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    2000 Oldsmobile Truck Bravada AWD 4.3L SFI 6cyl

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    1720 S RIDGEWOOD A
    (386) 427-1210

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    Timing Chain

    Inspect/Service/Replace

    The two types of chains are the roller chain and the silent chain.

    Timing chain types.

    Double roller chains are usually used on overhead cam engines. Roller chains are lighter and are more suited to high rpm than silent chains which are often used with pushrod engines.

    New silent chains are of the large pin design. These "floppy" chains are twice as strong as older silent chains. They are less susceptible to misalignment and overload problems than roller chains, due to greater flexibility. The large pin silent chain eliminates chordal action; a problem that roller chains have. Chordal action means the chain operates on a constantly changing diameter, which causes varying camshaft speeds.

    As an engine ages, its timing chain stretches. timing chains have a drive, or tension side and a side where slack accumulates. long chains, such as those used on ohc engines, use chain tensioners to take up slack as the chain stretches.

    Parts of an OHC chain drive. Courtesy of Nissan Motors.

    In an engine that is equipped with a non-tensioner timing chain, excess chain slack can occur and allow the cam timing to change (skip teeth on the sprockets).

    Long chains also have chain guides which have rubber surfaces that wear. they are usually replaced when the chain is replaced.

    Here are four ways to check a timing chain for excessive stretch:

    1. On engines without chain tensioners that have distributors driven by the camshaft, watch the timing mark while checking ignition timing at idle. If the mark moves back and forth, the chain is loose.

    2. To check a pushrod engine for chain slack while the engine is in the car:

    • Remove the distributor cap and turn the vibration damper in one direction until the distributor rotor begins to turn.
    • Then, turn the crank in the opposite direction until the rotor moves again.
    • Observe how far the damper moves before the rotor turns.
      • Because most distributors are driven by the cam, this is a good way to check for chain slack on these engines.

    The engine may still run acceptably with 10° to 15° of chain slack. But ideally, movement should be less than 5° at the crank.

    3. To check for a stretched roller chain, pull on the top of the chain while it is on the same sprocket. This can be done during a valve adjustment, while the valve cover is off. A worn chain will have considerable slack when tested in this manner; a new chain will fit snugly on the sprocket.

    4. The most obvious way to check chain slack is to measure the slack during engine disassembly, after removing the timing cover.

    • Be sure to turn the crank in one direction first, in order to tighten up one side of the chain.
    • Then measure the amount of slack in the chain.
    NOTE
    Large pin silent chains are so flexible that they appear excessively sloppy when new. Typical movement on the slack side is around 0.200" in and out (0.400" total). Replacement is suggested when total movement is 1.00".

    Freewheeling and Non-freewheeling Engines

    As an engine ages, its timing chain stretches. In fact, this is really the main obstacle to long engine life.

    Some engines will experience piston-to-valve interference if the timing chain or belt skips or breaks. These engines are called non-freewheeling or interference engines. When this happens on a pushrod engine, pushrods will usually bend.

    This pushrod was bent when a valve collided with a piston. Courtesy of Federal-Mogul Corporation.

    Valve Timing

    Valve timing can become retarded (late) when a timing chain becomes so worn that it skips a tooth.

    NOTE
    Late valve timing will cause poor low rpm performance. At higher rpm, engine performance might be acceptable.

    If the timing chain has skipped and valve timing is retarded, suction will be felt at the exhaust pipe. This happens because the exhaust valve is still open during the piston's intake stroke.

    Valve Train Noises and Worn Chains

    Valve train noises can also come from inside the timing cover. They can be caused by a bad timing chain or a loose sprocket or gear. The noise usually is a rattle or knock that becomes louder when decelerating.

    For engines with a timing chain tensioner, a worn chain can become loose enough on the sprocket to rattle whenever the engine floats or cruises between load and coast conditions. In severe cases, a chain can actually wear a hole in the timing cover resulting in an oil leak.

    A worn timing chain rubbed on the timing cover, causing noise and an oil leak.

    Severely worn cam bearings can also be the cause of excessive timing chain slack. Depending on the design of the lubrication system, this problem can also be accompanied by low oil pressure at idle.

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