A valve adjustment is a simple job that will familiarize you with your boat's engine, and can be done in less than two hours once you have learned the proper procedure.

On engines built before the 1980s, the valve stem to rocker arm clearance should be checked and adjusted after every 50 hours of operation. On later-model engines, they should be checked and adjusted every 250 hours or so, in accordance with the manufacturer's recommendations.

Many people may believe that the valves don't need adjustment so long as they are quiet. But I like to adjust my valves every spring, along with my annual impeller change and other maintenance.

ENGINE ANATOMY

Here's how the system works. The camshaft, which is turned by the flywheel, has lifters that lift a push rod for each valve. That in turn pushes the rocker arm, which depresses something called a tappet onto the valve stem. This depresses the valve spring, opening the valve. As the valve moves away from the seat, it lets air into the cylinder or exhaust out of the cylinder.CAPT. ALAN R. HUGENOTA valve head rocker arm assembly.

Clearances are very important. The valve clearance is carefully selected so that the valve and rocker arm assembly have room to expand as the engine components heat up. However, too much clearance means the components will "slap," which could cause them to overheat.

If the valve clearance is too small, as the engine heats up and all the metal parts expand, a valve may stay slightly open all the time. This results in lost compression with burned valves and seats.

If the clearance is too large, then valve openings will be delayed, the valve will not open far enough and it will close too soon. This can result in an incomplete intake or exhaust, or both.

All of this can damage your engine and require major repairs or even a complete overhaul.

ACCESSING THE VALVES

This procedure applies to both diesel and gasoline engines.

The tools needed are a slotted-head screw driver, a set of open-end wrenches (or at least those that fit your engine), and a set of feeler gauges to slip between the valve riser and the rocker arm. All are available at your local hardware or automotive store.

Before beginning, check your engine manufacturer's service data for clearance specifications and the exact procedure. This usually contains a picture showing where to place the feeler gauge on your specific engine. Remember too that the clearance for the fuel intake and the exhaust valves can differ, so check and recheck the manufacturer's recommendations for each type.CAPT. ALAN R. HUGENOTPush rod, rocker arm and valve spring arrangement.

Begin by warming up the engine under load for 30 minutes. It's easiest to do this by running at 1,500 RPM in gear against the dock lines. After it reaches operating temperature, shut off the engine. Valve clearance should always be adjusted on a warm engine.

Next, remove the head cover. Be careful with the bolts, making sure they don't go into the bilge. They are usually machine-threaded and not easily replaced. Also be careful not to damage the head cover gasket.

Rotate the engine until the piston of the cylinder you are adjusting is at Top Dead Center on compression stroke, with both valves closed. Finding Top Dead Center on a four-cycle engine is easy, because the fuel inlet valve opens on a downward stroke of the piston. Both valves then close during the next upward stroke.

To find top dead center for any cylinder, rotate the engine until the fuel intake valve is almost all the way down (open); at this point the piston is at the bottom of the inlet stroke. Put a chalk mark on the flywheel. Now rotate the flywheel another half revolution. At this point the piston in that cylinder will be at Top Dead Center.

Check the clearance on the valves for this cylinder and decide if adjustment is required. It is not always necessary to adjust all the valves, so always check the clearance of each valve before making any adjustment. If the engine manufacturer says that the valve clearance is to be 0.008 inch, then you should be able slip a 0.008 feeler gauge through the tappet clearance. But you should not be able to slip the 0.009 gauge through.

MAKING THE ADJUSTMENT

If adjustment is required, loosen the lock nut of the valve to be adjusted and turn the adjustment screw until achieving the correct clearance, as measured with the feeler gauge.

American-built engines have two types of tappet adjusting screws. One is the self-locking. the other, which is more common, has a locking nut to hold the adjusting screw in place. This locking nut must be loosened and the adjusting screw or nut held in place while it is re-tightened after the adjustment is completed. On most American engines the adjusting screw or bolt penetrates a threaded hole in the rocker arm.CAPT. ALAN R. HUGENOT

Foreign-built engines have a number of different arrangements. The Swedish-built Volvo marine diesel does not have a screw head that penetrates the rocker arm on the side opposite the spring. Instead, it has a bolt head that resides on the lifter below the rocker arm, opposite the spring. This bolt threads up from the lifter, and has an adjusting lock nut on the top of the lifter. The lifter also has two flats to hold a wrench. (See photo)

After the valve clearance has been adjusted and re-checked, tighten the lock nut.

Since you are at Top Dead Center, you can now adjust the other valve on this cylinder as well because both valves are closed. Simply repeat the steps for the other valve. It doesn't matter whether you do the intake valve or the exhaust valve first.

Perform the above four steps for all cylinders, by rotating the engine until the next cylinder in the firing order comes to Top Dead Center. When you are finished, reinstall the rocker arm cover using the undamaged gasket (or a new gasket), using high-grade gasket cement. Tighten all the cover bolts as recommended by manufacturer.