Hi,
here is a link that may helps with your problems. It's from the website of C2Marine which also makes very good professional c-tillers, like them from roostersailing.
First.. try McLubing it.. This MAY be all you need. Second.. If it still doesn't go in.. LIGHT block sand the sides one or two sanding strokes at a time with very fine sanding paper.
Re-McLube and try the fit. Do this till you can get it to go in all the way so the backend of the tiller extends slightly out the back. It should be hard to pull out but not impossible.
With it fitted this tight, there is no need for pin at the rudder head to lock it in. The rudder line keeps it pushed back as far as it will goes anyways.
Overtime the fit will become perfect to where McLube may not be required.
Remember your goal is to have a very tight fit so the rudder and tiller move as one in all conditions. When its blowing30knots and your going into a hyper-speed gybe while carving down the face of a monster wave.. if theres any slop.. well we all know what happens then...
Sail4sc is right on. Notice that this is not a problem, but how it is supposed to be. You want to have it fit in the best way possible, so they make them slightly bigger than they are supposed to be. Go with the instructions of sail4sc: sand it lightly and do not use any machine-tools!