.
Emissions and legality
Swapping an engine into a road vehicle is regulated. EPA treats removing or defeating emissions controls as tampering, and states add their own rules. Check with your state before you buy an engine.
Want a new engine instead of a used industrial one? See the Cummins R2.8 crate engine.
Where this comes from
- 4WD Mechanix: Cummins 4BT and 4-cylinder ISB diesel conversions
- CCJ: Cummins 6BT and 4BT among popular rebuilds at Colorado Engine Company
- 4BT Engines: 4BT vs 6BT, which is the better swap candidate?
- 4BT Diesel: 4BT vs 6BT Cummins
- Diesel World: Cummins 4BT 101
- U.S. EPA: Tampering and aftermarket defeat device enforcement policy
Last reviewed 2026-10-19. General information only: your owner's manual and service information come first.