Jeff Liebermann wrote:
. . .
You don't want to heat the outside. That will cause the outer tube to
expand, which will increase its compression on the inner tube. You
want to cool both tubes causing both to contract. If you can remove
an end cap, pour in some ice water.
. . .

If you heat a ring of material, both the OD and ID increase in
proportion. This loosens, not tightens, its hold on an inner object.

I recall from freshman physics that if you heat two concentric rings
made of the same material (having a positive coefficient of expansion),
all dimensions increase by exactly the same proportion. A little
geometry shows that any gap, even microscopic, between them will
therefore also increase proportionally. Cooling both objects of course
has the opposite effect, reducing the size of any gap and tightening the
fit. Heating the outer one and cooling the inner one, as Ed did, gives
you the best of both, increasing the ID of the outer tube and decreasing
the OD of the inner tube.

Roy Lewallen, W7EL