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The fittings are iron with a rubber lead look ish type seals with a male iron on the end so just use a 3/4" female iron to copper. I usually fit them just above ground so you can get to them again.
I used 54mm trac pipe on a job it is really easy to use, just remember to wrap the fitting with the tape that's provided after testing and remember to leave the fittings accessible!
If it was a straight 4m run i'd probably do it in galvy MI. 30 odd quid for a 6.5m length and 2 bends and 30 minutes cutting 4 threads and screwing it together.
Trac pipe will be easier tho. You can do it in copper if it is coated or wrapped.
Reply to the thread, titled "Trac Pipe, underground?" which is posted in Renewables on Plumbers Forums.
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