Intercooler piping

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I am in the middle of gathering the required parts for plumbing in my new iintercooler and the one thing I need to know is this.

What diameter hose do I need to fit on to the compressor outlet? This is about the last bit I need to source.

I dont want to remove the original untill I have to as it looks a bit old and fragile I still need to use the car and dont want it to come off in pieces.