1. 1/2" copper pipe is 1/2" bore. The outside diameter is usually around 15.8 mm.
2. 15 mm copper pipe is 15 mm outside diameter.
3. So 1/2 " is about 6% larger on the diameter than 15mm.
4. You can usually get away with a 15mm compression olive on 1/2" pipe, but may well require careful filing / sanding while keeping the diameter round.
5. I think there is a risk of damaging / over-compressing the O ring in a push fit fitting forcing it onto 1/2" pipe.
6. If it were me I'd:
6.1 Cut back the 1/2", fit a 1/2" to 15mm converter and a short length of 15 mm pipe. OR
6.2 Expand a short length of 15 mm to 15.8 mm outside diameter
Imperial is measure by ID so the 1/2" pipe is 13mm ID but nearly 16mm OD, so the push fit's internal diameter and O-ring is 15mm and therefore too tight for the imperial pipe. So that's a no to pushfit on imperial.
As suggested, compression will usually always fit, use copper olives as they'll give a little when fitting. use the compression fitting to tap the olive on if it's very tight.
I do know you get 1/2" press fit but I don't think they are for water, more for fridge units and other typically imperial sized appliances.
I don't use press fit so couldn't confirm if 15mm pre-fit fittings could be used on imperial but the O-rings may fall into the same specific size issues though there will be chaps on here that do use them and could confirm
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