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All In The Timing


5/21/2013
By Gary Goms

Because their remaining service life is difficult to estimate, timing belts must be replaced at specific mileage intervals to prevent failure.
 
The current project of a fellow I know who restores antique tractors is a 1938 John Deere equipped with a huge two-cylinder engine. The engine develops full torque at about 2,000 rpm, which is slightly above cruising speed for a modern engine. The engine itself is a statement of simplicity since the intake and exhaust valves are mechanically opened by eccentrics or lobes cast into the engine’s camshaft.

The Otto Cycle
Like a modern engine, the John Deere is an Otto Cycle engine. In the Otto Cycle, the intake cam lobe forces the intake valve to follow the piston downward as it descends to draw air and fuel into the cylinder on the intake stroke. As the piston begins to ascend on the compression stroke, the intake valve closes. Just before the piston reaches top dead center, the spark plug fires the air/fuel mixture, forcing the piston downward on the power stroke. Just before the piston reaches bottom dead center, the exhaust valve camshaft lobe forces the exhaust valve open, which releases the residual exhaust gas pressure into the exhaust system. As the piston ascends on the exhaust stroke, spent exhaust gases are pushed out of the cylinder in preparation for another intake stroke. A strong valve spring is required to force the valve, rocker arm, push rod, and tappet (also known as a lifter or cam follower) to follow the contour of the rotating camshaft lobe.

As you might suspect, the valve and piston events must be timed perfectly to develop maximum torque and to prevent the valves from colliding with the pistons. Fortunately, most “non-interference” engines have enough clearance between valve and piston to prevent piston-to-valve contact. On the other hand, many “interference” engines don’t have this clearance and, if the valve timing varies just a few degrees, the valves will collide with the piston.

Modern automotive engines are usually designed with overhead camshafts to eliminate the all but the camshaft follower and valve spring from the valve train. Aside from the few remaining push rod engines in production, all modern engines rely on a timing belt or timing chain to time the camshaft with the crankshaft. Metal timing chains are making a comeback because of their reduced maintenance needs.

Timing Belts
Because their remaining service life is difficult to estimate, timing belts must be replaced at specific mileage intervals to prevent failure. Early timing belts had to be replaced at 60,000-mile intervals while current production engines require replacements at 105,000 miles or as specified by the auto manufacturer. As preventive maintenance, most expert technicians prefer to replace all of the rotating parts in the timing cover, including tensioner and idler pulleys, water pump, balance shaft belts, and the camshaft and crankshaft oil seals.

Timing Chains

Imminent timing chain failure is usually heralded by a knocking noise from the front of the engine as the worn chain “slaps” the timing cover or by a rattling sound when the timing chain guides break and begin to rattle against the timing chain. When the chain breaks or jumps timing, the engine will stall. The worst-case scenario is bent valves on interference engines, which will require rebuilding the cylinder heads. As with rubber timing belts, it’s vitally important to not only replace all of the timing chains in the engine, along with the timing chain guides and tensioners.














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