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I worked as a light- and medium-duty truck fleet mechanic in my early career.  In this environment, vehicles must run well and safely.  I performed any and all mechanical service work and preventive care on 22 vehicles and pieces of construction equipment.  In the process, I found quick ways to "test" engines, chassis, brakes, steering, transmissions, clutches, axles and electrical systems.  Some of those fundamental diagnostic skills serve me to this day.


One area of testing was for internal engine wear, and at other forum topics, I discuss the use of compression, vacuum and leak down gauges.  None of these tests, however, pinpoint wear on the engine's timing chain, sprockets or timing gear mechanism.  The vacuum gauge comes closest, as it can show retarded valve timing or ignition retard effects, indicated by low vacuum at an idle with normal engine compression readings.  Retarded valve timing indicates a worn timing chain, sprockets or timing gears.


In the conventional ignition distributor (breaker point or HEI) era, there was a hint of timing chain wear when the ignition spark timing suddenly retarded (without a loose distributor housing).  At one time, OEMs tried nylon coated camshaft sprocket teeth for quieter engine operation, and it was not unusual for the engine to suddenly show retarded spark timing with mysterious "white plastic" bits appearing in the engine's drain oil.


Regardless of sprocket or gear design, I used my own quick test for engine timing chain wear without the need to remove the engine's timing cover.  With a breaker point or breaker-less ignition distributor with a cap and rotor, the test is quick and straightforward:


1)  Disable the ignition and starter, detach the battery negative cable if necessary...This timing chain or gear test will be easier if the spark plugs are removed, but this is not mandatory.


2)  Rotate the crankshaft pulley in the direction of engine cranking, using a socket and ratchet wrench at the crankshaft bolt; make sure the ignition is disabled or battery negative cable disconnected. Bring the pulley to the TDC mark.


3)  Remove the distributor cap.  Note the position of the ignition rotor to the distributor housing.  Mark the distributor housing edge with a crayon or marker if helpful.


4)  Rotate the crankshaft in the direction opposite cranking, very slowly; you will be watching for the slightest movement of the distributor rotor.


5)  As soon as the rotor budges, stop rotating the crankshaft.  Note the number of degrees that the crankshaft has rotated.  The distributor shaft and rotor will have rotated 1/2 that number of degrees, since this is a 4-stroke engine: There are 2 rotations of the crankshaft for each rotation of the camshaft. 


6)  This amount of movement should be slight for a timing chain or gears in good condition.  Since the distributor shaft runs off the camshaft, this movement reflects the play at the timing chain or gear set.


7)  Repeat this test if necessary, always bringing the crankshaft pulley slowly to the TDC mark in the cranking direction—without passing the mark.  If you pass the pulley mark, rotate the crankshaft back 45-degrees or so and carefully bring it to the TDC mark again.


This test indicates the play between the camshaft gear or sprocket and the crankshaft gear or sprocket.  It works on most OHV engines and also L-head engines.  From the earliest Willys engines with sprockets and a chain, through the gear set valve timing mechanisms, to the modern chain and sprocket sets, the principle remains the same.


If you're trying to translate rotor movement to wear, my experience taught that approximately 5/8" of crankshaft pulley movement (8" or so pulley diameter), at the O.D. of the pulley, was considerable for OHV or L-head engines with a timing chain and sprockets.  If the engine has timing gears, like the older Jeep L-head or F-head 134 four, movement should be less than this amount.  You're talking about play between gear teeth.


This is a rough test but useful when your ignition base timing has been retarding over time on a conventional distributor—and the distributor housing is not loose.  On all engines, including modern EFI engines where the PCM/ECU controls spark timing, the distributor's rotor movement determines the amount of timing chain and sprocket or timing gear wear.


The fleet engines I serviced and rebuilt at that time included Chevrolet, GMC, Dodge (slant six), Ford and I-H inline six-cylinder OHV types.  These pushrod engines used either a timing chain with sprockets or timing gears, all with a breaker point ignition.  Engines like the AMC/Jeep 232/258 and 4.0L are of similar design, beginning with breaker point, then electronic and finally PCM driven ignition systems.



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