Motorcycle Valve Adjustment Simplified

In both concept and practice, the motorcycle valve mechanism of any four-stroke engine is pretty straightforward. Essentially, valves act as the doors to the combustion chamber, allowing the fresh mix to flow in during the intake stroke, sealing the chamber during the compression and power strokes, and providing an exit for the spent gas during the exhaust stroke.

Despite the harsh environment in which they live, valves require relatively little routine maintenance. The average valve requires nothing more than a periodic clearance, or lash, inspection and the occasional adjustment to stay happy—tasks well within the grasp of those even with little mechanical skill.

Anytime a valve is closed there needs to be a gas-tight seal between the valve seat located in the cylinder head and the valve face. To ensure the valve is fully seated, there must be some slight valve clearance, or lash, between the valve stem and rocker arm or cam—think of it as free play, similar to that which exists in a clutch or throttle cable. Lash may be adjusted manually via a screw, shim, or automatically by a hydraulic device.

As the engine warms to operating temperature the various metals inside the engine grow slightly due to thermal expansion. As the valve train components are brought up to running temperature they expand along with all of the other bits and pieces. If there is insufficient lash between the valve stem and its actuating mechanisms when the engine is cold, there will be even less clearance when the engine is hot.

If the valve clearance is reduced beyond a reasonable amount, several unpleasant things occur. First, the valve timing is changed slightly, and this may affect performance and emission outputs. But there is an added complication: The exhaust valve stays alive because it's able to transfer a fair portion of the heat it absorbs from the combustion process directly into the cylinder head when it's resting on its seat. As clearance is reduced, the valves spend less time on their seats. This means the exhaust valves have less time to give up its heat, leading to anything from detonation, pre-ignition, and engine overheating, to a warped or burnt valve.

Finally, if the clearance is removed altogether, the valves may be held some slight distance from the valve seat, thus reducing, or even eliminating the seat-to-valve-face contact area, allowing combustion gases to leak past. Hopefully, at that point, the engine won't start and the valves will receive some long-overdue attention. If the engine still has enough compression to start up and run, the likely scenario is an overheated exhaust valve, followed shortly by a torched exhaust valve and then a long tow home. Valves can lose clearance for any number of reasons. The most common is the seat recession.

Seat recession occurs as the valve pounds itself into the valve seat, reducing operating clearance. At one time lead was added to fuel to prevent detonation. Nowadays the valve seat recession is most commonly a factor only on new bikes or very high-mileage bikes.

Loose valves create less of a problem, though if they become too loose, mechanical damage between the valve stem and its adjuster may occur. Loose valves tend to make a lot of noise, they also hammer themselves and the other valve train components, accelerating wear. In an extreme case, a loose valve in a pushrod engine may let the pushrod fall away from the rocker arm and destroy itself, as well as some of the other expensive bits.

Generally, loose valves occur due to normal physical wear between the components. If a valve develops a sudden affinity for large clearances, start looking for wear at the camshaft, rocker arm tip, or perhaps a collapsed or damaged pushrod.

Out-of-adjustment valves can make themselves known in a variety of ways, the most common being a distinct clatter from the valve area indicating loose valves, while tight valves may cause hard starting or a poor idle.

We recommend that you have these tools and some mechanical knowledge before you start digging into your bike:

Before undertaking any work of this sort it's always better to have all your ducks in a row and a thorough understanding of what you're about to do. Take the time to read through the appropriate texts. And gather up everything you might need for the task before you're up to your elbows in it.

There are three basic methods of adjusting valve clearance. The simplest and most common is the screw adjuster. It's been around for a long time and works just fine. Method number two involves shimming the valves. The shims are generally secured by a bucket-shaped tappet or lifter placed directly between the cam and the valve stem. The third type of adjuster is the hydraulic lifter, which normally requires no routine inspection or adjustment.

Begin by letting the bike cool until the engine is cold to the touch. In most cases, the fuel tank, and bodywork will have to be removed to gain access to the valve cover. Once the valve cover is exposed, wipe away any debris or dirt around it and use compressed air to blow any offending matter away from the spark plug wells—in a pinch you can use a soda straw and your lung power.

Remove the valve cover according to your shop manual's instructions. Valve covers differ greatly in their construction. If the valve cover won't come loose after all the fasteners have been removed, presumably, the gasket has glued itself to the head and cover. If you are sure all the fasteners have been removed, place a block of wood, angled slightly upward, on one corner of the valve cover, and give the block a sharp blow with a hammer. That should dislodge the cover.

