Identifying M20 single vs twin mass flywheel for swap

[otoolb]

Hey all,

I purchased an M20 flywheel for the infamous flywheel swap.  The part came in and it appears to be a "twin mass" flywheel according to realoem.  I know realoem can be wrong, but it weights 24lbs.  

I am positive that this is not the single mass flywheel that I was looking for because several sources have quoted this at about 18lbs stock (bmwman91, tuner motorsports, ...).  I am looking for some semantics help and general clarification in case i run into trouble returning the part.  

First of all, does anyone have a PN for the true single mass flywheel? I have PN 11221706573, but can't find a PN for the 18lbs flywheel.

Second, can anyone help clarify if there is a difference between the term "twin mass" and "dual mass"?  I would have assumed that they mean the same thing, but my "twin mass" flywheel is a single piece of steel.  The supplier said that all dual mass flywheels have two masses connected together with a "dampner" (yes, he said dampNer and I laughed) so he is going to try to tell my that my 24lbs beast is a single mass flywheel just as advertised.  

anyway, I am excited about this swap.  Just looking for a little help.  

Thanks

[dude8383]

Can you take a pic of what you received? It's fairly easy to tell the two apart.

[otoolb]

Sorry, my post is a little misleading.  I know that I have the heavier twin mass flywheel for the m20.  I know it is an M20 flywheel because I recognize it, and weighed it.  

My two last questions are the most important.  I am looking for a PN for the lighter flywheel so that I can eliminate any confusion for the supplier.  Also, I am interested to know if it is common for either a dual mass or twin mass flywheel to be manufactured out of a single piece of steel.  Anyone I have talked to so far says that dual mass is ALWAYS two pieces joined together.  This would hurt my case in returning the flywheel because it would mean that their advertised "Single Mass Flywheel" was actually more or less correct.  Meh, if it comes down to it I may be able to machine it down to the same dims as a true single mass. I'm not sure how I would track down accurate dims for a flywheel that I don't have... :-)

Thanks for the response though.  I remember seeing your posts around here.  You have provided a lot of good info as I have done research for my projects.  Did you do the swap too? Just curious.  Thanks again.

[dude8383]

Haha well I still recommend posting up a pic of this flywheel anyway because like I said, one of us can probably identify it.

I had the 323i flywheel which was definitely a single mass flywheel. I certainly see where there might be some confusion but its hard to picture this without a physical picture of what you're describing.

In any case, you can always bring the flywheel to the machine shop and have them shave it down. I don't recommend going lower than 13lbs though...you might experience some clutch chatter which will sound like a bunch of marbles getting shredded.

[otoolb]

o_O shredded marbles??!?!  I don't want that.  Thanks for the info.  I will try to remember to upload a few pics once I get home.  

Is anyone else interested in having inspection quality dims off of this flywheel? I might talk to the guys in inspection to see if they will let me put a mic on it.  It seems like it could help finally answer some of the questions about spacers, 323 TO bearing, ring gear engagement, ...

I would need to pull dims for a full clutch assembly for the M42 and the M20, but you never know what will turn up. :-D

[DesktopDave]

I can haul mine out for some dimensions & p/n's if you like.  No dual mass flywheel is made from a single bit of steel.  There are two parts to the flywheel joined by steel or rubber depending on the make & model.  The dual mass keeps it in dynamic balance...one part spins firmly bolted to the crank.  The sprung outer or rear part is permitted to move a bit, damping out slight balance problems.  Makes shifting easier but cuts available power on acceleration.

Harmonic dampeners on the nose of imbalanced V6's or over-sized V8's do the same thing, just on the other end of the crank.

Automatics have a self-balancing feature with the torque converter.  The oil will always distribute itself perfectly, balancing out any lateral forces by flowing into equilibrium.

Somewhere on here it's said that an entire late model m20 flywheel & TO bearing can be used if the slave cylinder is adjusted to suit.  I've not found any way to do that yet, but if/when I'll be sure to posty.

[otoolb]

Thanks DesktopDave

I did a little more research on which flywheels are available one which e30's and I am getting close to sorting it all out.  I found two sites that confirm the 18lbs weight for the 325i flywheel.  

bmwman91 shows the flywheel on the scale at e30tuner.com (http://bmw.e30tuner.com/my318is_pic_rebuild6.php).  Pretty convincing

Then over on revlimited another owner posts measured weights for 325i and 325e flywheels.  http://r3vlimited.com/board/showthread.php?p=397397 Again, very convincing.  

My scale shows 23.8 lbs for the flywheel in question.  Pictures are on the way, but the forum won't accept my pics.  

As for pulling dims off of your flywheel, that would be awesome, but you should wait until I get off my lazy butt and inspect both of my setups.  I just realized that I have two full clutch assemblies (one M42, and one M20 if this is a stock flywheel). Once I have everything written up I will let you know.  My plan is to have rough dimensioned drawings, that way we can be sure we are talking about the same dimensions.  

