Author Topic: M20 ring gear not interchangeable with M42 (Read 4056 times)

bflan2001 · « **on:** November 13, 2011, 11:15:58 PM »

I read from a few posts here that they are the same size, but they are not. I had both flywheels and ring gears side by side today and the M20 ring gear is about 2mm larger inner diameter. So I guess you have to machine down one of the mating surfaces. Not sure how much it costs but I will found out soon. :/

Here they are, the inner surfaces are aligned 180* from this point:

wazzu70 · « **Reply #1 on:** November 13, 2011, 11:59:32 PM »

Lame. I was planning to swap the M42 ring gear onto the M20 wheel as I was told it was a direct swap. I suppose it must be the M50 ring gear?

Bummer it didn't work out, but thanks for posting/clarifying!

DesktopDave · « **Reply #2 on:** November 14, 2011, 08:02:42 AM »

I wasn't sure on that either, thanks for posting the info.

I guess this makes sense as the M20 SMFW is larger than the M4x DMFW (228mm vs. 215mm)...IIRC the later model >87 has the correct tooth pitch for the M42 starter.

I wonder if the 240mm M3 SMFW could work?

Warsteiner · « **Reply #3 on:** November 14, 2011, 10:20:53 AM »

My M42 ring gear was not a direct swap onto my M20 FW!! The FW had to be turned down a bit. The M3 FW does not work, wrong number of teeth. The M20 FW's ring gear is also off by 24 compared to M42.

Cheers,
Ralph

wazzu70 · « **Reply #4 on:** November 14, 2011, 06:10:22 PM »

True, late model and early model M20 flywheel have the same tooth count. I have a flywheel from a 91 in mine.

DesktopDave · « **Reply #5 on:** November 14, 2011, 07:49:26 PM »

That's not right, is it? I'm pretty sure the early model M20 isn't the same tooth pitch as the later one. I have an early one with a matching starter sitting in my garage. Never know when you'll need a SMFW, eh?

bwawuz02 · « **Reply #6 on:** December 14, 2011, 07:34:41 AM »

The M42 ring gear can be swapped to the M20 FW, I had it done by rebello racing in antioch. Call Metric Mechanic and they'll tell you how to do it, well they told me, but I did buy a bunch of stuff from them.

bflan2001 · « **Reply #7 on:** December 14, 2011, 11:08:08 AM »

bwa, do you think you could share the info here? are you saying it can be done without modifying the ring gear or flywheel?

Warsteiner · « **Reply #8 on:** December 14, 2011, 08:36:42 PM »

You need to machine the flywheel down.

Cheers,
~Ralph

bwawuz02 · « **Reply #9 on:** December 17, 2011, 12:55:23 PM »

Quote from: bflan2001;108889

bwa, do you think you could share the info here? are you saying it can be done without modifying the ring gear or flywheel?

I don't remember the specifics, I was standing in rebello's shop when I called MM and just relayed the data. pretty sure it was a small turndown of the FW as others have mentioned.

KenC · « **Reply #10 on:** December 17, 2011, 06:19:54 PM »

Just get a 4-wire M20 starter and be done with it. Used ones are like $30.

Warsteiner · « **Reply #11 on:** December 17, 2011, 08:58:28 PM »

If you don't want to change your starter you must measure your M20 FW, then your M20 ring gear and see what the difference is. Then you take the M42 ring gear and apply the same math to make the M20 FW fit the M42 ring gear. I just can't remember the number that mine had to be cut down. If I had to take an educated guess it was about .085" then you just heat the ring gear and it drops right on. Cutting the flywheel on that edge will eat up your machinist's diamond bits. It's really tough stuff.

You have to do your math on this one and be accurate.

~Ralph