Are there any issues in using 28 mm copper pipe in the cooling system. It would include two soldered 90 degree bends. I have used 15mm in the heater circuit with no problems in the past.

It's been o.k. in mine for a few years now. Put a straight coupling on the end and clamp the pipe behind the solder ring for security.

Use a compression gland soldered in place on the pipe to retain hoses (with a suitable clip of course!)

i have used a mix of 10, 15, 22 and 28mm copper on my car. All soldered with no problems at all, its more than adequate for the job. To help retain hoses, i used a compression fitting to crush the appropriate olive onto the pipe's end, then removed the fitting and nut before i soldered it all up. I also managed to solder 22mm to 28, by cutting a yorkshire (solder ring) joint in two and using it to fill the gap! Its held up well on trackdays so i guess it worked.

I'm glad you all said OK cos I started today. Used a compression olive cut in half on heater hose joins and cut a yorkshire joiner as below. Both give a good 'barbed' retainer.
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