just did this a couple months ago on an e30. The odometer is mechanical & the gears crumble. Fairly simple DIY with new gears:
http://www.z3bimmer.com/OffTopic/E24Bimmer/OdoGear/
I'm not sure about the e24 but the e30 could have 2 different gauge cluster suppliers, either VDO or MotoMeter. Makes a difference for which gears you order. If you get new gears from Odometer Gears, call him on his cell & he'll walk you thru the entire repair process, pretty straight forward once you get the cluster out and the odometer innerds opened.