Intended for copper tubes, this expander is easily made in any size required.

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The parts (from left to right) assembled on a suitable fine-thread bolt are:

A steel washer

A stack of hard neoprene washers

A second steel washer

A thrust washer & nut.

In my case, I had a ball thrust race in the parts box but two or three greased brass washers should work ok.

The secret is in the neoprene washers, when they are compressed between the steel washers by tightening the nut, the rubber displaces sideways with sufficient force to stretch soft copper. My source for washers under 3/4" diameter is the local hardware store's supply of common faucet washers. They can be ground to size after assembly on the bolt by rotating the assembly against a belt sander.

When using the expander, be careful to locate the rubber where the tubeplate is. Failure to do so will result in a bulge in the tube on the inside of the boiler.