Warning !!!  This is a draft, this is only a draft, please do not use these instructions to perform this procedure as there are mistakes and misspellings and a whole bunch of bad stuff that will surely blow up your engine.  Please report any comments and corrections to DanO - Thanks. 

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Valve Clearance

Overview

Valve clearance is the distance or gap between the cam and the top of the valve.  Because the NSX has a rocker arm between the cam and valve, we measure this clearance as the distance between the cam and the rocker arm pad (where the cam lobe actuates the rocker arm).  We adjust the clearance by turning a set-screw located at the end of the rocker arm.  The bottom of this screw effectively rests on top of the valve, so turning the screw clockwise moves the rocker arm up towards the cam (less clearance), and falls away (more clearance) if we turn the screw counterclockwise. We need to occasionally check and adjust valve clearance because the these components wear slightly with use and the clearance may fall out of the specified range resulting in extra wear (not enough clearance) or lost performance and noisy engine (too much clearance).  Its been our collective experience that the NSX's valve clearance stays well within its service limits between service intervals.  Honda raised the service interval for the 1996 cars to every 30k miles and 1997+ models are adjusted at 30k and then every 60k miles thereafter. we can do some damage to our engines if we set our clearances too tight, along with extra wear we can make it so the valves don't seat properly, which can result in damaged valves, which is not good$$$.  Fortunately, with a little time and attention to detail, this is easy to avoid.  Our goal is to keep the clearance within the service manual specifications and try to be consistent with our adjustments and measurements. The first few clearances we check and adjust can be a little frustrating and take a bit of time, but we'll soon get the hang of it and it really becomes quite easy.  After I've finished all 24 measurements/adjustments, I usually go back and quickly re-check them again, just to make sure everything's OK. Please read more about valve clearance from www.nsxprime.com The NSX FAQ.

Tools & Supplies

Prepare Tools: We'll need a 10mm deep-well and regular socket, 12mm socket for OEM strut bar or 9mm hex wrench for most aftermarket strut bars, a 19mm socket (same as your lug nut socket) with a long extension (at least 16") and a 17mm combination wrench (not pictured).  Well also need feeler gauges (.006 and .007 inches).  A micro torque wrench is helpful if you're not used to tightening screws to low torque values.  

Prepare special tools: 07MAA-PR70120 tappet locknut wrench and 07MAA-PR70110 tappet adjuster.  These special tools allow easy adjustment of the valve clearance.  Tip: draw an "X" on the top and sides of the tappet adjuster with a paint marker.  This visual reference is a real time saver.

Prepare special parts:  It's a good idea to have some head cover seals and some Honda bond to replace any seal that's damaged or deteriorated.  I've replaced mine only once and have had my covers off several times so you may not need to replace your seals.  Each cover has two seals, one small one, which seals the timing belt bay, and a large one, which seals the rest of the cover.  There are also three round doughnut-type seals, which seal the ignition holes.  I have never replaced mine, however I'll probably replace them just for fun next time I have the covers off.

Prepare car and mechanic: The service mat is a worthwhile investment for this procedure--plus it looks cool.  Make sure you allow yourself enough time and energy to do a good job with this procedure.  If you find yourself in a time crunch or are getting frustrated you can always close the engine hatch and come back to the project later.  You may want to find a short stool or something similar to stand on to help reach the exhaust cams.  I stand on the spare tire with a non-skid carpet underneath.  Note: we have to perform this procedure with a cool engine (under 100F).  Remove your belt or any jewelry that will scratch the paint as you lean up against the car.

Remove valve covers

Step 1: Jack up car as low as possible and remove right rear tire.

Step 2: Remove coil covers and coils (Steps 2 - 5b)

Step 3a: (Remove front cover) remove the PCV valve, driver's side wiring harness, passenger's side  harness covers, and brass spacers (see pictures above and below)

Step 3b: disconnect the front oxygen sensor (press on tab and pull apart) and clear the ignition connectors out of the way

Step 3c.  Remove front valve cover:  We'll use our 10mm deep-well socket to remove the two 2 nuts located at the bottom of the threaded rods sticking out of the cover (see picture above).  Using the same wrench, we can remove the 4 black acorn nuts on each corner of the cover.  Once the nuts are removed, try to raise the cover a few centimeters, this will break the gasket seal and break free the six rubber grommet washers.  These six washers are a real pain, I like to collect all six of them before removing the cover as they have a tendency to get lost; there are 4 of these washers: 1 under each acorn nut you removed and two in the center around the 2 threaded rods.

