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I have an MTB that is quite old. The biggest chainring with 42 teeth is now done and I need a new one. It's a Shimano FCMC30, but it seems nobody is selling that new anymore. The chainring in question says "Shimano Hyperdrive C".

Since I can't find the original, is there maybe a single chain ring I could buy that is compatible to my type?

If so, where would I find such information e.g. online?

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si.shimano.com has Shimano's technical documentation. Searching for your crankset FC-MC30 produces the Exploded View (EV) document https://si.shimano.com/pdfs/ev/EV-FC-MC30-1351.pdf This lists the part numbers and descriptions of the components of the crankset. The good news is that your chainrings are bolted on and so are replaceable. A replacement ring must have the same mounting hole number and position (BCD - diameter of the circle of bolts). The bad news is that the large chainring mounting holes apparently fit a short-lived, obsolete standard that is unlikely to be found.

A recent reddit post is relevant:

semyorka7 6 months ago · edited 6 months ago

So, look closer at that diagram. STX FC-MT30 through MT33 were weird shit - there's actually only bolts on the crank for the 58mm bolt circle. both the inner and middle rings bolt through with the 58mm circle, the outer ring actually attaches to the middle ring.

You can probably get away with a 94mm 5-arm outer ring, but the bigger issue is that if you want to replace the middle ring, you need a NOS middle ring - otherwise there's no place to attach the outer ring

Both MT32 and MT33 call out 94mm instead of 95mm for the outer ring, so if you want to guarantee that a non-STX outer ring will bolt up without having to do any work with a metal file, those are the ones I would look for. Shimano PN 1SC3200

Unfortunately, people usually spend most of their time in the middle ring and NOS middle rings for old shimano MTB cranksets are basically unobtanium compared to outer and inner rings. Frankly I'd recommend replacing the crankset with a nice 110/74 triple that you can easily source a variety of chainrings for - FC-M730, FC-MT60, FC-M560, FC-M900, etc are all plentiful on ebay.

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  • Thanks for linking that post from Reddit. Actually, in that thread, they also call out that the outer chainring actually has a 95mm BCD on the spec sheet. I measured bolt to bolt and it's 56mm or 57mm. So the BCD is 56*1.701 = 95.26 or 57*1.701 = 96.95. It's quite annoying that there seems to be no standardization for these things. What a mess. I guess I'll now somehow have to change the crankset :/ – user10030 Apr 24 at 13:27
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There are NW 94mm rings for 1x if you want to go that path. Wolf Tooth makes one. In practice though a whole new repair crankset with the same sizes might make more sense, especially since having to buy a new BB for it is negligible if the old one is due for replacement anyway. If you 1x and decide that you need more range, you're then having to reinvent the bike more than may be worth it.

That original replacement chainring is rare but it's also a dusty chainring box queen at several of the shops I've worked at, so finding one isn't impossible.

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