My opinion if I dare share it with only one cup of coffee in my system...
I've seen both loose and tight valves when checking valve clearance many times on ATV and motorcycle engines. Valve stem stretch does happen causing tighter valves as Rick explains. There is also the possibility of the valve seating further into the head upon closing as it wears into the seat or the valve head slightly dishing out from heat around the outside perimeter of the valve head and spring pressure. Loose valves would generally be caused by valve train wear on the cam or rocker arm and also wear on the end of the valve stem where the rocker arm contacts it. Common sense would make one believe tight valves are more common but not always the case either...... Tight valve increase or advance valve timing (valve open sooner) and loose valves decrease valve timing (Valves open later). I seem to remember reading that 1 thousands difference in valve clearance can equate to a change of up to 5 degrees rotation in valve timing and 10 degrees crank rotation. Not sure about that though???? Although this may explain why Sanny's high compression engine starts better with tighter valves. There definitively would be a change but not clear on how much.
Heat cycles on the engine may result in metal movement in a new engine. There could be changes in the head, valve train, or valves as it gets accustomed to normal operation or maybe the assembly worker was in a hurry and didn't set valves properly to began with?????
Loose valves make a lot of noise, accelerate wear on the end of the valve stem and can affect performance some but generally cause no long term damage that cannot be adjusted out unless there is excessive cam wear. May be an indication of improper lubrication in the cylinder head. Oil wears out. Long chain oil molecules will break down into shorter ones with use, heat and age. Scientists call it viscosity loss or oil breakdown. Old contaminated oil with gasoline, moisture, metal particulate from transmission and blow-by impurities does not lubricate as well either.
Tight valves may make no noise, affect performance due to compression loss, accelerate wear on the valve seat and valve, and can destroy valve seat seal. Can cause long term damage requiring valve replacement valve or seat replacement, or a valve grind job.
Properly adjusted valves will make a little noise, when the level of noise increases or reduces it's time to check valves. I'll generally check valves at 300-500 miles on a new engine and have never found an engine that didn't require a tweak in adjustment on at least one or more valves during this initial adjustment to my satisfaction. I'll recheck if I notice a change in sound or somewhere between 1000-2000 miles or so. In my experience I've often found no adjustment necessary but not always the case. I also believe an air cooled engine should be checked more often then a water cooled engine. They run hotter. I've always set mine in the middle of the recommended range. If the specs were 4 to 6 thousands, I'll set at 5 thousands and reset to 5 if I find one at either 4 or 6 thousands. No particular reason other then a habit. Four to six thousands isn't much considering the hair on your head may be 3-5 thousands in diameter on average.
I've seen both loose and tight valves when checking valve clearance many times on ATV and motorcycle engines. Valve stem stretch does happen causing tighter valves as Rick explains. There is also the possibility of the valve seating further into the head upon closing as it wears into the seat or the valve head slightly dishing out from heat around the outside perimeter of the valve head and spring pressure. Loose valves would generally be caused by valve train wear on the cam or rocker arm and also wear on the end of the valve stem where the rocker arm contacts it. Common sense would make one believe tight valves are more common but not always the case either...... Tight valve increase or advance valve timing (valve open sooner) and loose valves decrease valve timing (Valves open later). I seem to remember reading that 1 thousands difference in valve clearance can equate to a change of up to 5 degrees rotation in valve timing and 10 degrees crank rotation. Not sure about that though???? Although this may explain why Sanny's high compression engine starts better with tighter valves. There definitively would be a change but not clear on how much.
Heat cycles on the engine may result in metal movement in a new engine. There could be changes in the head, valve train, or valves as it gets accustomed to normal operation or maybe the assembly worker was in a hurry and didn't set valves properly to began with?????
Loose valves make a lot of noise, accelerate wear on the end of the valve stem and can affect performance some but generally cause no long term damage that cannot be adjusted out unless there is excessive cam wear. May be an indication of improper lubrication in the cylinder head. Oil wears out. Long chain oil molecules will break down into shorter ones with use, heat and age. Scientists call it viscosity loss or oil breakdown. Old contaminated oil with gasoline, moisture, metal particulate from transmission and blow-by impurities does not lubricate as well either.
Tight valves may make no noise, affect performance due to compression loss, accelerate wear on the valve seat and valve, and can destroy valve seat seal. Can cause long term damage requiring valve replacement valve or seat replacement, or a valve grind job.
Properly adjusted valves will make a little noise, when the level of noise increases or reduces it's time to check valves. I'll generally check valves at 300-500 miles on a new engine and have never found an engine that didn't require a tweak in adjustment on at least one or more valves during this initial adjustment to my satisfaction. I'll recheck if I notice a change in sound or somewhere between 1000-2000 miles or so. In my experience I've often found no adjustment necessary but not always the case. I also believe an air cooled engine should be checked more often then a water cooled engine. They run hotter. I've always set mine in the middle of the recommended range. If the specs were 4 to 6 thousands, I'll set at 5 thousands and reset to 5 if I find one at either 4 or 6 thousands. No particular reason other then a habit. Four to six thousands isn't much considering the hair on your head may be 3-5 thousands in diameter on average.
