The other day I discovered a water leak in the iron elbow (#38) in the lower rad hose on the CBB GT. It was leaking at the threaded nipple (#40) for the heater hose. When I removed the hose nipple I discovered that the paper/fiber gasket (#41) had perished. To give that connection a more permanent fix I used a bonded washer instead of the fiber gasket. A bonded washer is a metal washer with an o-ring bonded into the inside diameter. Problem solved.
A bonded washer can be obtained from McMaster-Carr, Grainger, eBay, or most any auto parts store. The part I used is a Perkins part number 025100016. The thread size for the nopple is 11/16".