Step 4: (Remove rear cover) Remove rear oxygen sensor connector, rear cover breather hose (use pliers on spring clamp), 4 acorn nuts, 2 center nuts and grommet washers.  Lift head up slightly and carefully rotate 90 degrees rotating the passenger side of the cover to the rear.  Be careful not to damage the timing belt or rubber cover seal, then carefully pull cover out.

Measuring and Adjusting Valve Clearance 

We measure valve clearance by measuring the gap or free play between the cam and the corresponding rocker arm.  To measure this gap, we need to make sure the valves for the cylinder we are measuring are in the closed position.  We do this by positioning the appropriate piston at TDC (top dead center) of the compression stroke, which insures that both intake and exhaust valves are fully closed and the cam lobes are not engaging the rocker arms.  Fortunately. it's easy for us to position our valves in this manner.  The numbers 1 through 6 are stamped on the outside of the intake cam pulley on the front cylinder bank.  When the number on the cam pulley is lined up with the timing belt cover (see picture below right) then that piston is at TDC.   

Positioning the cam for measurement/adjustment
To position a piston/cam at TDC, make sure the transmission in neutral and use the 19mm socket and long extension to rotate the crankshaft pulley clockwise until the desired cylinder number stamped on the cam pulley is lined up with the timing cover.  The crankshaft should turn with moderate force, so if you find yourself really leaning on the wrench then double-check to make sure the transmission is in neutral.  Rotating the shaft counterclockwise is not a good thing as it may skip a tooth on the timing belt, which is bad.  The picture below shows that cylinder #6 is at TDC and the valves are ready to be measured/adjusted.

We will be measuring/adjusting the clearance on each of the four valves per cylinder--two intake valves and two exhaust valves.  The intake valves tend to be easier to measure/adjust because they are relatively accessible--located towards the center of the engine where the intake plenum runs.  The exhaust valves are a bit challenging, especially at first, but they get easier.  Tip: it may be easier to start with cylinder #4 as it is the most assessable.

 
Measuring intake valve clearance (looking from driver's side at the rear cylinder bank)
 
Measuring exhaust valve clearance (looking from driver's side at the rear cylinder bank)

To measure valve clearance we need to insert the feeler gauge between the rocker arm pad and the cam we are measuring.  Study the two Service Manual diagrams above and notice how we measure from the center of the cylinder bank out.  The tip of the gauge should poke out from under the cam and point at the adjusting screw--we can see this when we adjust the intake valves.  Sometimes the feeler gauge has a tendency to migrate towards the mid rocker arm so double-check the position of the gauge to make sure it's under the proper rocker arm.  Tip: to make measuring easier I remove all but the .006 and .007in gauges from my set.

...better pictures coming soon (as soon as I can get this damn cast off my right hand <g>)

One of the most frustrating tasks involved with this procedure is figuring out how to properly position the tappet wrenches by feel on the exhaust side.  I always find that the locknut tool needs to be angled more horizontally than I expect to fit on the exhaust locknuts.  

Adjusting order
There are several ways to go about this procedure: we can start by adjusting just the intake valves on all the cylinders, or do one cylinder bank at a time or move from bank to bank in sequence with the numbers on the cam pulley--as long as we hit all 24 valves, order really doesn't matter.  If it's your first time then it might be easier to start with cylinder #4 and work down the front cylinder bank #4, #5 and #6, which gives you time to get the hang of it before tackling the rear bank, which is less accessible.  

Now let's take a detailed look at measuring and adjusting the clearance for a valve:

Measuring 
We'll start with an easy one, cylinder #4, which is located on the front bank passenger's side.  Turn the crankshaft so that #4 is at TDC position.  Let's use our .006 inch feeler gauge to measure the clearance on the intake valve closest to us.  Slide the gauge between the cam and rocker arm until you can see the point of the gauge coming through the other side and pointing at the adjusting screw.  You'll probably have to monkey around with the gauge a little bit to find the right angle that allows the gauge to slide in there.

