M42 single mass VS M20 single mass conversion

[peerless]

I keep hearing about this M20 conversion so I decided to do some research into what the real advantage was.

Flywheel weights
------------------
5/90 US 318is M42 5sp. A/C. Originally Twin Mass 21 21 1 223 550 12.250kg / 27lbs
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3/89 European 318is M42 5sp. NON-A/C. Single Mass 11 22 1 739 315 9.9kg / 22lbs
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3/90 US 325is M20 5sp. A/C. Single Mass 11 22 1 706 573 8.5kg / 19lbs
__________________________________________________  ______________

Clutch Kit weights (Includes Clutch Cover, Disk, Throw-out bearing)
-------------------
5/90 US 318is M42 Dual Mass 21 21 1 223 571  5kg/11lbs
3/89 European 318is M42 Single Mass, 21 21 1 223 570  5.6kg / 12lbs
3/90 US 325is M20 Single Mass, 21 21 1 223 102 6.98kg / 15lbs
__________________________________________________  _______

Combined weights as rotating assembly
-------------------------------------------
5/90 US M42 Dual Mass. 17.25kg / 38lbs
3/89 Euro M42 Single Mass. 15.5kg / 34lbs
3/90 US 325is M20 Single Mass 15.48kg / 34lbs
____________________________________________

Total weight savings of M20 single mass to M42 single mass  0 lbs.

__________________________________________________  _______

With the M42 specific flywheel you can use a M42 starter. No need to change the starter pinion gear as required with the M20 conversion. This can be a problem if your starter shits the bed at the track or some parking lot. You can go to a parts store and buy a starter and throw it in the car. What if the car changes ownership and the new owner takes it to a shop and the tech is scratching his head why the new M42 starter doesn't work.

In regards to cost I have broken that down as well. Here the M20 conversion does have and advantage. M20 single mass flywheels are plentifully available in the US. These can be found used in mostly good condition for $50-100. The Euro M42 single mass flywheel is not a common item here in the states so they most likely would be purchased new. Pelican has them for $323. We purchased ours through a private party for $200 shipped, new in the factory BMW box. I haven't checked my dealer pricing yet.

Price Comparison (new parts, pelican parts for pricing)
______________________________________________
5/90 US M42 Dual Mass,  Flywheel $685 Clutch Kit $209 ($894)
3/89 Euro M42 Single Mass, Flywheel $323 Clutch Kit $230 ($553)
3/90 US M20 Single Mass, Flywheel $367 Clutch Kit $216 ($583)

Total Cost saving of M20 vs M42 flywheel conversion using new parts, none, it would cost $30 more. But again that using new flywheels and as mentioned before the US single mass flywheels are fairly plentiful in good used form.

Also as one last note of interest, the M42 single mass because of its design leaves a LOT of meat that can be milled off for a very good weight savings, maybe more so then the M20?.

A nice trait of the M20 is it has more surface area. M42 215mm/8.46" to M20 228mm/8.97" diameters. Thats 13mm/.500" (1/2") difference. For the forced induction guys, thats a good thing.

Well that about wraps up my research on the subject. I hope you find it useful and informative.

If you see an error in what I researched please point it out and provide references to included information. Something of this nature needs to be correct.

Part numbers and weights where provided by bmwfans.info

M42 single mass advantages:

Lighter weight over the dual mass unit and just as light as the M20 conversion.
Use factory M42 starter with no modifications
Use factory clutch kit complete
Uses factory length flywheel bolts
All parts readily available.

Disadvantage:
Cost involved in acquiring single mass flywheel
====================================

M20 single mass conversion advantages:

Lighter then M42 dual mass, but same weight as M42 single mass.
1/2" more clutch surface diameter for improved load capacity on high HP M42's.
Less cost for flywheel as they are plentiful used in good condition.
All parts readily available.

Disadvantages:
Must purchases additional throw out bearing as the M20 bearing is too short. (323i)
Must use modified flywheel bolts
Must use spacer in back of flywheel for proper spacing
Must modify existing starter and install M20 pinion gear

For complete M20 conversion instructions and modifications read here,  http://www.blackforestindustries.com/m20flywheel.htm

Comments?

[tjts1]

Excellent info. Thanks for doing all the research. This deserves a permanent place in refference. Did the later m42/m44 engines in the e36 chassis also use the same dual mass flywheel?

