The radiator core is made of many tubes, usually oval shaped, that transfer coolant from the top tank down to the botom tank. Because these tubes are small the radiator actually serves as a fairly efficient filter by traping lumps of crud that are liberated from the block.

This crud slowly but surely blocks the small tubes in the radiator & reduces its ability to shed heat by restricting coolant flow.

Rodding the radiator refers to a process where the top & bottom tanks are removed from the radiator, usually by a radiator specialist, & a small rod is pushed along the tubes in the opposite direction to coolant flow in an attempt to restore flow in the radiator core.

If the core is badly corroded, or damaged, or too solidly blocked to clear, then replacement is the fix. If a heavy duty core is available, then it is often a good idea to spend the few extra sheckles & have it fitted. I use a cheezecloth condom kind of coolant filter in the top tank of my new radiator & this has captured much more crud than I had believed it would. Now my 1200 runs right on the money even when cruising at highway speeds during even the hottest days.

Spend what it takes to fix it now rather than wait untill your engine is damaged when you need to pay to fix both the radiator AND the engine.