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Thread: m10 single mass flywheel for an m42?

  1. #1
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    m10 single mass flywheel for an m42?

    I've read speculative posts, and also others that imply some firsthand knowledge that they "may" be swappable, but these all are posted with the 2002 guys. None are nearly as definitive as the m20 swap which I have done on my 318is. I see the weight for the m10 flywheel for mine is somewhere around 14 lbs. vs the very heavy 27 lb dual mass m42. I believe the m10 flywheel will require the m42 spacer to clear some bolts much like the m20 swap. If I understand correctly the flywheel I have will be a smaller diameter than the m20, but the max power I plan to put down with the m42 will be slightly north of 170 so that's probably not much of an issue. Or is it?

    Anyway, I'll be pulling the m10 in a week or two and hopefully can document the process. If it's a fail at least I'm not out any parts or money.
    91 318is
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  2. #2
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    This is the most definitive info I have found so far. I would need to run the stock m10 starter I think or find a lightweight solution.
    OK, the results are in. I bolted up both flywheels to the M42. The M10 FW fouled against the rear seal housing bolt heads. I then removed the 1/8" spacer from the dual mass FW, drilled it out to fit the dowel on the crank, and bolted up the M10 FW. The ring gear sits .03" farther away from the engine than the M42 FW. Looking good, but then I checked the tooth count, and the M10 has 3 more teeth than the M42, so the pitch is slightly different.
    Edit: Looks like a sr441x or sr440x will work and reman ones are less than $70 shipped. I may see if any e32 or e34 are at the local U Pull It as well.
    Last edited by captain awesome; 09-17-2018 at 12:37 PM.
    91 318is
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  3. #3
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    Quote Originally Posted by captain awesome View Post
    This is the most definitive info I have found so far. I would need to run the stock m10 starter I think or find a lightweight solution.


    Edit: Looks like a sr441x or sr440x will work and reman ones are less than $70 shipped. I may see if any e32 or e34 are at the local U Pull It as well.
    Fan of the sr440x; ltw for size and nice torque bump for turning over any motor. I never checked to see if the m10 flywheel fit a M42 using the hub spacer. My M42 car has the ltw M20 flywheel and clutch kit, no spacer, shorter standard m20 bolts, custom longer push rod for the slave cylinder, m20 standard throwout bearing. The m20 ring gear was pressed off and replaced with a m42 ring gear (yes can do this) and use a standard m42 starter. Anything is lighter than a m10/m20/m30 starter.
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  4. #4
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    I'm thinking I can use the m10 flywheel bolts with cleaned up threads and some loctite. From the pics I've seen the throwout bearing fork engagement is considerably different in height between the two, so I think you are right that the slave cylinder rod will need to be extended a bit. From what I see about 1/4-3/8" or so should do the trick. Do I need to take the flywheel to a shop for the ring gear or can that be done in my garage? Balance issues?

    flywheel3.thumb.jpg.f100fee9831e2a263ec02a9ca3d9d339.jpg
    91 318is
    83 320is

  5. #5
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    I bought mine that way so couldn't honestly tell ya. Had more funds then time back then. DynoTech Motorsports(CA) sold as a full M20 kit for the M42. The clutch rod was included but it's just a piece of 1/4 rod cut and ground to an extended length. This way works with a stock M20 throwout bearing with the stack height.

    For an M42 setup a single mass is noticeably faster than dual mass and worth extra effort.
    88 M3
    91 318is
    91 318i
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    08 X3 3.0si

    "If it flys, floats, or f*cks, rent it!"

  6. #6
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    Yeah I've got an m20 flywheel on my 318is and love it. Didn't have to extend the pushrod on the slave though.

    From my cursory googles a bit of heat will pop the ring off and some heat to put the new one back on. I've got a spare m42 flywheel to test on I guess before committing.
    91 318is
    83 320is

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