I'm just using water pipe out of the MSC catalog. It's a bit tougher, but don't forget that lifter will still rub against that tube.
I bought an X block (for damn cheap) that was tubed with aluminum. Several bores had been worn through. So I just ran the reamer down there and got rid of the aluminum and used brass.
Tnx. So just regular copper water piping, like you would sweat together with solder, right? Kinda makes sense..... it would probably be impossible to 'drive' soft copper tubing without it just mooshing.