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Valve Adjustment


A normal maintenance item for most Hondas that is often overlooked is the valve adjustment. Also known as setting “valve clearance” or “valve lash,” this adjustment is an important maintenance item to insure lengthy, proper, and efficient operation of your engine.

The valves in your engine open and close to allow intake air to flow into and exhaust gases to flow out of your engine. The method that Honda uses is a rocker/follower that rides on the cam lobe (with the crankshaft and timing belt/chain turning the cams) that actuate the valves. The cam has a "lump" for lift and a "base circle" for the duration of the engine rotation where the valve is closed. In order to insure that the valves close completely the "valve clearance" or "valve lash" is set so that there is a known clearance between the valves and the rocker arm.

The valve adjustment is important for two reasons. One, if the valves do not close all the way, you can lose a substantial amount of power since the intake mixture will be "squeezed out" during the compression cycle (hence less mixture burned, less energy resulting from combustion). This might also result in burned valves as the exhaust valves depend partly on contact with the valve seat (head) for cooling. Second, if the valves do not open all the way you will get poor flow through the head (less mixture) and again lose power.

As the engine ages and more miles are put on it, the valve clearance can change. Repeated thermal cycling of the mechanism as well as mechanical wear will alter the adjustment slightly, resulting (typically) in increased clearance. This increase in clearance is usually characterized by an increased “ticking” at idle. While Hondas generally run wide clearances and tick some at idle, if you notice a more pronounced ticking noise you should probably consider having this adjustment performed.

The normal service interval for the adjustment has changed wildly over the years. In the late 80’s, Honda recommended it every 15,000 miles. In the late 90’s Honda changed that interval to every 30k miles. Now, with the S2000, this has been changed to 90,000 miles.

The adjustment usually takes 1-2 hours on a typical Honda. Since the adjustment needs to be done with the engine cool, there is usually at least a 3-4 hour cooldown period that must preface the actual adjustment. Most dealers and service stations will only charge for the work done (and not the wait time) or simply charge a flat fee. Regardless, this is typically a job that costs in excess of $90 or $100.

Depending on the motor and the methodology, you may set this adjustment by measuring the clearance between the valve and rocker or the rocker and the cam. Since the two are related geometrically by the length of the rocker (and where it pivots, it is crucial that you know the clearance for your engine and where exactly to measure it. It is also crucial that you know what temperature the spec is at (100F for both the Prelude and S2000). The valves must be adjusted at near the same temperature, since as little as 30 or 40 degrees F will change the clearances.

This document is written with both the F20C (S2000 engine) and the H22A (Prelude engine) in mind. While the process is very similar, there are a few differences. These differences will be noted where applicable.

What You Need
- 10mm socket and driver
- 10mm wrench
- Honda valve adjustment tool (optional)
- Feeler gauge (good to .002"), angled is helpful!
- Flathead screwdriver
- 5mm hex (allen) key (S2000 only)
- Small needlenose pliers (Prelude only)
- Torque wrench
- Jack (factory one will work)
- 19mm and plug socket (optional, see below)

Getting Started
The basic procedure will require that the engine be turned so that the #1 cylinder is Top Dead Center (TDC, piston all the way at the top of it's stroke), then #2, #3, and #4 (the firing order), adjusting the valves for each when TDC. There are two ways to do this. You can jack up one drive wheel and put the car in high gear, then turn the wheel to turn the motor. This method requires little effort though is a pain on a RWD car like the S2000 (since you have to come back to the front of the car to check cam position). You can alternately remove all 4 plugs and use a 19mm socket and breaker bar to turn over the motor from the crank pulley. This can be difficult on a FWD car (like the Prelude) with limited access to the crank pulley, or on the S2000 where the factory airbox can get into the way. Either way, you simply need to be able to turn over the engine to get the pistons to TDC.

If you have decided to jack the car up, put the car in high gear and do so (on level ground) so that one drive wheel is off the ground. By simply turning the drive wheel in the direction of forward movement, you will be able to turn the motor over easily. If you have decided to turn the engine over by hand, remove the plugs with the plug socket and place the 19mm socket and ratchet on the engine (see below).

Removing the Valve Cover
The first thing you need to do is pop the hood and remove the valve cover. On the S2000, this means you use the 5mm allen key to undo the 4 bolts securing it in place. Once you do this, you need to disconnect the 4 spark plug coil connectors and the VTEC solenoid connector (on the side, towards the front), then swing their wiring harness out of the way. You will need to remove the 4 10mm bolts that hold the coils in place, then remove and set them aside.

On the Prelude, the process is a little more complicated. Using the 10mm wrench, remove the negative battery cable. This is necessary since we need to disconnect the alternator cable, which if you short it out, will blow the main 100A fuse which is available only at the dealer. Once you disconnect the battery, pull back the gray boot on the harness on the alternator. This will expose a 10mm nut, use the wrench to remove it. You will then need to use the needlenose to compress the barbs on the clip that holds the wire in place. Lift up, then undo the 10mm nut that holds the plastic from that goes across the valve cover. Once you remove that, the wire should be free. In this area you will also see a power steering hose that needs to be pulled out of a clamp, and a 10mm nut that holds a ground wire in place (undo it). Remove the 4 spark plug wires.

There are 8 10mm nuts on the Prelude and 5 on the S2000 that need to be removed before you can remove the valve cover. Remove them (be careful not to lose or drop them), as well as the large washers that go between the nuts and the valve cover. Once you remove them, lift up on the valve cover and set it aside. Depending on the last time yours was removed, it may take some work to break the seal on the gasket. It is sometimes easier to remove the oil fill cap and pull up from the hole. Be careful on the S2000 as there are several sensors on the head that make removing the valve cover a little tricky.

