It seems like a joke. I was winding down the rebuild of my 1983 240D transmission, Manual. I had replaced all the main bearings, synchros, shims and large outside snap rings succesfully.

On literally the last gear I had to put back on, on the outer countershaft, i noticed the gear was upside down. When I removed the snap ring,(172) I dropped it into the transmission through a very small hole on the bottom of the housing.
The snap ring is relatively small, about a 1/2' in diameter and I think made of soft steel.
I know this sounds ridiculous, but would it be SMARTER to leave the snap ring in there and reorder it and install a new one on the end of the countershaft? I noticed on my rebuild that the 3rd Gear synchro spring had been eaten alive and digested by the transmission and that thing is 5 or 6 times bigger!
If I take the chance to reopen the transmission to get the ring out, I will have to remove and reinstall the main bearing snap rings and main bearing shaft and shims. I think I was just lucky to put the thing back together correctly the first time~~~~~~

I dropped a small snap ring in my transmission --can I leave it? 83 240D