Loosen the locknut
Now, place the locknut wrench over the adjusting screw locknut on the rocker arm and slide the adjusting wrench down inside the locknut wrench and turn the head (of the adjusting wrench) until the blade of the wrench engages the adjusting screw.  Now we're ready to loosen the locknut by placing the open end of the 17mm combination wrench on the locknut wrench and turning it counterclockwise about a 1/4 turn.  In the pictures above you can see that I use the closed end of the wrench, but using the open end is probably easier at first, especially for the exhaust valves.

Adjust
Now that the locknut is loose and the feeler gauge is in between the cam and rocker arm, we can play around with the clearance.  If we turn the head of the adjusting wrench clockwise (it will turn quite easily by hand) it will reduce the clearance, counterclockwise will increase clearance.  As we turn the wrench, we can move the feeler gauge back-and-forth to feel the changing resistance.  Specified clearances for the intake valves are .006 - .007 and .007 - .008 for the exhaust valves.  Turning the adjusting wrench just a few degrees will move the clearance in/out of these specified ranges.  To set the intake clearance, use the .006 (.007 for exhaust) gauge and turn the adjusting screw so that there is a bit of resistance or drag on the gauge when you move it back-and-forth.   If you're not familiar with using feeler gauges then it's good practice to double-check your work by using several blades.  Try the .007 gauge, if it fits that's OK, but it should be tight.  However, the .008 (.009 exhaust) should not fit.  The .005 (.006 exhaust) should fit quite easily and have no resistance.  We'll get quite proficient at this after doing 24 of these babies and probably be able to set the clearances perfectly using only the.006 (intake) and .007 (exhaust) gauges. 

Tighten the locknut 
Once the clearance is set, then we need to tighten the locknut.  Place the open end of our 17mm combo wrench and tighten (14 lb-ft. clockwise).  Keep an eye on the adjusting wrench head to see if it moves when you tighten the locknut, if it does, then most likely you'll have to re-adjust the clearance and re-tighten--you may also be over-tightening the locknut.  After tightening, always re-measure the clearance and re-adjust if necessary.

Repeat
Repeat the above: position, measure, loosen, adjust and tighten for each of the 24 valves.  Take your time and have fun with it.

Re-check
When you've finished measuring/adjusting all the valves, it's a good idea to go back and quickly re-check all the clearances.  This is a good way to catch any mistakes or variations in measuring/adjusting consistency.  Now that we're so good at it, it only takes a few minutes.

Valve Clearance Summary

Start with cold engine
Jack up car and remove right rear wheel, strut bar, 12mm socket or 9mm hex
Remove coil covers and coils 10mm socket
Remove wiring harnesses that connect to head covers, PCV valve, breather hose, O2 sensor connectors
Remove 4 black acorn nuts, 2 center nuts and rubber 6 grommet washers, 10mm socket, deep-well 10mm
Remove head covers, rear cover rotate passenger's side 90 degrees back and then pull out
Move desired piston to TDC and set clearances .006 - .007 inches for intake, .007 - .008 for exhaust, 14 lb-ft. locknut torque
Re-check all when finished 

Installation

Installation is basically the reverse of removal.  Check the seals on your head covers to make sure they're not deteriorated or damaged.  If they are then spot-dab a little Honda bond between the head cover seal channel and the new seal to help keep it in place during installation. Before I install the covers, I like to give the heads a once-over with an inspection mirror to check for anything that may have fallen in there during the procedure.  After you have installed the covers, but before you have started to tighten them down, spend some time with an inspection mirror and follow the head seal around the cover.  Look for a good seal and watch out for wires that may be caught in there, like that nasty rear oxygen sensor wire, which likes to get in there and mess up an otherwise perfectly good seal. 

Torque values:
    Head cover acorn nuts and center nuts - 7 lb-ft.  Over-torque these babies by too much and you'll snap a stud.
    Ignition coils - 9 lb-ft.
    I like to re-torque the head cover nuts after I've installed the ignition coils

Make sure you've got all everything put back together, like the front and rear O2 sensor connectors and the grounding cable on the front driver's side acorn nut. 

Once everything is back on, run the engine for about 5 minutes and check for oil leaks around the cover seals.

Way to go!  That was so easy let's do it again!  ...OK, maybe next year.