[nomad]

Great info. The way people write about the conversions you'd think it'd be at least 10lb difference... not so.
What about cost and weight of aftermarket flywheels? Would it be worth it to get the flywheel if the clutch is still relatively new?

good question about E36 clutches, although, didn't they get a different transmission? Somewhere I thought it said those newer trannys were better?

So how much weight can be milled off the M20 flywheel?
If I have to do a clutch job i'll convert but for 4lb it's not a huge deal now.

[peerless]

We purchased a 93' E36 M42 as a core engine and it had a dual mass flywheel.

It turns out that the single mass flywheel was also used in early US E30 318is models, from 11/89-3/90. Hell most catalogs don't even show a US M42 until 91'.


http://bmwfans.info/part-11221739315/&detail_usage=E30|2-T|318is-M42|USA|L|M
http://bmwfans.info/part-11221739315/

[Wise Old Dog]

You can mill a M20 flywheel down to about 13 lbs. A M42 down to 17 lbs. I have one of each. The M20 is prefered if you have a modified engine as it can handle more power. Otherwise the M42 is just fine. There is an e-bay vendor that has a BMW junkyard in GA that sells the M42 single mass flywheel every once and awhile. Goes by the name of VINES. I got mine for $50

[batsbats]

I'm aware of the rattling that occurs with BMWs and dual mass FW setups changed to lighter FWs.  I read a lot of complaint on bf.c about the E36s.

Does it 'chatter' at idle?  Can you hear it from outside the car?

[dude8383]

I have a ltw 323i flywheel that weighs roughly 11lbs.

I paid less for the whole deal that the M20 clutch kit.

This by far is the best option because the flywheel is lighter than ALL three you've mentioned. Not sure about cost or availability but I got lucky with my purchase.

323i Flywheel : $75
323i TOB: $63
M20 Clutch: $100


ZERO chatter with this setup.

[peerless]

I forgot about the 323 flywheel. I have one as well. I scored it for $40 at the local pick a part.

I will agree thats a light weight flywheel. As you mentioned its lighter then all 3 compared.

Interesting, might have to update my comparisons. I would assume that the starter still needs to be modified?

I have a ltw 323i flywheel that weighs roughly 11lbs.

I paid less for the whole deal that the M20 clutch kit.

This by far is the best option because the flywheel is lighter than ALL three you've mentioned. Not sure about cost or availability but I got lucky with my purchase.

323i Flywheel : $75
323i TOB: $63
M20 Clutch: $100


ZERO chatter with this setup.

[dude8383]

I forgot about the 323 flywheel. I have one as well. I scored it for $40 at the local pick a part.

I will agree thats a light weight flywheel. As you mentioned its lighter then all 3 compared.

Interesting, might have to update my comparisons. I would assume that the starter still needs to be modified?

Yeh I had to swap pinion gears from the m20 starter and throw it in the m42 starter...pretty easy to do though, just gotta heat it up like crazy.

[batsbats]

I have a ltw 323i flywheel that weighs roughly 11lbs.

I paid less for the whole deal that the M20 clutch kit.

This by far is the best option because the flywheel is lighter than ALL three you've mentioned. Not sure about cost or availability but I got lucky with my purchase.

323i Flywheel : $75
323i TOB: $63
M20 Clutch: $100


ZERO chatter with this setup.

I'll have to keep my eye open for 323i parts!  Where did you score yours?

[dude8383]

I'll have to keep my eye open for 323i parts!  Where did you score yours?

I scored the flywheel on bimmerforums, but it was pure luck because someone PM'ed me asking if I wanted one.

The TOB I ordered brand spankin new from Patrick @ BMA Parts.

[jayzc529]

Can you use the dual mass clutch with a single mass flywheel?

[Wise Old Dog]

No, will not fit.

[E30nate]

Euro 323i throw out bearing:   21517521471

US 325i throw out bearing:    21517521471

US 318is throw out bearing:    21517521471

so, unless realoem is wrong, you dont need to change the bearing, right?

just wanted to mention how much of an idiot i am, as about a month ago there was a 323i at pick n pull, and i didnt even think to pull the flywheel. just throwing that out there.

[nickmpower]

You can just use an m20 starter

I bought a m20 jb racing 8lb flywheel and m20 clutch disk and m3 pressure plate. Should be able to give feedback on the setup soon