Just finished rebuilding a 1018(72C) TRS transmission. I felt really good about it except 1 part was left!!!! Looks like I left out the "expansion cup" which seems to go in the coupler from the transmission side. This means taking apart the whole deal which doesn't seem to be too appealling to me to say the least. I think I know what I need to do but I wanted to make sure this was a necessary part.(Pretty sure they wouldn't throw it in there if it had no purpose.)

It is 100% necessary. But why was it removed in the first place? Or was it out already? Look at it carefully, is there an imprint on it from the end of the drive's u joint shaft?
Put some Loctite retaining compound on it when you put it back.

Thanks for the reply and information. Also thanks for getting the parts to me. Can you tell me how the cup should be oriented? I can’t tell from the diagram on the website. It looks like it goes in the coupler from the velvet drive shaft side? Should the cupped side go toward the drive shaft or the other way?
I wasn’t sure really where it came from during disassembly. It just appeared! I’ll shoot a photo of it in the morning.

Yes it goes in from the transmission side. Normally it goes in with the cup facing the transmission.
Before you hang the drive grind 1/4" or so off the end of the U joint shaft. Most likely that is what knocked it out. Do not forget the loctite.

Thanks this is a completely new rerig. The boat had a Merc trans 2 that the coolant lines had failed on and needed to be rebuilt. So I thought I would swap out to the velvet drive which I would be able to get parts for. I found the velvet drive used so I don’t know the history on it. I do know it looked good on the outside but had taken a bath inside. Everyone replace those coolers before they leak.

I plan to run the TRS drive initially. If I enjoy the boat, I might go to a Konrad in the future.

Marc you were right on you can see the marks on the cap!
Back apart.

We sometimes flip it over so the cup is pointing out for more clearance

captain ron says "always use your cup boss"

We have one ZF63A-1,56 on stock if you need it. It about 12 years old, but it have less than 50 working hours. It is like new.

I have two super cyborg 900’s with TRS tailstocks and less than 50 hours if anyone needs them.