"Tom Gardner" fired this volley in
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The piston IS pushed away by the spring, these are pull-off cylinders
so the spring naturally extends them. I might try the TIG, thanks!

Tig can concentrate its heat in a small area. Wrap the whole body of the
cylinder in a wet cloth, or better, suspend it all but the end-cap in a
vessel of water, then weld away.

LLoyd