Adjusting the Valves
The first thing that you need to do is bring the #1 cylinder to TDC. On the S2000, TDC is notated by two marks on the timing chain sprocket (the center one) that are flush with the lip of the head at TDC. There are also marks on the cam gears which point towards each other at the 3 and 9 o'clock positions. On the Prelude, there are arrows with "TDC" on them pointing straight up at TDC. Turn the wheel or crankshaft until you bring the cam gears around to TDC. At this point, you should notice that where the rocker arm contacts the cam, the lobes should be pointing just to the side of the rockers.

Unless it was removed (or the hood replaced), there should be a sticker under the hood that has the proper valve clearance on it. The intake side is the side nearest the intake tract, and the exhaust the side where the exhaust manifold/header is. For the Prelude, the intake faces back and the exhaust front, for the S2000 the intake is on the driver's side and the exhaust on the passenger's. As of October of '02, the clearances for the S2000 and Prelude are:

S2000 (F20C)
.008" to .010"
.010" to .011"
Prelude (H22A)
.006" to .007"
.007" to .008"

Note that a range is specified. For inspection, it is fine for the valve clearance to be within this range. For setting the valves, it is fine as well, though setting them on the tight edge of the spec will reduce valvetrain noise slightly (see Advanced: Reducing Clearance? below).

With the cams in position, you should be ready to adjust the valves on the #1 cylinder. Start with the Intake first. Use the 10mm socket and a driver to loosen the locknut on the rocker. It is easier to adjust (finely) the set screw if you leave the nut slightly snug (i.e., you don't need to loosen the locknut much). Once you loosen the locknut, you are ready to insert the geeler gauge. On the S2000, you will need to insert it just under the adjuster screw, between it and the valve stem (hidden inside the retainer on top of the valve spring). It's hard to miss, simply aim under the adjuster and you should find it. For the Prelude, you need to insert the feeler between the rocker pad and cam lobe. You should see 3 rocker pads, the outer two being over the valve springs- those are the two you want. See if it fits with some resistance, if it won't fit or is too loose (no resistance), you will need to adjust that valve.

A good method to use is the "go/no go" method. Select the "proper" feeler gauge as well as the next- larger one. The intention is to adjust the set screw so that the "proper" gauge slides with some resistance but that you cannot insert the next larger size at all. With experience, you should become adept at determining the proper resistance without needing the next larger feeler.

When you are done adjusting that valve, hand-tighten the locknut. You will find that depending on how snug the locknut was while adjusting the valve (or how loose it was) that the adjustment changes. You may find it easier to hold the adjustment screw stationary while tightening the locknut. Once you have the locknut hand-tight, tighten it to 14 ft-lbs with the torque wrench. It is a good idea to check the adjustment after torqueing to insure it did not change. If so, loosen the locknut and readjust. Repeat this procedure for the next valve on that cylinder.

Once you are done with that side, you are ready to move to the other (exhaust). Adjust the exhaust valves as you did the first, paying attention to the different clearance (intake/exhaust are almost always different specs).

Once you are done with that cylinder, you will need to rotate the cam gears another 90 degrees (1/4 turn) to get to cylinder #3. You can do this by rotating the drive wheel in the "forward" direction (normal forward rotation), or by turning the crankshaft clockwise for the S2000 or counterclockwise for the Prelude (the Prelude's H22A engine rotation is backwards from the S2000's F20C). Once you have the lobes pointed away from the rockers (similar to how they were on the #1 cylinder), you can adjust #3. Simply repeat the above process to adjust intake/exhaust valves on #3. This entire process is repeated in the firing order, which is cylinder #1, #3, #4, and then #2. 1-3-4-2. If you get lost, simply go back to TDC on #1 (easy to find) and re-time yourself to the next cylinder. The cam gears should rotate 90 degrees with TDC on each cylinder position.

After you do all the valves you should be ready to put the valve cover back together. Installation is the reverse the order of removal. The only note is that if your valve cover gasket was leaking, it should be replaced. In most cases it will not be, and it is perfectly fine to reuse the old one. Be sure there is nothing stuck to it and that the mating surface is clean, then reinstall. One note on reinstallation, the bolts that secure the valve and plug covers are supposed to be torqued to 8.7 lbt-ft. If you do not have a torque wrench that goes that low, this should only be hand-tight with a normal 10mm wrench. Imagine hanging 8.7 lbs off a foot long wrench, or 16 off a 6" one... it's not much (be careful!!!).

Advanced: Reducing Clearance?
One question many people have, stemming from racing, is the effect of reducing clearance. Since the size of the "lump" on the cam determines how much lift and duration there is on the valve, you can directly impact the lift and duration of the valve opening by changing the valve clearance. With a 1:1 rocker arm ratio, you can increase lift by .002" of an inch if you decrease the clearance by .002" of an inch. This is NOT recommended for the novice, as many motors do NOT have 1:1 rocker ratios, and you can easily lose power or damage the valvetrain by adjusting the valves too tight.

That being said, it is generally safe to adjust most Hondas to .002" tighter than the tight side of the factory spec. Many racers do this to get maximum lift and duration out of a stock cam. It should be noted that on OBD2 vehicles, going tighter than this can alter the dynamics of the engine such that the ECU detects the change and throws a Check Engine light. Again, this is not recommended for the novice!

This page last updated 10/25/02.
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