In article , Martin wrote:
Are you using a pipe bending spring?
If not, you should be.

Not with a pipe bender ....

Kinking used to be a problem with some continental-sourced tube but
I've not had this problem in 20 years.

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Tony Bryer SDA UK 'Software to build on' http://www.sda.co.uk
Free SEDBUK boiler database browser http://www.sda.co.uk/qsedbuk.htm