Resist the temptation to pry the cover off. Chances are all you'll do is break something expensive. Don't try to hammer a screwdriver or chisel between the valve cover and the head. You will ruin the gasket surface in a very short order.

Before the valves can be set you'll need to find the top dead center (TDC). Manufacturers generally imprint either the flywheel, the alternator rotor, or the ignition rotor with a datum mark indicating top dead center. Depending on the engine design, there may be one mark or a separate mark for each cylinder. Your manual will fill you in on the exact procedure.

To make turning the engine over easier, most bikes have some kind of bolt threaded into the crankshaft which may be used to rotate the engine. Sometimes the bolt is used to hold the alternator rotor. Occasionally it's installed specifically to use when turning the engine over.

Every so often you'll run into an engine that has no convenient way of turning it over. The easy solution then is to position the bike so that the rear wheel is clear of the ground. Remove the spark plugs, pop it into gear, and turn the rear wheel by hand to position the engine.

Valve clearances are checked and adjusted when the crankshaft/piston of the cylinder you are adjusting are at top dead center on the compression stroke. Start by rotating the engine in its normal direction of travel. Watch the intake valve as you rotate the engine. The valve will open on the intake stroke and close on the compression stroke. When the valve begins to close, start to look for the TDC indicator. When it lines up with its reference mark, check both rocker arms for free play.

If the intake and exhaust rocker arms have some lash and the TDC marks line up, the engine is on the TDC of the compression stroke and you can check your valve clearance. If one of the rocker arms is depressed you're on the wrong stroke (even if the marks line up) and need to rotate the engine around again until both rockers are free. Your shop manual will list the appropriate valve clearances. If you don't have one, the clearances may also be found on the tune-up/emission information sticker located somewhere under the seat, on the frame, or in the owner's manual.

To measure the valve clearance select the appropriate feeler gauge, and place the blade of the gauge through the gap between the adjuster and the valve stem. The blade should have some drag on it, but still, be free to pull through. Using a feeler gauge takes some practice and is always somewhat subjective. Until you get used to doing it you can cheat a little. Say, if the manual calls for 0.004 (four-thousandths of an inch) clearance, slide your 0.004 gauge through the gap. If it feels a little loose try a 0.005 blade. If it won't go, you know the adjustment is as good as it's going to get. If the 0.005 slides through the valve, it's still a little loose. Readjust it.

Because the exhaust valve runs so much hotter than the intake valve, it's normally given more clearance. However, there are some oddball exceptions to the rule. So if something doesn't read right, cross-reference your info.

If the valve needs adjustment, loosen the lock nut and, using the appropriate tool, turn the adjuster to decrease or increase the lash. You'll find that leaving the adjuster nut a little snug will keep some tension on the adjusting screw, making it easier to accurately set the clearance. When the clearance feels good, hold the adjusting screw, and tighten up the lock nut.

Recheck your lash with the gauge. If you find the clearance has changed, don't panic. Tightening the lock nut often changes the clearance slightly. When the valves of the first cylinder are finished move on to the next and repeat the procedure until all of the valves have been adjusted. Novice mechanics may feel a bit intimidated by all of this. But with a little patience, the correct tools, and the right attitude, anyone clever enough to ride a motorcycle is certainly clever enough to maintain it properly.

There are two basic styles of shim-adjusted valves. The more common places the shim atop a small inverted bucket. The bucket rests on top of the valve, over the valve stem and spring, and the cam acts directly against the shim. Generally, when a shim-over-bucket design is employed, the shims can be removed and replaced without disturbing the cams.

Less convenient is the shim-under-bucket style adjuster. This design places the shim underneath the bucket, between the valve stem and the underside of the bucket. This system complicates adjustment, as the cams must be removed to change shims. Since the shims are only available in set increments, valve clearance tolerances are usually a little more generous.

The big disadvantage here is that not many owners are willing to go out and buy a box of valve adjustment shims. What you'll end up doing is measuring all of the clearances. If any are off, you'll need to determine what size replacement shim you need and head down to your local dealer to buy one. That is unless you are swapping one out that happens to be the size you need, or you have one from a previous adjustment.

Keeping a record of which shims are in the bike can simplify this. If the correct size isn't in stock, you're screwed. You can reassemble and ride the bike the way it is, which means doing a large portion of the job twice, or leave it apart until the shim arrives, which makes a great argument for screw-type adjusters.

For more articles on how to maintain and modify your motorcycle, visit our tech section.