I am (obviously) new to GM mechanicals... further GM does not seem to cover assemblies they purchase in their manuals, so I am flying partially blind.

I am trying to make my Muncie M21 not leak by replacing all the oil seals. Which leads to my dilemma: the input shaft seems to not have any seal nor anyplace to put a seal? I am guessing that the oil level is always supposed to be lower than the input shaft and any oil that works through the bearing up there is supposed to drain back into the transmission. I further guess that this is actually how this thing is supposed to breath as it heats up and cools down. This sounds ok I guess... so long as I do not park nose down on a hill?

Anybody know what the story is here?

I think the oil level is low enough and there is an oil slinger on the input shaft. The most fun in reassembling the trans is installing all those roller bearings.

I bet... I avoided taking this one that far apart.

Instead I took a tour through the Saginaw recirculating ball steering gear. Getting all the balls in the right place is probably just about as much fun.

I also guess that the drivetrain angle helps keep oil away from the front and that the "oil" they used just does not run out. In addition to the slinger there are the threads to pull oil back in. I noticed that the speedo geer uses the worm oil retrieval method as well.

I would probably find a much more robust sealing setup on a modern transmission filled with ATF-weight oil.

I'm drawing a blank here, but did you take off the input shaft bearing retainer? Is there an oil seal under there or not? I can't remember. I've replaced that bearing on a Super T-10 and a Saginaw cast iron 4spd, it wasn't a big deal, but I can't recall if there are any seals. Pop the bearing retainer off, it's only 4 bolts, have a look ... nothing should come shooting out, LOL. I just had an M21 in the garage, there was no play in the input shaft so I never even looked at the bearing, just installed in my father's 1967 Camaro.

Sorry that I can't be of more help

Yep, been there done that. Removing the front bearing retainer is what lead to the question. When I asked for parts at the store they sold me a seal but when I took it apart there no seal and nowhere to put one. It might be something they added in later years.