The issues are:
1) The M20 flywheel is a slightly larger diameter than the M10 or M30 flywheel.
As you mount the starter to a fixed point - ie. cast into the block - then the starter position can't be moved further out to accommodate the larger flywheel.
The M10 based engines - i.e. M10, S14 and M30 all use the same smaller diameter flywheel, with course, deep ring gear teeth.
The M20 based engines - i.e. M40, M42, M50 etc. all share the larger diameter flywheel, which has finer, shallower ring gear teeth.
The main difference seems to be in the ring gear, so you might be able to put an M10 / M30 ring gear onto an M20 flywheel, although the M10 guys on 02FAQ etc. suggest that the ring gear doesn't fit, the M20 flywheel is just slightly too big.
2) the M20 starter bendix is different to the M10 / M30.
It has a spacer to limit the travel of the starter gear and when mounted to the M30 block, the starter only barely engages with the flywheel. Apparently the spacer can be removed from the bendix, but again, it requires modification & even still, the starter isn't real happy trying to engage with the larger flywheel.
Anyway - pics speak louder than words...
Tooth comparison on ring gears, note M10 flywheel (top) is much courser and deeper than the M20 flywheel (bottom) - this effects the diameter of the flywheel.
Here, you can see how the M20 ring gear failed after being mounted to the M30 (on right) - note that the starter only barely engages with the ring gear, and that the profiles of the flywheels are quite different - the M10 flywheel (left) has meat on the starter side of the ring gear, the M20 flywheel doesn't.
Unfortunately I didn't snap a pic of the differences between the two starter bendix, but if you get hold of an M20 & M10 or M30 starter then the difference is quite obvious.
If you still want to attempt it, you need the e21 323i 30mm throw out bearing as the common 25mm one (fitted to most BMWs) is too short, and ultimately causes the slave cylinder to over extend and fail. I attempted to get around this by lengthening the slave cylinder rod 7mm, which did work, but then the ring gear failed.
In any case - I tried hard to defeat the internets, but ultimately I lost & fitted an M10 flywheel with an HD 215mm clutch kit, which has worked fine.
1) The M20 flywheel is a slightly larger diameter than the M10 or M30 flywheel.
As you mount the starter to a fixed point - ie. cast into the block - then the starter position can't be moved further out to accommodate the larger flywheel.
The M10 based engines - i.e. M10, S14 and M30 all use the same smaller diameter flywheel, with course, deep ring gear teeth.
The M20 based engines - i.e. M40, M42, M50 etc. all share the larger diameter flywheel, which has finer, shallower ring gear teeth.
The main difference seems to be in the ring gear, so you might be able to put an M10 / M30 ring gear onto an M20 flywheel, although the M10 guys on 02FAQ etc. suggest that the ring gear doesn't fit, the M20 flywheel is just slightly too big.
2) the M20 starter bendix is different to the M10 / M30.
It has a spacer to limit the travel of the starter gear and when mounted to the M30 block, the starter only barely engages with the flywheel. Apparently the spacer can be removed from the bendix, but again, it requires modification & even still, the starter isn't real happy trying to engage with the larger flywheel.
Anyway - pics speak louder than words...
Tooth comparison on ring gears, note M10 flywheel (top) is much courser and deeper than the M20 flywheel (bottom) - this effects the diameter of the flywheel.
Here, you can see how the M20 ring gear failed after being mounted to the M30 (on right) - note that the starter only barely engages with the ring gear, and that the profiles of the flywheels are quite different - the M10 flywheel (left) has meat on the starter side of the ring gear, the M20 flywheel doesn't.
Unfortunately I didn't snap a pic of the differences between the two starter bendix, but if you get hold of an M20 & M10 or M30 starter then the difference is quite obvious.
If you still want to attempt it, you need the e21 323i 30mm throw out bearing as the common 25mm one (fitted to most BMWs) is too short, and ultimately causes the slave cylinder to over extend and fail. I attempted to get around this by lengthening the slave cylinder rod 7mm, which did work, but then the ring gear failed.
In any case - I tried hard to defeat the internets, but ultimately I lost & fitted an M10 flywheel with an HD 215mm clutch kit, which has worked fine.
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