VALVE MECHANISM

The valve mechanism consists of many associated components. The valves, valve springs, rocker arms, and camshaft just to name a few. It is important to conduct a thorough inspection of each component. The failure of just one valve spring can have an impact on the engine's performance.

1. Valve Inspection and Repair

Begin the repair of the valve mechanism by repairing the valves. Clean the valves thoroughly with a wire brush or buffing wheel to remove all carbon and varnish. When this is done, inspect the valves for pitting, burnt surfaces, scoring, stem warpage, wear, and cocked condition. When you have determined that the valve is serviceable or have replaced it with a new one, check the valve face runout. This is necessary to determine whether the valve will form a pressure- and vacuum-tight seal with the valve seat. To perform this check, use the Prussian blue dye as explained here.

2. Valve Spring Inspection and Repair

To ensure that the valve seats properly, forming a pressure and vacuum tight seal, you must not only have a lapped fit between the valve and seat, but a valve spring of the proper pressure and squareness. To measure the pressure, use a spring tester has pictured below.

Checking valve spring pressure.

If the valve spring is at the specified height, check the spring strength on a pressure gage and compare it with manufacture's requirements in TM. If the valve spring has become weak, discard it and obtain a new one. To determine valve spring squareness, simply place a square alongside the valve spring in the vertical position and measure the point of greatest distance between the two. Valve springs are not repairable; therefore, just as the case with the weak spring, the spring must be replaced. Now, you are ready to install the valves in the cylinder head or in the cylinder block, as the case may be. Insert the valve stem into the valve guide from the combustion chamber side of the head and place the valve spring over the end of the valve stem. Now place the valve spring retainer on the valve spring and attach the valve spring compressor in the same position as when you were removing the valves. Compress the valve spring and insert the valve stem locks. Release the valve spring compressor and the valve is installed. With the valve installed, there is still one check which must be made, the "installed height" of the valve spring. This is done by placing a machinist's alongside the valve spring. Check your measurement against the tolerance listed in the TM. If the installed height does not meet specifications, either the valve or the valve seat insert must be replaced.

Checking valve spring installed height.

Note: If you are repairing an L-head engine, the procedure is the same as outlined on previous page except you will be working with the engine block instead of the cylinder head.

3. Valve Train Inspection and Repair

Unlike the L-head engine, the I-head and F-head engines are equipped with rocker arms which directly activate the valves. Of course, the F-head engine also has valves in the block as well as the head. The in-block valves are not activated by rocker arms. If rocker arms are used, there are a number of components required to operate them. These components make up the "valve train" and each must be inspected and repaired as necessary. Let's begin with the rocker arm shaft assembly, which consists of a hollow shaft with a series of

rocker arms, shaft supports, and springs. To inspect and repair the rocker arm shaft, first disassemble it. Mark the rocker arms to identify their position on the shaft. They should not be interchanged. Now, remove the cotter pin or

other retaining device from each end of the shaft and remove the rocker arms, rocker arm supports, and springs simultaneously. Check the adjusting screws in the rocker arms for damage or excessive wear. Remove the screws ONLY if they need to be replaced. If you will recall from earlier discussion, wear between the rocker arm and the rocker arm shaft will make it necessary to adjust the valves more often than normal. Therefore, before reassembling the shaft, check for this wear. If you can see wear on the shaft or in the bore of the rocker arm, then you know the defective part must be replaced. If not, measure the outer diameter of the shaft and the inner diameter of the rocker arm bore. These measurements are taken with T-gage (telescopic) and micrometer. Check your measurements against the maximum wear limits in the TM. If equipped with locating springs, ensure that none are broken and make sure that the ends of the oil tubes are not split. If everything is in good shape, reassemble the rocker arm shaft and move on down the valve to the valve train to the pushrods. Check the ends of the pushrods for nicks, scores, burrs, and apparent excessive wear by cleaning them thoroughly and giving them a good visual inspection. Check them for a bent condition also. In some cases, nicks, scores, and burrs can be corrected with an oil stone. A bent pushrod must

be replaced. Valve tappets cannot be repaired. Therefore, if the tappet is damaged or excessively worn, it must be replaced. Damage can be checked by visual inspection as can excessive wear on the bottom of the tappet.

4. Camshaft Inspection and Repair

Clean the camshaft in cleaning solvent and blow all oil passages clear with air pressure hose. Check the machined surfaces of the camshaft for nicks, scoring, burrs, and excessive wear. Eliminate all defects possible with crocus cloth or a smooth stone. If you cannot eliminate defects, the camshaft must be replaced. Next check the cam lobe lift. Place the camshaft in a set of "V" blocks and attach a dial indicator. Turn the camshaft until the dial indicator plunger rests on the lowest part of the lobe. Now, set the dial indicator at zero and turn the camshaft until the plunger rests on the highest point of the lift. Compare your reading to the specifications in the TM. If the lobe lift on all lobes does not

meet specifications, replace the camshaft with a new one. The camshaft is checked for runout by placing the dial indicator plunger on the center camshaft main bearing journal. This test is to ensure that the camshaft is in proper alignment to minimize bearing wear. The TM lists the allowable limits of runout. Defective camshafts must be replaced. Another check which must be made before installing the camshaft is the camshaft bearing running clearance. The inner diameter of the bearing bore is measured with the bearing installed. The procedure is the same as was used to remove the bearings. MAKE SURE THE OIL PASSAGE HOLE POSITION IS MARKED. Align the hole of the bearing with the mark and draw the bearing into position. After installing the bearings, measure the inner diameter of the camshaft bearings with a T-gage and micrometer. Now, measure the outer diameter of the camshaft main bearing journals on the camshaft. If they are within acceptable limits according to your TM, you are ready to reassemble the engine. Remember that the running clearance (the difference between the bore measurements and the journal measurement) must be within manufacturer's allowable limits. The outer diameter of the journals is measured with a micrometer as were the piston pin, crankshaft journals, etc.