short follow up on the valve clearance post
Why be so precise on these simple adjustments?
In a nutshell; balance, performance and longevity. lets just assume for a second that with perfect machining practices and a zero tolerance level of assembly your 120hp machine is getting that 120hp equally from each cylinder. It would run very smoothly, produce power equally from each cylinder, every power pulse would be exactly the same so acceleration/deceleration forces would be identical; ie the depicted pic it would be 30hp per cylinder. If the balance is thrown off by having some of the valves at the bottom of clearance, some in the middle and some at the top (but all within the clearance spec as allowed by all manufacturers) you could end up with a distribution of say; 31,28,29,28.... Now if you could measure crank harmonics and acceleration/deceleration pulses, heat generation, efficiency and tons of other data it would show an engine that will wear out sooner, produce less overall power and not run as smoothly. Cam timing will be slightly changed, valve lift slightly changed, combustion efficiency slightly changed, it also affects throttle body sync and exhaust scavenging pulses... Now we all know there are tolerances and it is highly unlikely your 120hp engine is so balanced, as each individual part has a tolerance at the manufacturer, each combustion chamber may be slightly different in overall size and more.... But those you nor I can control without pulling the engine apart and going through great efforts to make everything perfectly sized and balanced.... But-- Don't you suppose it makes a big difference to keep those assembly differences to a minimum and not add to the problem by not doing what is possible and easy when one is already in doing the valve adjustment?--This is one performance improvement that costs you nothing extra, just requires a little more effort than "close enough" when doing the valve adjustment service. That is the condensed version; in a nutshell because this discussion could really go on and into much more technical science of how the engine works which would be hard to articulate effectively without diagrams, actual demonstrations and hands on with some engines.