On a side note, I like your detail about the dual mass flywheel.  My experience (admittedly not much) leads me to believe that harmonic balancers and dual mass flywheels serve slightly different purposes.  The harmonic balancer is used to balance the first and second vibrational modes which are inherent in the engine.  These are partly from design, and partly from manufacturing tolerances.  The dual mass flywheel however is a low pass filter (checking, checking.... thinking.... uh, yeah, low pass) which would filter out any high frequency vibrations.  So I guess this would in fact filter out vibrations from the first and second modes, but I was thinking that it had more to do with the fact that you have so few cylinders firing and less even power application than, say, a 6 or 8 cylinder. But hey, the eta appears to have a dual mass flywheel too, so that blows my theory out of the water.  I had no idea about the automatics, but that is awesome (well, other than the fact that you are stuck with an auto tranny) :-) Anyway, I am WAY off topic.   I just hope I don't sound like a pompous engineer straight out of school... just sharing what I have pieced together with my limited tools.

Thanks for the info and I will let you know what happens with the inspection dims.

[deekay]

sounds like you got ahold of a single-mass M42 flywheel... i have one and that's how much it weighs.

[otoolb]

I am pretty sure it isn't an M42 flywheel.  It looks damn near identical to any of the pics I have seen for the M20 flywheels.  It is just the weight that is throwing me off.  

I have all the pictures ready to post, but I am having trouble uploading them.  Not sure if I need authorization from an admin or not.  I will go ask around on the general discussion board or the site specific board.  Hopefully I will have pics up soon.  Thanks again for all the help and info.

Here are the images in my album.  Hope it works

[DesktopDave]

You need 100 posts for pictures IIRC.  I suspect the reason that BMW spec'd the dual-mass flywheel was to damp out the 2nd-order NVR buzziness inherent in a four cylinder.  They used hydraulic motor mounts as well.

The M20 clutch swap isn't all that much lighter.  To save the most weight a custom alu flywheel is the way to go.  But since we're not made of money, the M20 is the best bang for the buck.  I'm also told that there is a non-A/C (Canada/Euro spec) M42 flywheel that fits perfectly and might work as well.  I believe it's also sold as the standard M40 SOHC part.  Valeo has also engineered replacement OEM style clutch kits specifically to replace dual-mass flywheels with a single-mass, but have not released a kit for the e30M42 yet.

From what I recall, the engineering behind the whole deal is that an inline six is a perfectly balanced and durable motor from an engineering standpoint (with crankpins staggered 120deg), but a packaging problem from a styling/packaging perspective.  V6's and V8's suffer vibration caused by the requirements of shared crankpins.  One could have perfect even-fire timing on oddball motors (VW VR6 & W8...GM 3800...Ford/Volvo/Yamaha "SHO" V8) but the split crank throws would either be too long, too flexible, or require unacceptably narrow bearings. Secondary order vibrations are far easier to deal with than primaries from what I've seen.

[otoolb]

Thanks again for all the great info.  I am going to throw chips tonight so we will see what happens.  I have no idea what I am getting myself into, but here is the plan in case anyone cares.  

I am going to grab the bolt pattern and center hole dims to make a lathe fixture.  (may skip this and just chuck it up directly, but I really don't want to mess up the "bearing" surface on the flywheel.  I plan to pull as much weight out of the engine side as I can.  Once I am satisfied, I am going to attempt to swap the ring gears so I don't need to mess with the starter pinion ever.  I am thinking blow torch and bearing press...

I may also try to remove the clutch alignment pins and do a "proper" resurfacing. I actually have no idea how to resurface a flywheel, but hey, what could go wrong.  If I don't get the pins off then I am just going to pull a little material off of the friction surface.  Has anyone done this before and have surface finish suggestions?

mmmm, I guess that is more than I will have time for tonight.  I am also a little concerned about the crank position trigger.  I noticed that it doesn't line up between the M42 and the M20.  Oh well?

I know most of this has all ready been discussed on the stickied thread, but I still have a few unanswered questions.  I will post again if I make any significant progress.  (otherwise I probably had an accident with a 450 degree flywheel...)

[DesktopDave]

The M42 and M20B25 'i' engine don't have crank position sensors on the clutch bellhousing.  You might have an M20B27 eta clutch there.  Your measurements should still be valid, they're identical part numbers IIRC.

[otoolb]

Oh... wait, you are right about the crank position sensor.  I remember pulling a sensor off of the block, but I can't picture where it actually lines up on the driveline.  I saw a sensor, I saw a reference mark, so I connected the two in my head. My bad. Sorry for the assumption.

[otoolb]

Sorry to Bump, but I wanted to give an update on the flywheel comparisons.  I took the mystery flywheel into the inspection room and got pretty accurate dimensions off of it.  I have a nice drawing that lays out most of the important ones, but I have to go back and double check a few.  Once I get the correct 325i flywheel I will be able to post the comparisons.  

There are a few 528i sitting in a junk yard near by.  I am going to try my luck and hope that one of them has a flywheel still.