Yamaha - Radiator Stopper Cooling System Component YF1-89613-08-20. Overview. When you're working on Yamaha cooling systems, it's often the small parts that end up making a big difference. I've seen engines lose pressure or run hotter than they should just because a stopper wasn't sealing properly. The Yamaha Radiator Stopper YF1-89613-08-20 helps keep the cooling system sealed so everything flows and pressurizes the way it's supposed to. It's a simple part, but it plays a key role in keeping th