M20 single Mass Flywheel conversion for E36 M50/M52: Basics

A cheap single mass flywheel (SMF) conversion for M50 and M52 (not M52TU) engines is the use of the flywheel for the M50’s predecessor, the M20 engine. The M52 is always equipped with a dual massflywheel (DMF) The benefits of this conversion is a reduced rotating mass (in addition to the rededuced regular mass) and RPMs will drop faster when the clutch is disengaged and the gas lifted, which allows fast shifts without the loosing traction on the rear axle when cornering because of mismatching RPMs of engine and drivetrain. The downside is a loss of comfort while driving and a loss of smoothness while Idling. For the M52 beeing smooth as butter even with high mileage when maintained properly, i was not able to notice any serious differences for idling and driving comfort. The mathematics and calculations for the lower rotating mass will be covered in another blog post. The M50 engine single mass flywheel can be used aswell and fits plug-and-play into the M52 when used with M50 clutch, pressure plate and release bearing, but it is quite rare beeing only used from January 1988 in the E34 and from August 1990 until July 1992 in the E36 for M50B20 Engines without air conditioning.

Additionally, compared to the M20 Flywheels, it is very heavy (10,5 kg vs. 6,3kg or 8,5 kg) and only offers the same clutch Diameter (228mm), but unlike the M20 clutch, there is no SACHS performance clutch (181862168232) available. Using a stiffer pressure spring and reinforced clutch plate, the Sachs performance clutch can take up to 350 Nm of torque, which equals the torque of the 3.2 E36 M3. Anyway, the M52 328i models use 240 mm clutches, while the M50 325i and M52 323i are only equipped with 228 mm clutches. Keep that in mind when converting an M52 328i engine to the M20 flywheel.

The M20 is straight-six, single overhead camshaft, 12 valve engine used in the BMW E21 and E30 3-Series and E12, E28 and E34 5-Series. The M20 was always equipped with a single mass flywheel, which can be used in M52 engines quite easily. There are two different M20 flywheel versions:

The flywheel used until September 1984: 11221264517 weighting 6.3 Kg
E21: 320i, 323i
E30: 320i, 323i
E12: 520i
E28: 520i

Theflywheel used from september 1984: 11221706573 weighting 8.5Kg
E30: 320i, 323i, 325i, 325iX
E28: 520i
E34: 520i, 525i

The 6,3 kg Flywheel will fit the M52 engine without modifiying, but you can still take some weight out by milling. The 8,5 kg Flywheel needs some turning on the engine side, where there is a exalted ring. For M52 Engines, there is not enough clearance to the crankcase, and this ring needs to be removed on the turning lathe. This way we also will get the weight of our flywheel down, but not as low as 6,3 kg due to other difference in shape. You could take some more weight out in the inner area, but at this point you really should know what you are doing, and a CNC lathe would be helpful aswell. We will be happy just with the removal of the ring For the additional weight removal by milling both flywheels offer the some potential, so the overall lighter version will be the 6,3 kg flywheel. But because it is the older Part (26 years!) it will be harder to find.