-- Put the rod into the freezer for a while. As it cools, it contracts (gets smaller) slightly.
-- Put the cylinder into hot hot water for a while. As it heats, it expands (gets bigger) slightly.
-- Bring the rod and the cylinder togther quickly, before the rod has a chance to warm up or the cylinder has a chance to cool off.
-- I bet it'll fit now.
-- But be careful . . . get the rod exactly where you want it as fast as you can. Once both pieces come back to the same temperature, and the rod expands a little and the cylinder contracts a little, the fit will be so tight that you'll probably never get them apart again, or even move the rod.
To make a metal rod fit into a tight hole in a metal cylinder, cool the rod in a freezer and heat the cylinder, allowing both to adjust in size. Assemble them quickly before they return to room temperature. This method utilizes thermal expansion and contraction to create a perfect fit.
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