Author Topic: Some DEFINITIVE pics of M42 vs M20 FW conversion options (Read 33155 times)

irish44j · « **on:** November 12, 2012, 09:39:10 PM »

So did a bit of mocking up, since I hadn't seen any good comparison photos of the various M20 FW conversions. Here goes:

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 crank. 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 (off an M42 FW) 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.

Also, 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. I have both sitting on my bench as well.

M42 vs M20 without spacer, with 323i TOB.

Hope this is of use to someone, and comments/thoughts are welcome.

Oh also, I weighed the assemblies on a well-calibrated scale. The full M20 setup is almost exactly 5lbs lighter than the M42 setup by my scale.

irish44j · « **Reply #1 on:** November 12, 2012, 10:24:21 PM »

I'll add that DJ Buttchugg over on R3V (here as well?) is using the spacer AND the 323 TOB and it works fine as well. So that's option #3 if you're like me and happen to have a brand-new 323 TOB sitting around

irish44j · « **Reply #2 on:** November 13, 2012, 06:18:12 PM »

Here are shots with the level of the "ears" on the TOB, which matter more than the overall height. As you can see, the 323i TOB on the right sits "higher" by exactly 3mm. According to those who use it, it still works either with or without the spacer.

DesktopDave · « **Reply #3 on:** November 13, 2012, 09:03:31 PM »

Great post! Especially since I haven't done a SMFW yet. Very timely!

I stickied it too, easy to lose good info otherwise.

thedguy · « **Reply #4 on:** November 14, 2012, 12:19:54 AM »

So, if I buy all m20 parts, I'm good? Can the m20 starter be used with an m42, or how hard is it to swap the pinion?

The other threads have confided the crap our of me to the point where I have up on this swap years ago

irish44j · « **Reply #5 on:** November 14, 2012, 10:54:12 AM »

You should be able to buy all M20 parts, provided you take the spacer off the M42 flywheel and use it.

I can't verify the M20 starter yet, but I did mock it up in the casing and it seems to fit fine and correctly. My install isn't done yet, though, so I'm not 100% of that YET.

fumstoo · « **Reply #6 on:** November 15, 2012, 12:18:38 AM »

i run an m42 starter, m42 jb aluminum FW, m20 clutch and pressure plate with a g260...... And i definitely had a point i was going to make but after typing that i have forgot what it was... Ill edit this post when i remember

Geoff · « **Reply #7 on:** November 15, 2012, 05:48:09 AM »

it must be 4:20:D
Geoff

thedguy · « **Reply #8 on:** November 15, 2012, 10:04:16 AM »

The idea I was curious about, but got shit on by r3v at the time was...

What about swapping the ring gear from the m42 flywheel to the m20?

While you got the flywheels on the table, can you see where the tool inserts for cylinder 1 on the m42?

Thanks,
Dustin

irish44j · « **Reply #9 on:** November 15, 2012, 05:02:51 PM »

Swapping the ring gear seems like a lot more trouble than just swapping the starter (or starter pinion gear), IMO.

That said, I just measured and the inside diameter of the M42 and M20 ring gears are EXACTLY the same, so they should interchange if you want to go that route.

I think I see where the tool inserts - it's kind of a little "divot" (there is one on each side). There is not a similar "divot" on the M20, from what I can tell. IDK, I haven't needed to use it, so I never really looked for it.

irish44j · « **Reply #10 on:** November 15, 2012, 05:03:44 PM »

Quote from: fumstoo;117667

i run an m42 starter, m42 jb aluminum FW, m20 clutch and pressure plate with a g260...... And i definitely had a point i was going to make but after typing that i have forgot what it was... Ill edit this post when i remember

you point was that there are other ways that work as well, if you know what they are. I can only speak to the two or three setups I mention in the original post

or that you're just trying to confuse people

romkasponka · « **Reply #11 on:** November 15, 2012, 05:36:56 PM »

Quote from: irish44j;117684

Swapping the ring gear seems like a lot more trouble than just swapping the starter (or starter pinion gear), IMO.

That said, I just measured and the inside diameter of the M42 and M20 ring gears are EXACTLY the same, so they should interchange if you want to go that route.

I think I see where the tool inserts - it's kind of a little "divot" (there is one on each side). There is not a similar "divot" on the M20, from what I can tell. IDK, I haven't needed to use it, so I never really looked for it.

M20 starter has different modifications and when I did my first swap I had to modify starter pinion gear. I had to make it shorter 5mm or something. When I checked my single mass flywheel ring gear inside diameter it was different from m20, it was smaller few mm. IMHO m20 starter is not a good choice because it is very rare at the moment to find and every day it becoming much more difficult to find it and because of this I would stick with m42 starter motor. I had an idea to modify m42 single mass flywheel to take m20 clutch and I think it is possible

thedguy · « **Reply #12 on:** November 15, 2012, 08:16:28 PM »

Quote from: irish44j;117684

Swapping the ring gear seems like a lot more trouble than just swapping the starter (or starter pinion gear), IMO.

That said, I just measured and the inside diameter of the M42 and M20 ring gears are EXACTLY the same, so they should interchange if you want to go that route.

I think I see where the tool inserts - it's kind of a little "divot" (there is one on each side). There is not a similar "divot" on the M20, from what I can tell. IDK, I haven't needed to use it, so I never really looked for it.

I haven't used the "divot" but if I every do the mood, would be nice to have.

Since I plan to get the flywheel shaved for weight, I doubt it's much more to have the rung gear swapped by the machine shop.
This had a major benefit in not requiring knowing all the funky special parts need when the started inevitably fails.

I'd rather just have to order m20 clutch parts and everything else be as much factory m42 as possible.

irish44j · « **Reply #13 on:** November 15, 2012, 10:42:16 PM »

Quote from: romkasponka;117686

M20 starter has different modifications and when I did my first swap I had to modify starter pinion gear. I had to make it shorter 5mm or something. When I checked my single mass flywheel ring gear inside diameter it was different from m20, it was smaller few mm. IMHO m20 starter is not a good choice because it is very rare at the moment to find and every day it becoming much more difficult to find it

Did you use the spacer? If not, then that may be why you had to shorten the pinion throw, in theory, since it may not retract enough to clear.

As to M20 starters, I only plan to use it because I have a brand-new one sitting on my workbench

Galbūt jie yra sunkiau rasti Baltijos šalyse....they're pretty easy to find here in the US :confused: My local auto parts store has a couple of them in stock.

irish44j · « **Reply #14 on:** November 15, 2012, 10:43:42 PM »

Quote from: thedguy;117689

I haven't used the "divot" but if I every do the mood, would be nice to have.

Since I plan to get the flywheel shaved for weight, I doubt it's much more to have the rung gear swapped by the machine shop.
This had a major benefit in not requiring knowing all the funky special parts need when the started inevitably fails.

I'd rather just have to order m20 clutch parts and everything else be as much factory m42 as possible.

You could always also have a new "divot" milled into the M20 flywheel. You could actually just do it with a drill (though the FW would have to be re-balanced afterward, probably)