vw heater core to 1/2 (13mm) hose adapter

this is a fitting for adapting a 22mm inlet, 27mm flanged heater core to a 1/2" (13mm) hose.
it needs an o-ring, size isnt crazy important, i found 3 sizes that all seal in one small pack of o-rings. i recommend going to your local hardware store and just picking it out by feel. youre looking for aproximately a 20mm ID and 1.5-2mm thick, again, alot of o-rings fit and seal just fine