a bit of clarification on M20 flywheel/clutch conversion

[irish44j]

EDIT: Skip to post #13 for the DEFINITIVE ANSWER to the question of "what parts to use and what fits" for the M20 flywheel/clutch conversion on the M42.

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I managed to come by for cheap all the parts for the M20 conversion from someone who was going to do it but then did an engine swap and didn't need to. It includes:

M20 single-mass flywheel
M20 clutch and pressure plate (new)
323i throwout bearing
M20 starter (new)

So....all of the threads on this are so freaking convoluted with BS (I've read all of them at this point), just figured I'd ask a simple question to get a simple answer, hopefully.

With the above parts, all I need to do is:

1. get shorter flywheel bolts
2. machine the engine side of the flyweel a bit to clear the sump bolts
3. use the M20 starter as-is.
4. do NOT use the spacer off the M42 flywheel

Is that correct, or am I missing something?

thx,
Josh

[bmwman91]

Buy M20 flywheel bolts for an M20 equipped with the single mass flywheel. Get new ones, don't try to find bargain ones at the hardware store. They are critical fasteners.

Use the spacer ring from the M42 flywheel between the crank and M20 flywheel. The pressure plate might not disengage properly without it since the slave cylinder has to push further. Don't machine the flywheel, unless you know a reputable shop that is known to do a good flywheel lightening job and can balance it up to at least 8000RPM.

I don't know if the M20 starter unit will fit the G240 bellhousing. All you really need is the pinion gear & planetary gear assembly from the M20 starter. Those can be installed into an M42 starter pretty easily. You will need to do that if the entire M20 unit does not fit.

[atmobenzin]

I don't know if the M20 starter unit will fit the G240 bellhousing

It will ;)

[Dj Buttchug]

Buy M20 flywheel bolts for an M20 equipped with the single mass flywheel. Get new ones, don't try to find bargain ones at the hardware store. They are critical fasteners.

Use the spacer ring from the M42 flywheel between the crank and M20 flywheel. The pressure plate might not disengage properly without it since the slave cylinder has to push further. Don't machine the flywheel, unless you know a reputable shop that is known to do a good flywheel lightening job and can balance it up to at least 8000RPM.

I don't know if the M20 starter unit will fit the G240 bellhousing. All you really need is the pinion gear & planetary gear assembly from the M20 starter. Those can be installed into an M42 starter pretty easily. You will need to do that if the entire M20 unit does not fit.

this
and this

It will ;)

both are correct

[irish44j]

excellent, thanks. So in summary:

- don't modify the flywheel at all
- use the spacer off the M42 flywheel (the one with the rivets holding it in)
- get new M20 flywheel bolts (was going to do that anyways)
- use M20 starter as-is with no modifications
- use 323i TOB
- use all the rest stock M42 stuff (slave cyl, etc)

all correct?

[ForcedFirebird]

I ground the oil pan bots to clear the m20 FW, no spacers and didn't have any issues with the slave not extending enough. Also didn't use the 323 TOB.

I know your pain. We spent a whole weekend trying to put a g260 behind the m42, all the online info was dead wrong. They said to use a 325i drive shaft but the splines for the two halves only engaged a tiny bit. Ended up finding a used g240 and left the m20 clutch parts in it.

[Dj Buttchug]

excellent, thanks. So in summary:

- don't modify the flywheel at all
- use the spacer off the M42 flywheel (the one with the rivets holding it in)
- get new M20 flywheel bolts (was going to do that anyways)
- use M20 starter as-is with no modifications
- use 323i TOB
- use all the rest stock M42 stuff (slave cyl, etc)

all correct?

yes

[ForcedFirebird]

Why is it people use the 323 TOB? We definitely didn't use one and the car had almost a year in service and brought one of my shop mates all the way through HPDE, sold the car to another shop mate and he made it to HPDE3 with the same car (both owners on daily status), all the way up until the m42/g240 was pulled in preparation for Spec e30 just about a month ago. I know we didn't use the 323 bearing because we used what was on our shelf and we certainly never had a 323 TOB for any reason.

EDIT: And we put the m20 cone on the m42 starter...

[irish44j]

The 323 TOB is a bit longer - so I perhaps people use it with the flywheel without using the spacer (and with the back lip of the flywheel shaved to clear those bolts)?

