I have a 1959/1960 Envinrude 35 HP lark long shaft. I have removed the lower screw on the skeg to drain the lower unit oil. This screw also has about a half inch non threaded shaft and an O ring to seal it against the port. My problem is, I removed the pin to put on a new O ring, but the screw/pin will not insert all the wat to catch the threads. It seems as if something dropped and is preventing it from going in? What do I have to do to get this back in?