VALVE TIMING

The relationship between the camshaft and the crankshaft determines the valve timing. Gears, drive chains, and reinforced neoprene belts are used to drive the camshafts that open and allow the valves to close in relation to the position of the pistons in the cylinders. The gears, drive sprockets, or cogs, as the case may be, of the camshaft and crankshaft are keyed in position so they cannot slip.

With directly driven timing gears (fig. 3-24), one gear usually has a mark on two adjacent teeth and the other, a mark on only one tooth. To time the valves properly, you need to mesh the gears so that the two marked teeth of the one gear straddle the single marked tooth of the other gear.

In chain-driven sprockets, you can obtain correct timing by having a certain number of chain teeth between the marks or by lining up the marks with a straightedge, as shown in figure 3-24.

Engines using a continuous neoprene belt have sprockets, or cogs, attached to the camshaft and crankshaft. The belt has square-shaped internal teeth that mesh with the teeth on the sprockets. All engines with this system use a timing belt tensioner. Timing marks on this system vary with each manufacturer. Before setting the valve timing on any engine that you are overhauling, always check the manufacturer's specifications and instructions.

CRANKSHAFT SERVICING

Most modern engines have main and connect- ing rod bearings of the precision-insert type, which can be replaced without removing the crankshaft. However, if oil passages are blocked, journals are tapered out of round, or the crankshaft is bent, simply replacing the bearings will not correct the trouble.

If the bearings appear to have worn uniformly, probable the only requirements are crankshaft journal checks and bearing replacement. If bearing wear appears uneven, then the safest procedure is to remove the crankshaft from the engine and check it.

BEARING CAPS REMOVAL

When removing bearing caps, if they are not already marked, be sure to mark them so they will be replaced on the same journal from which they were removed. If bearing caps stick, carefully work them loose by using a soft-faced hammer,

Figure 3-24. - Driving the camshaft.