The M20 and M42 starters are exactly the same size and have the same "throw" on the gear engagement from what I can tell. The only difference I can see (other than cosmetic things) is the gear teeth. I don't see why the M20 starter wouldn't just be drop-in in place of the M42 starter, frankly.

a few pics comparing the setups:

EDIT: see a few post down for better pics.

starters (M20 left, M42 right)


TOBs (323 left, 318 right)

[ForcedFirebird]

It is the ring gear count that the starter gear has to match. The m42 FW and m20 FW can't be compared laying on a bench. You have to check them from the crank mounting height. The m20 has more meat on the back - the part that hits the oil pan bolts - the very same reason it's not a bolt on for the 24v motors.

I am just curious as the g26 swap cost a whole weekend on my lift with 2 sold guys working on it and one a few hours each day - just to end up with an m20 clutch pack and g240, only because we weren't going to drop the trans to swap back after pulling the trans about 8x - even with my lift, that was a looooong weekend :(

[irish44j]

It is the ring gear count that the starter gear has to match. The m42 FW and m20 FW can't be compared laying on a bench. You have to check them from the crank mounting height. The m20 has more meat on the back - the part that hits the oil pan bolts - the very same reason it's not a bolt on for the 24v motors.

I am just curious as the g26 swap cost a whole weekend on my lift with 2 sold guys working on it and one a few hours each day - just to end up with an m20 clutch pack and g240, only because we weren't going to drop the trans to swap back after pulling the trans about 8x - even with my lift, that was a looooong weekend :(

yeah I realize that about the ring gear count. But with the spacer on the M20 FW mounting surface, the ring gear is in virtually the same relative place. I measured it with my engineering tools between the two FWs. The photo I posted above was a terrible illustration of anything at all, lol and didn't show what I meant it to :P

[irish44j]

I'm going to go try something more illustrative. back in a bit.

[irish44j]

ok, looks like we were both right in different ways:


My workbench is PERFECTLY level, first of all.
I used some blocks of EXACTLY the same thickness underneath the assemblies, simulating the mouting face of the block. So this display shows the relative heights of the entire assemblies AS MOUNTED on the engine.

All bolts are tight, and the pressure plates are cinched down to spec, with clutch plates inside.

FIRST TEST:
This is the M42 stock assembly on the left.
On the right is the M20 assembly WITH the little spacer underneath. As you can see, the ring gears on each are perfectly even. Both have a standard E30 TOB on top. As you can see, they are PERFECTLY level with each other.

So, the bottom line on this setup is that the full M20 assembly here, with the spacer and stock TOB, is exactly the same overall thickness as the stock M42 stuff.

M42 vs M20 w/spacer and stock TOB:




TEST 2:
In this test, I took out the spacer ring from below the flywheel, and put the slightly longer 323i TOB on top instead of the stock one.

Results: The ring gear is a bit off. The overall height is exactly the same. So basically, this setup should work fine as well. I'm thinking that the 323 TOB method was developed before someone thought of using the little spacer to allow use of the M42 TOB. The only thing here is that the backside of the FW would have to be milled a bit to clear the engine/sump bolts. So this method involves a BIT more work.

Looking at the starter pinion gear travel, it appears that the M20 starter OR the M42 starter with the M20 cone should both work perfectly.

Thoughts?

M42 vs M20 without spacer, with 323i TOB.

[Dj Buttchug]

this is interesting. First off these pictures are gold. Second, It seems that both set ups work fine. The difference in ring gear height really isnt too much of an issue I dont think because the starter throw will still reach out and grab the flywheel in both situations.

I however am running the spacer AND the 323 TOB, and I have no issues to report other than a slightly stiffer clutch pedal (which I like) however it could be the aftermarket burly clutch I am running.

[irish44j]

this is interesting. First off these pictures are gold. Second, It seems that both set ups work fine. The difference in ring gear height really isnt too much of an issue I dont think because the starter throw will still reach out and grab the flywheel in both situations.

I however am running the spacer AND the 323 TOB, and I have no issues to report other than a slightly stiffer clutch pedal (which I like) however it could be the aftermarket burly clutch I am running.

good to hear. Since I have a brand-new 323 TOB (and two old/nasty M42 TOBs), I'd just as well use those as well. So to get the ring gear alignment perfect I may do exactly as you have done.

So now we have 3 setups that work :)