after much digging and reading I finally found a logical post on this mod with part numbers!!!
Reposted from: http://forums.bimmerforums.com/forum/sh ... i-flywheel
From my research on M42 flywheels, it is probably best to convert to an M20 flywheel. The M42 has a dual-mass flywheel. The clutch disc is solid...no spring plate to dampen vibrations & help on starts. The springs are between the 2 halves of the flywheel. Converting to a LTW aluminum flywheel on the M42 means a SOLID CLUTCH since the LTW ones are 1-piece. That is not necessarily a good thing. When I do my flywheel, I am going to use an M20 flywheel & clutch since it uses a sprung-hub clutch disc. All I need to change other than that is the starter pinion. Then I can have a dampened LTW clutch setup. Without that dampebing you are exposing your tranny to a LOT of unnecessary vibrations & stress, and starting from a stop might be tougher. Just my $.02.
Kos-motate139...do you autoX in Marina with the BMWCCA? I think I met you there once...the only other M42 in the house.
Here is a writeup for converting to the M20 setup by D. Coop:
"M20 Flywheel Conversion for the M42
Parts Needed: 323i Release Bearing BMW PN: 21 51 1 204 525
M20 Clutch Disc
M20 Pressure Plate
88-91 325i Single Mass Flywheel
88-91 325i Starter Pinion Gear BMW PN: 21 51 1 204 525
*NOTE : Steps one through five are covered in the Bentley Manual and this invaluable guide to servicing your car is highly recommended when you are attempting to DIY. The steps below are just a general outline based on my experience.
Before I begin I would like to thank Stu Mchenry, Jeff Kinney, Dave Ruiz and a damaged pressure plate from an aftermarket company(photo 1), who will remain nameless at this time, for providing me with the motivation, information, and technical assistance for this project.
Step One: With your car on the ground disconnect your battery. After disconnecting the battery secure the vehicle and elevate it to a comfortable working height(photo2). It is highly advised to make use of a professional lift when executing this procedure, yet it is possible to accomplish using jack stands.
Step Two: First unscrew your O2 sensor from the exhaust and either hang it out of the way or disconnect it all together at the harness. Next, remove the catalytic converter section of your exhaust by unbolting it from the down-pipe and rear portion of the exhaust. This will make room to drop the transmission. Since the rear section of my exhaust was welded to the catalytic converter, I was able to unbolt the exhaust at the down-pipe only and slide the exhaust clear of the transmission letting it hang. This may or may not work for you so use discretion in choosing a course of action. Next you will want to remove the heat shielding above the driveshaft and the support below it, then move onto the three large bolts connecting the driveshaft to the transmission. Carefully separate the drive shaft from the transmission using a pry bar.
Step Three: Now it is time to disconnect everything from the transmission (photo 3). You will need to remove the small circlip that connects the shifter to the selector rod, and then slide the stud free from the hole. There is a support running along the top of the transmission that is connected to the transmission and the body of the car. You can either remove this at the transmission or the body. Removing it at the body was found to be more accessible. Next you will need to locate and disconnect your reverse light plug. After you have disconnected the reverse light plug, you will need to unbolt your clutch slave cylinder from the transmission. After unbolting the slave cylinder locate the two long bolts that attach the starter to the transmission. This proved to be very difficult from underneath the car. In the end a combination of accessing the starter from the above and below with someone spotting where the socket was in relation to the bolt proved to be the most effective method. Once the starter is unbolted, disconnect all electrical lines running to the starter, including grounds and remove the starter from the car entirely.
Step Four: With everything removed from the transmission, it is now time to unbolt it. To be safe it is suggested that you support the engine with a stand underneath the oil pan. Next remove all of the bolts connecting the transmission to the engine block. Next, using a transmission jack, support the transmission and secure it to the jack using straps or tie downs. It is important that you make sure that the transmission is strapped down snugly and securely. Now unbolt the rear transmission mounts from the car. The transmission should now be free at this point. Gently slide the transmission back away from the clutch assembly. It will be necessary to adjust both the height and angle of the transmission jack so that the splines on the input shaft do not bind and will come free with less resistance. With the transmission free, lower it and roll it clear of the car.
Step Five: Now it is time to remove the clutch assembly (photo 4). Remove the bolts around the perimeter of the pressure plate and pull it off of the flywheel. The clutch disc will come out with it so be careful not to be struck by it if it should fall. After you have removed the pressure plate and clutch disc it is now time to unbolt the flywheel from the crank (photo 5). Locate the bolts in the center of the flywheel and remove them. Once removed gently pry the flywheel away from the block. Once again be careful, as it is heavy and sometimes awkward.
Step 6: Now comes the part the Bentley Manual will not cover. With the new single mass flywheel placed side by side with the old, dual mass flywheel, you will notice a distinct difference in thickness (photo 6). To compensate for this thinner flywheel, you must make three specific modifications to the existing parts. The first modification is the flywheel bolts themselves. Since the flywheel is now thinner, the original flywheel bolts (which are not recommended to be reused but can be if you feel comfortable doing so) will need to be either cut down, or replaced all together so that they do not go all the way through the flywheel (photo 7). The recommended length is just under one (1) inch when measured from below the bolt head, that is to say not including the thickness of the bolt head. The third modification we found to be necessary was unexpected. When we test fitted the flywheel to the crank it would not spin freely because protrusions from the oil pan and part of the block came in contact with the flywheel. To remedy this issue we needed to find a spacer that was the right thickness and had the correct diameter holes. Luckily the BMW Gods were smiling on us that day, and we were able to remove an exact fit spacer from the center of the old, M42 flywheel (photo
. To remove this ”aspacer”a you simply drill out the two rivets that hold it in place. Drill from the top side, the same side as the spacer and it will come free very easily. Voila, you have a fix.Step 7: Next you will need to replace the old release bearing with the new, longer one to compensate for difference in thickness (photo 9). This is as simple as sliding the old release bearing off and replacing it with the new one (photo 10). Once again, with both flywheels side by side, you will notice another difference, this difference being the teeth on the flywheel. The single mass has finer teeth, which will not match up with the original starter pinion gear. To correct for this situation the M42 starter must be taken apart and the gear must be replaced with the new starter pinion gear (photo 11). You must remove the shift lever, which looks like a wishbone,(part number 8 on the diagram) from the M42 starter pinion gear and attach it to the new M20 starter pinion gear. This is relatively straightforward. See the diagram for details (photo 12).
With these modifications completed you are ready to reinstall the flywheel, the clutch assembly, the transmission, and the exhaust system in the reverse order of removal. Make sure to follow torque specs and double-check each step. It is also very important to make sure that the flywheel spins and does not bind. Assuming all goes well you will notice a great difference in acceleration in all gears when cruising, and a particularly marked difference off the line in first, and in second gear. Smoothness of shifting suffers noticeably but it is not intolerable. The idle, which so many people are concerned with, hardly changed at all. In my case it almost seemed to improve. Overall this modification is extremely worthwhile and is a great bang for the buck considering the cost of an aftermarket lightweight flywheel. We at Black Forest Industries highly recommend it!"
and so do most of the others I have seen.
