quote:plans for fixing the swirl to vent to a surge and have the expansion tank converted to accept vents and drain to pump suction as a surge. I was wondering if 5/8" for the top radiator vent was appropriate to stay with similar hose as heater
On mine I (MarkIV/4280) used standard barb fittings and 5/8" radiator hose for the bleed from the swirl tank to the expansion tank. The fittings welded into in the tanks were whatever was convenient (5/8" Pipe?) to provide that nipple.
The hardline from the radiator was smaller, maybe 3/8", that was because of the bends, and having to run and restrain it. It has to run all the way under the car, up the back bulkhead, and around behind the passenger-side frame sill to get to the Overflow tank.
As you are just bleeding air, what you need is a constant flow, not massive volume.
In my opinion, 1/4" line would work, but 3/8" was the better choice.
Rocky