To convert the M42's clutch/flywheel setup to that of the M20 engine.

A while ago I decided that when the time comes to replace the clutch, I would upgrade to a lightweight flywheel. Why not just use an M42 lightweight unit? Well, the M42 uses a VERY heavy dual-mass flywheel (28lb), and a solid clutch disc. Between the two halves of the flywheel contain the springs that help absorb some of the impact/vibrations from the clutch being engaged (these springs are USUALLY found on the clutch disc, on other cars). The M42 aftermarket unit is a single-mass one, and using this would mean having a solid clutch setup, which is uncomfortable, and more importantly, very hard on the driveline.

The M20 uses a sprung-hub clutch disc setup. The M20 clutch friction surface is only 13mm larger in diameter (228mm), and will fit the M42 with some modification to the starter & a different throw-out bearing. The outside diameters of the two flywheels are identical. This conversion has been performed before, and is reasonably well documented. There are lightweight flywheels available for the M20 at MPD Motorsport (8lb) for about $400, or you can use a stock M20 flywheel. At 19lbs, the stock M20 unit is a "lightweight performance" flywheel for the M42 and could be used as well! I opted for the M20 stocker as I was offered a used one for $60.

M20 flywheel (Stock or Lightweight) for a 08/1987+ 325i/iS ONLY

M20 clutch disc & pressure plate

Starter pinion/gear from an 08/1987+ 325i/iS ONLY

Throw-out bearing from an E21 323i [PN 21 51 1 204 525]