Here is a site that lists outside diameter and wall tolerances for the tubing they supply. 

http://www.speedymetals.com/information/materialframe.htm

I think you're right, Phil - given the (supposed) exact match between the ID of oner and the OD of the other, it would not take much in the way of imperfection to interfere with the fit. I'll measure the two as accurately as I can with a vernier before proceeding.

I think I have a method of sanding the interior of the extension tube for a few feet sufficient to remove the oxide.

Chris

(Edit:  I just read the last couple of posts after posting this. Yes, using epoxy when you are clear that it is going in sounds sensible. Also, West System makes a product to help epoxy bond to aluminum. )

When I worked for a couple of years as a millwright we used to polish shafts for an easy fit with long strips emery cloth and the shaft sprayed with WD40 oil. When I was planning to convert a CP 16 I needed to slide 3.25 OD outside diameter aluminum tube inside the 3.25 inside diameter mast tube for additional strength above the partners. On saw horses I sanded the tube this way, shoe shine style, turning the tube as I went and polishing with the wd40 for a very smooth finish. I made up a sanding block for the inside of the mast as well since the inside surface of the tube can be slightly less than perfect according to a site that sells industrial tubing.

I left the tube outdoors overnight at about 0 degrees F. and the mast in a heated garage. Ready for a struggle with the one end of the mast against a wall, protective wood blocking fitted in the tube and a sledge hammer standing by the tube slid easily in by hand and became a non removable fit. I must have sprayed it with WD40 as well but don't recall.

With your mast if it's a bet it's one you don't want to loose. But if it did not go well and it was attempted without epoxy you might carefully cut along two or three sides to remove the extension in case of a failure. However, without glue it would require something at the top of the tube / mast joint to ensure no slippage in use.

Chris,

I worked on aluminium cargo hatches on a live aboard barge, gluing foam and ply to them using a two part PU glue.

Epoxy would also stick to aluminium, but you have to sand very well to take off the tough oxidation layer. You will really be able to feel the softer aluminium as the friction goes up once you sanded through the oxide. I used an orbital disc sander

Sanding the inside of the extension tube would be very hard, but if you succeed in scratching the surface well with course sandpaper there would at least be keying of the epoxy in the scratches.

Chris,

Have you considered sanding down the inner tube untill it fits more or less easily?

Put the two together with epoxy, if it doesn’t bond it will close the clearance you made by sanding down the inner tube.

 I would try to avoid welding.