brewer,
I can't help with snap ring size other than tell you to call Clausing and order one. I think that the part number that you are talking about is 641-084. You have a ways to go before you have 30 posts and get access to Downloads but if you will PM me your email address, I will send you the operation and parts manual. You just need to tell me whether your machine has the early or the late carriage. The late carriage has adjustable mesh between the gear driven by the carriage traverse handwheel and the larger gear that it drives. On the early one, there is no adjustment.
If the PO removed the entire compound assembly (two major parts and a bunch of smaller ones), and if he didn't lose the two beveled pins in front of the two socket head set screws in the upper swivel below and to the left and right of the compound feed crank, then yes, you just tighten the two set screws until the beveled pins bear on the pintle on top of the cross slide. Turn the compound upside down and run each screw in until either the tip of each pin is visible or you see the tip of the screw. If the latter, you will either have to find, make or buy the two pins. If the former, confirm that with the compound inverted, the flat of the bevel is down. Hold your finger against the end of the pin and retract the set screws until the pin tip is barely inside the swivel base. Turn the assembly back over and set it down on the pintle. Rotate the compound around to 30 degrees and tighten both screws.
