OK, the speedometer. Since the tranny I am swapping in is from a W123, the transmission tailshafts are not identical because the W126 has an electric speedometer while the W123 has a mechanical one, and the speedometer is fed from the tailshaft. There were two reasonable ways to solve this problem:
1) Swap tailshafts. This IS possible going W123 (transmission 722.315) --> W126 (transmission 722.303) but IS NOT possible going W126 --> W123 because the mechanical speedometer needs to fed tranny fluid from a special line coming from the main body of the tranny that is not in the 722.303 because it does not need it. Despite the fact that I could have gone this route, I chose not to.
2) Put the mechanical speedometer from the W123 into the W126 cluster! This is a bit more difficult than it maybe sounds because the clusters are very different. However, going on Charlie's thought that "with enough hacking anything is possible", I poked ahead and it turns out that it's actually not too terribly difficult to fit it in. Here is the partially modified speedometer:
I trimmed the bottom of the speedo to squeeze the face into the cluster, and the one other thing that I will have to do is trim a little bit of metal off of the backing plate -- the steel cup that the speedometer/odometer mechanism sits in. The top is
just a little bit too high for the W126 cluster.
Here comes my question: how should I take this metal off? It looks to be about -- oh, I don't know -- 1/8 of an inch thick? Should i use a grinder? A saw? What do you guys think? It's a very thin length all across the top, so I was thinking of using a grinder, but the only hitch is that I don't have one (rats). I'll get a better picture up tomorrow morning.