M42 Lightweight Flywheel Upgrade

If you have considered a lightweight flywheel for your M42, then there are a few choices.  You can use an M20 flywheel, or you can select an aftermarket M42 flywheel from someone like JB Racing.  The M20 choices are numerous, but there are two that would already be considered a lightweight in stock form.  These are the flywheel from the euro e21 323i and the 1982 528e. These particular flywheels weigh 13.6 pounds and should be a significant upgrade over the very "massive" M42 flywheel. In order to use the M20 lightweight flywheel, some other things had to be taken into consideration.

M42 Left, M20 right

When using an M20 flywheel, you need to account for the difference in tooth count between the M20 and M42 ring gear. You can change the starter drive gear on the M42 starter, or you can just use an M20 starter. We used a Bosch unit from an e30 325i. There has been some controversy over the use of the m20 starter on an M42. I can assure you that the M20 starter works with no issue. It bolted right up and the wiring is an exact match as well. In order to use the M20 flywheel, you will need to have the engine side of the flywheel machined very lightly in order to clear the 10mm bolts that secure the rear main seal housing to the block. The other option is to use the spacer plate that is on the transmission side of the M42 dual mass flywheel. It is secured by two rivets and they are drilled out in about five minutes. It is placed between the flywheel and the crankshaft.

One complication will be that the spacer from the M42 flywheel is not machined with one large hole to fit over the sleeve in the crank. You will need to increase the diameter of one of the bolt holes in the spacer. This is not difficult, but it is necessary. In order to secure the flywheel to the crank, you will need the corresponding shorter flywheel bolts to use with the M20 flywheel. The M42 flywheel bolts are now significantly too long. As for the clutch, we used a new OEM Sachs E30 325i clutch kit for a single mass flywheel. However, we chose not to use the 325i throw out bearing. The e21 323i throw out bearing is optimal because it will maintain the proper throw of the clutch when using a much thinner flywheel. I also thought I would mention this because I have seen it omitted, but all BMW single mass flywheels use a shim plate between the flywheel bolts and the flywheel.

Shim plate sitting on flywheel

Summary of the m20 flywheel upgrade:

E21 323i flywheel (13.6 pounds)

e30 325i clutch disc and pressure plate

E21 323i throw out bearing

E30 325i starter

E21 323i flywheel bolts (M12 x 1.5 x 28mm)

Flywheel shim plate (part number 1122126282)

Round spacer plate from transmission side of M42 flywheel (or minor machine work to M20 flywheel)