Thanks for all of the posts in this thread. The radiator shop owner did not know that this problem is fairly common and that there are some fixes out there. I finally went to my local Volvo supplier [who runs a Volvo wrecking yard as well as two service outlets in Nanaimo and Victoria here on Vancouver Island. She is promising me a fix for tomorrow morning and I will drive the approximately thirty kilometres with a wine cork in the hole, with the pressure cap loosened, and will see what she has for me. I will bring some water bottles with me just in case the cork blows out on me. It is highway driving but will keep to 80 km/hour

This is not a threaded hole and is for the auxiliary fan sensor for a later model Volvo as most of you have pointed out to me.

I expect she will have a sensor waiting for me and I hope I can install it before returning home. However, if the cork is still holding I will probably just buy the sensor and return home with it so that I can install it myself.

What a bummer that people have blown their engines over this stupid setup. If I can ever find out who manufactured the radiator I will send them a piece of my mind.

My thanks again to everybody who supplied me with suggestions for dealing with the problem.

Bob in Parksville, Canada