I am in the process of reassembling a Parker 51 Aerometric (1948 marked nib and barrel, no O-ring) after replacing the breather tube and sac. I had really no problem getting all the parts disassembled but I am now concerned with how tight the feed is getting it back into the collector. It feels much tighter going in than it felt coming out. And it seems like I can't get it as far in as it needs to be because one, it's so tight that I can't even feel the feed hitting a stop point and, two, when I then put the nib in as far as it can go the tip of the feed is too far forward compared to the tip of the nib.
I managed to get the feed back out of the collector to start over, but I'm stopping here for some advice. Would it help if the parts were wet maybe? I'm guessing silicone grease would be a bad idea :-) My browsing hasn't yet come across this issue for any insight.