This anode rod is used on the earlier furnaces that had no welded anode threaded ports .
Linked anode rod was lowered down the vent tube and secured with a bolt and nut at top of vent tube .
Anode is 3/4" dia. Some furnaces had a smaller hole for 1/2" rod. You will have to enlarge the opening inside of the vent tube at it's base to allow 3/4" anodes.
This anode is also purchased for home made hot water furnaces and furnaces that never had a anode rod for boiler protection.