How Often Should I Check Valve Adjustment?
[A/H] Experience has shown that if the valves are not excessively noisy then they are usually OK. They don't move to the tight side unless someone adjusts them incorrectly by mistake. They usually get more loose as things wear in. Then you would hear them.
In the 1997 maintenance schedule it says to do it at 30k miles, then again at 90k. The 1996 manual says 30k ,60k, 90k. It is normal to hear the valves when the engine is cold. Some of what you hear is the oil pressure relief valve in the oil pump, as the oil warms up the pressure drops a little and the relief valve does not work as much.
If it is going to keep you awake at night wondering if you should do it.... do it.
[LE - 99/2/22] The specified time to check/adjust valves has increased steadily over the years because Honda realized they weren't going out of spec as quickly as expected.
Originally it was recommended that they be done every 15k miles. Experience showed that this was overkill. Then it went to every 30k miles in '95 or '96 (forget which). It seems this too was determined to be more than necessary, because since '97 the schedule is to do the first valve check/adjust at 30k and then every 60k after.
Of course if you race your car or something you should be doing all the maintenance more frequently than the schedule suggests.
[JF - 99/5/15] I have 11k miles on my '97 NSX and as I am planning to go to NSXPO in May, I asked my NSX tech if I should do my first valve adjustment before the trip or after the trip when I will have a little more than 15k miles (I live in Texas). He advised me not to do the first valve adjustment before it is recommended at 15k and that it would be better for me to do it a little later than sooner. He told me it is important for everything to seat and wear in correctly before making the initial adjustment.
[MBA - 2000/8/11] Valves get tight with age, rather than loose because as they wear into the seats, they get closer to the rocker causing the free play to diminish. There has been a lot of speculation and discussion about how often they need to be adjusted, and in fact Honda has changed the recommendation in the book from 15 k to 30 k in the new models, but the older models have had no update bulletin or service news. I still do mine every 15k, and I still recommend it to my customers.
What Is "Valve Lash"?
[RB] Valve lash is the mechanical clearance between the cam lobe and valve stem or transfer rocker when the valve is fully closed. It's usually checked with a feeler gauge and is some non-zero value on with "mechanical" non-self adjusting valve lifter mechanisms.
Differential expansion characteristics cause the cold clearance to be different than the running clearance and margin is built in to make sure their is always some clearance, especially on exhaust valves. An exhaust valve that doesn't close completely doesn't transfer enough heat back to the head and can "burn".
Cam lobes have entry and exit profiles (called ramps) which are designed to limit the opening and closing acceleration of the valve to limit mechanical stress and also noise. Still, the tighter you set the valve lash (less clearance) the more open valve duration you get which tends to boost top end performance slightly. The looser you set the valve lash the more bottom end is boosted and the more valve train noise you get.
If you have all the valves adjusted too tight idle quality and low end performance may suffer slightly with an attendant small gain in top end horsepower. Looser valve lash does the opposite and results in a noisier valve train.
If you have some tight and some loose that might also effect smoothness since different cyclinders will have different power contributions at different RPMs. Some of the above is a little simplified but you get the drift.
Why Does Valve Lash Need Adjusting?
[LE - 99/12/28] Too much lash can accelerate wear on the rocker arms, cam followers, cam lobes, valve stem tips and valve seats. How much it does depends on the engine and how far out of spec the lash is. Too much lash also tends to make more noise so you're more likely to know it needs adjusting.
Too little lash is more dangerous both because it leads to more serious problems and because it doesn't make any noise. Luckily the NSX valves do not tend to get tighter so unless they are adjusted incorrectly it should not be a problem.
But just for your info, too tight a lash may allow the lash to close up as heat builds up in the engine - especially if the engine begins to run hotter than normal because it's being worked hard. This gets worse as the engine gets hot because valve clearances tend to close up as the parts heat. That's why the NSX shop manual specifies that you should do the gap adjustment when the engine is cold. Some other engine service manuals will specify seperate hot and cold adjustments. The loss of lash can prevent the valves from fully seating which results in loss of compression and valve overheating. Exhaust valves rely on cooling through the valve seats much more than intake valves, so loss of lash in the valvetrain often results in a burned exhaust valve.