COMPONENTS-WHAT TO NEVER REUSE AND WHY

You are after reliability,and used seals, rusty valves ,and springs are not reliable.So here are the basics to rebuilding a Yamaha outboards cylinder head.Seals, well that's obvious they don't cost much,replace them.Springs, 2 reasons, #1 a spring that has run an average of 3500 rpm for 500 hrs has compressed and relaxed approx. 105 million times,this is a cruising rpm and a low hour engine,metal fatigues and fractures. #2 more than likely the head is being overhauled because it corroded,passing water into the cylinder and engine oil causing rust and pitting,pitting on a spring creates a stress riser (weak point) and causes the spring to break and drop the valve into the cylinder now you have major damage over a $10.00 spring.

Valves,its not just the face we are worried about it's the entire head ,neck and stem between the combustion chamber and valve guide,especially the exhaust valve ,this valve has to endure all the heat and corrosion from exhaust gasses and salt water intrusion, all of these factors cause pitting on the face ,neck and stem, again creating a weak point and causing the valve head to break off and drop onto the piston. Another thing to think about is Yamaha 4-stroke valve stems are only 5-6 mm in diameter and are always under spring tension, any pitting will cause a catastrophic  failure. And at last the intake valve. Many people don't replace them,they are under the impression that they stay cool and don't live in a corrosive environment. In automotive this is a true statement but not marine, yes they stay cool, however the intake valve is exposed to everything that comes in the intake such as corrosive salt air, corrosive fuel blends ,and saltwater from an external leak, like a hole in the thermostat portion of the engine block.Facing valves is a very normal and acceptable practice in an automotive machine shop but not so much in marine