Embrace Your Inner Noob
Adjusting valve clearances is simple, right? They can be, as long as you follow what Honda has to say about them. If there's one thing Honda says to do when it comes to adjusting those valves, it's to do them when the engine is cold. Look at any Honda service manual and they'll all tell you as much. Unfortunately, engines are often blazing at full temperatures when the thought of a valve adjustment occurs, which makes attempting to adjust those tappets, despite the heat, all too tempting. Stop. Have a bit of self-control. Let that cylinder head cool off before doing anything; the results will be better for it. Metal objects like camshafts, rocker arms, tappets and even the cylinder head expand and contract as temperatures change. When they're hot, they get bigger, reducing the clearances between parts. Oftentimes camshaft manufacturers and even service manuals will offer recommended valve adjustment clearances to be used during hot conditions, but this just isn't the right way to do it. A typical Honda cylinder head reaches temperatures above 200 degrees F. Who's to say what temperature it's at by the time you reach the last adjuster? It'll inevitably be less than when you started. Remember, as the top end and valvetrain cools, clearances change accordingly, slightly, but enough to matter.
Do yourself a favor and allow the engine to cool down some prior to adjusting those valves. Point a high-powered fan toward the cylinder head until it drops to, at most, 100 degrees F or is warm to the touch. With the cylinder head cooled down, follow the service manual's maximum clearance specifications and adjust to the higher number. Erring on the high side helps put the valves in the middle of the range, as if the engine were stone cold. Don't have a service manual? Valve clearance specifications can be found underneath the hood on the emissions label. Scraped the label off, got a carbon-fiber hood? You can always purchase the label at your local Honda dealership.
It's important to note here that if you're trying to correct a valve clearance-related problem, like excessive valve noise, idle misfiring or slight power losses, don't adjust the valves unless the engine is completely cooled down. Allow the car to sit overnight. Excessive valve clearance causes noise and insufficient valve clearance may cause misfiring, even permanent valve damage so it's important to get it right.