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Clutch/starter combos on M30b35's

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    The issues are:

    1) The M20 flywheel is a slightly larger diameter than the M10 or M30 flywheel.

    As you mount the starter to a fixed point - ie. cast into the block - then the starter position can't be moved further out to accommodate the larger flywheel.

    The M10 based engines - i.e. M10, S14 and M30 all use the same smaller diameter flywheel, with course, deep ring gear teeth.

    The M20 based engines - i.e. M40, M42, M50 etc. all share the larger diameter flywheel, which has finer, shallower ring gear teeth.

    The main difference seems to be in the ring gear, so you might be able to put an M10 / M30 ring gear onto an M20 flywheel, although the M10 guys on 02FAQ etc. suggest that the ring gear doesn't fit, the M20 flywheel is just slightly too big.

    2) the M20 starter bendix is different to the M10 / M30.

    It has a spacer to limit the travel of the starter gear and when mounted to the M30 block, the starter only barely engages with the flywheel. Apparently the spacer can be removed from the bendix, but again, it requires modification & even still, the starter isn't real happy trying to engage with the larger flywheel.

    Anyway - pics speak louder than words...

    Tooth comparison on ring gears, note M10 flywheel (top) is much courser and deeper than the M20 flywheel (bottom) - this effects the diameter of the flywheel.

    Here, you can see how the M20 ring gear failed after being mounted to the M30 (on right) - note that the starter only barely engages with the ring gear, and that the profiles of the flywheels are quite different - the M10 flywheel (left) has meat on the starter side of the ring gear, the M20 flywheel doesn't.

    Unfortunately I didn't snap a pic of the differences between the two starter bendix, but if you get hold of an M20 & M10 or M30 starter then the difference is quite obvious.

    If you still want to attempt it, you need the e21 323i 30mm throw out bearing as the common 25mm one (fitted to most BMWs) is too short, and ultimately causes the slave cylinder to over extend and fail. I attempted to get around this by lengthening the slave cylinder rod 7mm, which did work, but then the ring gear failed.

    In any case - I tried hard to defeat the internets, but ultimately I lost & fitted an M10 flywheel with an HD 215mm clutch kit, which has worked fine.
    Last edited by lukeADE335i; 01-20-2015, 04:15 AM.
    My e30: OEM+ with M30B35


      Holy information!
      Thx Luke:-)
      Originally posted by flyboyx
      i have watched my dog lick himself off a few times


        Okay, that's some great information right there.

        In the picture of the failed ring gear, it's only broken on one side. Looks like the M20 starter isn't reaching out far enough to contact across the whole tooth. If that was a contributing factor in its failure, maybe machining the starter mount on the block so it sits closer could get it contacting across the tooth and somewhat prolong the life of the ring gear.

        You've made it pretty clear that the M20 starter/flywheel isn't ideal, however, I think I'll still run it but only because I have hoards of 323i parts laying around. If I can get a year of Mallala/Tailem Bend drift practices out of it, I'll be pretty happy. Can always push start it when she inevitably fails.
        Last edited by Hooffenstein HD; 01-20-2015, 06:14 PM.


          Yes - the starter only engages about 1/3rd of the ring gear. There is a spacer in the M20 starter bendix that you could remove that would get the gear to engage with more of the ring gear, but it still doesn't solve the diameter issue.

          When I first ran the M20 flywheel and starter, I didn't think it sounded too bad, but now that I'm running the correct setup, the starter was really struggling to engage, and sounded awful when the engine was turning over. Reminded me of the air start on an old Mack truck (if you've never heard one, they're extremely loud!)

          If you have a good M20 clutch kit already that you'd like to use, you'd be better off sourcing a 228mm M10 flywheel from somewhere & then reusing the M30 starter. You'd just need to then work out which throw out bearing you need (either 25mm or 30mm).

          EDIT... just reread your post - if it's just for a drift pig type thing, then it would work. Remove the spacer in the starter bendix - you'll freak them out in the pits though when you start the car! I'll be interested to hear how long a Getrag 240 lasts with that kind of use. I've daily driven mine for going on 3 years now with some spirited driving etc. but it doesn't cop the kind of thrashing it will doing drift.
          Last edited by lukeADE335i; 01-21-2015, 01:04 AM.
          My e30: OEM+ with M30B35


            sorry to haul up an ond thread but im trying to find out abou the 2 different ring gear pitches of m20 FW! almost all m20 FW have fine pitch 137 teeth but do some have a different tooth count???

            For info here is the complete run-down on all interchangeability and fitment! We make FW so i have a lot of info about the differences.

            There are only 2 different fitments big block m10/m30/s14/s38 etc they all have the same Fw ring gear with 116 teeth and are all interchangeable, as are the gearboxes and starters.

            Then there are the small blocks starting at m20, then m40 right through to m54 (and maybe beyond!) they all have the same pinion to crank dimension and ring gear offset and are all interchangeable as are all the gearboxes and starters BUTT!!!>>>>>

            The m20 has a unique tooth count of 137 teeth where as ALL the others only have 113 teeth, SO as long as you match m20 starter to m20FW or match any m40,42,43,44,50,52,54 starter to any m40....54 FW then they are all interchangeable on all small block engines.

            you can NOT interchange small block and big block FW!!
            Last edited by e21jps; 05-19-2016, 08:44 AM.

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              there are three basic starters that will work on the m30,....

              1. the huge, heavy, slow turning original as found on the m10, and early m30s through the e28 chassis.
              2. the smaller cased fast spinning model as found on the e32 and e34 m30s
              3. the reduction gear style starter as found on the m70 v-12s

              all will fit it used with either an m10 or m30 flywheel.