During most engine rebuilds, a completely new timing assembly should be installed. If wear exists on any component, replacement of the entire assembly is necessary. Wear in the chain, gears, or sprockets means a timing lag, which results in poor engine performance.
The timing chain or belt is generally installed with the gears in their correct positions. Before installing a new chain, soak it in oil.
OHC (Overhead Cam) Engines
Some OHC engines use a chain drive; others use a belt drive. Removing the cover on some OHC engines that have timing chains is more difficult, because the cover often fits between the oil pan and the cylinder head. There are special procedures for replacing cam timing components in these engines.
Before a chain repair job, perform a leakage test on non-freewheeling engines to check for bent valves, so that an accurate repair estimate can be made.
OHV (Overhead Valve) Pushrod Engines
On many OHV pushrod engines, the crankshaft sprocket is installed on the crankshaft nose and the crankshaft is rotated to position piston #1 at TDC. At this point, a mark stamped onto the crankshaft sprocket is pointing directly upward (toward the camshaft).
The camshaft sprocket is then temporarily bolted to the cam and used to rotate the cam until a mark stamped on the cam sprocket is pointing directly downward (toward the crankshaft). The sprocket is then removed from the cam (without allowing the cam to rotate).
The timing chain is looped over the cam gear, the mark on the cam gear is positioned directly downward, and the chain is looped around the crankshaft sprocket. When the cam sprocket is attached to the cam, the timing marks on the crank and cam sprockets should be pointing toward one another.