TL;DR

  • What: The cooling-system changes needed when an M50 goes into an E36 that came with a four-cylinder.
  • The key difference: the four-cylinder radiator has its expansion tank built in; the six-cylinder system uses a separate expansion tank mounted on the six-cylinder fan shroud. So the radiator, shroud, tank and their hoses all come from a six-cylinder E36.
  • Also needed: the six-cylinder lower radiator mount, the six-cylinder auxiliary fan, and new wear parts throughout.
Inline-six engine on a stand with expansion tank, thermostat housing and coolant hoses on the bench
Swap engine on the stand with the cooling parts it needs on the bench.AI illustration

Radiator, shroud and expansion tank

Four-cylinder E36Six-cylinder E36 (M50)
Expansion tankIntegral with the radiator (Bentley)Separate tank on the fan shroud
Main fanBelt-driven with viscous clutch, or electric on some four-cylinder modelsBelt-driven with viscous clutch
Cooling system capacity (Bentley)6.5 L10 L (M50/M52)

What to fit, per owners who have done the swap:

  • Six-cylinder radiator and hoses. A four-cylinder radiator can be modified to work in a pinch, but it isn’t recommended.
  • Six-cylinder fan shroud and expansion tank, with the long hose from the engine to the tank and the short hose from the tank to the radiator.
  • Driver’s-side lower radiator mount: the four-cylinder radiator is narrower, so its mount is larger and the six-cylinder radiator won’t sit in it. Use the six-cylinder mount, or modify the old one. The other side is reported to be the same for both.
  • If your four-cylinder car had an electric main fan behind the radiator, it is replaced by the M50’s belt-driven fan with its viscous clutch on the water pump.

Part numbers: look them up by VIN for a six-cylinder E36 of your body style on RealOEM.

Auxiliary fan

Every E36 has a two-speed electric auxiliary fan in front of the radiator, behind the grille. Bentley: it serves mainly the A/C, but it also runs when the coolant gets hot, switched by a dual temperature switch in the radiator:

SpeedSwitches on at
Low91 °C
High99 °C

Fit the six-cylinder auxiliary fan (it mounts on the bumper side) and connect the radiator’s temperature switch. The DME doesn’t control these fans.

Heater circuit

The M50’s heater hoses connect at the rear of the cylinder head and run to the heater valve and heater core. Connect both, check the heater valve while you can reach it, and set the heater to full warm when filling. That opens the heater circuit so it fills and bleeds too.

Wear parts: replace everything

The cooling system on a donor engine is as old as the engine. While it’s on the stand, fit a new water pump (with a metal impeller), thermostat and seal, hoses, clamps and expansion tank cap. See the cooling system overhaul for the details and torque values.

Filling and bleeding (Bentley)

Use BMW-approved, phosphate- and nitrate-free antifreeze, 50/50 with distilled water: about 10 litres of mix for the M50.

  1. With the engine cold, set the heater to full warm.
  2. Fill slowly to the expansion tank’s COLD (KALT) mark.
  3. With a bleed screw: loosen it, switch the ignition on without starting, and keep adding coolant until it flows from the bleed screw free of bubbles. Then tighten the screw (the one on the thermostat housing to 8 Nm).
  4. With a separate tank and no bleed screw: Bentley notes the system bleeds itself through the vent hose on the tank.
  5. Run the engine to operating temperature, let it cool, and top up to the mark.

If the temperature gauge acts up after the swap, suspect trapped air or a missing tank hose, and bleed again.

Checks after the first drives

  • Coolant level cold, against the tank mark, for the first few drives
  • Leaks at every new joint after a full heat cycle
  • The auxiliary fan comes on when the coolant is hot or the A/C is switched on

Related: M43 to M50 parts list and conversion guide.