Author Topic: How to Evaluate Timing Chain Condition  (Read 1112 times)

Online bd

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How to Evaluate Timing Chain Condition
« on: October 29, 2017, 12:32:26 AM »
by bd


The purpose of the engine timing chain (Figs. 1 and 2) is to maintain a fixed relationship between the timed rotations of the crankshaft and camshaft to precisely control the opening and closing of the valves with respect to the position of the pistons as each piston reciprocates in its cylinder.  This fixed relationship, in conjunction with the camshaft profile, prevents the valves from colliding with the pistons and allows for consistency of valve operation and engine breathing between cylinders.  Bear in mind that the camshaft rotates at one-half crankshaft RPM.  Thus, one camshaft degree represents two crankshaft degrees ... for each 360 rotation of the camshaft (one revolution), the crankshaft rotates 720 (two revolutions).  The ignition distributor is gear driven by the camshaft at a 1:1 ratio, so the distributor rotates at the same RPM as the camshaft.

Figure 1 - Three styles of timing chain used in GM V8 engines, a) an OE(A) Morse or silent chain, b) an OE and aftermarket, moderate-performance single roller chain, and c) an aftermarket, high-performance double roller chain.

Figure 2 - Schematic illustration of the fixed relationship between the crankshaft sprocket, camshaft sprocket and timing chain.  The crankshaft sprocket is exactly one-half of the diameter of the camshaft sprocket for a 2:1 drive reduction ratio, meaning that for every two revolutions of the crankshaft, the camshaft revolves once.  A timing chain with zero wear exhibits virtually zero slack between the crank and cam sprockets, maintaining precise timing control between the crankshaft and camshaft.

There is a persistent and longstanding misconception that engine timing chains gradually "stretch" with use.  Timing chains do not stretch - they wear out!  Wear gradually occurs between the timing chain and its sprockets, and between the pins and pin bores of the chain hinges, which increases the effective pitch(B) of the chain (Fig. 3).  While sprocket pitch remains constant, if chain pitch increases sufficiently, a link of the chain can climb high enough up a cog of a sprocket to crest a tooth and jump time, break and/or be ejected from the sprockets entirely.  Jumping time can bend or break valves, causing severe engine damage, especially in high-performance engines with decreased clearances between valves and pistons.

Figure 3 - Schematic cutaway of a timing chain, illustrating the elongation that results from gradual, cumulative, asymmetric wear between the pins and pin bores of a chain with normal use.  Cumulative wear effectively increases chain pitch or the distance between adjacent links.

As previously expressed (and illustrated in Figure 3), chain elongation results from wear that mimics an increase in pitch.  Since the crankshaft and camshaft have a fixed axial separation controlled by their relative positions within the engine block, chain elongation, manifesting as slack in the chain, retards camshaft and ignition timing with respect to the crankshaft (Fig. 4).  Retarding the camshaft effectively shifts the engine's power curve toward higher RPM while decreasing low RPM torque.  Retarding the valve train also brings the exhaust valves into closer proximity with the pistons while shifting the intake valves further away.  In addition, timing chain elongation introduces erratic camshaft acceleration.  Due to the slack in the chain the camshaft pulses back-and-forth as it rotates to the extent and limit of wear.  Inconsistent drivability issues may result, as camshaft and ignition timing become unstable and rapidly vary up to the amount of the timing chain slack as measured in crankshaft degrees.

Figure 4 - Schematic diagram illustrating the effect of chain wear on camshaft timing.  a) A new timing chain with zero wear maintains an exact relationship between the crankshaft and camshaft and exercises precise control over camshaft timing.  b) The worn timing chain is elongated and flops, allowing camshaft timing to retard and camshaft acceleration to pulsate, degrading camshaft and ignition timing stability.


To evaluate timing chain and sprocket condition (net cumulative wear) in gasoline engines used to power the full production line of light-duty GM trucks during the 1973 - 1987 (91) model years, prepare by cleaning the timing marks on the harmonic balancer and the ignition timing plate, so they can be read and compared accurately.  Use a white or yellow crayon to mark the TDC(C) and 0 timing marks.

Remove the distributor cap and bump the engine over using the starter, or by hand, until the distributor rotor "approaches" the normal position of the #1 cylinder spark plug wire.  If you overshoot TDCC(D) for cylinder #1, rotate the engine two more revolutions until the rotor is just short of the #1 cylinder spark plug wire.  Slowly, continue rotating the engine by hand in a clockwise rotation, until the TDC and 0 timing marks are aligned.  While observing the distributor rotor, slowly and smoothly rotate the engine by hand in a counterclockwise rotation until the rotor just barely begins to move.  Either read the degrees of separation between the TDC and 0 timing marks directly from the timing plate OR measure the separation between the TDC and 0 timing marks using a dial caliper or precision ruler.  The measured separation directly reflects accumulated timing chain wear and elongation, and relates to crankshaft degrees of camshaft and ignition retardation, as follows:

For an 8" diameter harmonic balancer, every 0.100" of separation measured between the TDC and 0 timing marks represents 1.432 of crankshaft free rotation (chain slack), or 1.432 of crankshaft decoupling from the camshaft.  Since camshaft, valve and ignition timing specifications are always provided in crankshaft degrees, camshaft and ignition retardation are also 1.432.  If the engine uses other than an 8" harmonic balancer, you can derive crankshaft degrees from the separation measured between the TDC and 0 timing marks, using the following formula:

(360 / (π x Harmonic Balancer Diameter in Inches)) x Measured Separation in Inches = Crankshaft Degrees of Free Rotation


360 S / 3.1416 d = Crankshaftdeg

where 360 is the number of degrees in a circle, S is the separation between the TDC and 0 timing marks measured in inches, π is pi (3.1416), d is the diameter of the harmonic balancer measured in inches, and Crankshaftdeg is the number of crankshaft degrees of free rotation (chain slack and camshaft/ignition retardation).


Dependent on chain length and sprocket diameter, the limit of tolerable timing chain elongation for automotive gasoline engines falls within the range of 1% - 1.5%.  There is no magic mileage after which the timing chain should be replaced.  Timing chains can provide useful service lives anywhere between 100,000 and about 350,000 miles.  Factors controlling useful service life include the vehicle's operating environment, the frequency and consistency of engine lube maintenance, the quality and suitability of lubricants, oil filters and parts that are used for lube maintenance, and the demand typically placed on the engine by its operator.  A workable routine is to evaluate timing chain wear after an engine has accumulated 100,000 miles of service and perhaps every 50,000 miles, thereafter.  For the engines discussed herein, a timing chain exhibiting >6 crankshaft degrees of slack (≥0.420" of separation measured between the TDC and 0 timing marks for an 8" balancer) is considered worn out and should be replaced.  High-performance street engines adhere to tighter constraints limited to ~3 crankshaft degrees (~0.210" of separation measured between the TDC and 0 timing marks for an 8" balancer).  In any event, if measured separation is ≥10 crankshaft degrees (≥0.700" of separation measured between the TDC and 0 timing marks for an 8" balancer), the timing chain has far surpassed its useful service life and should be replaced without delay!


When replacing a timing chain, always replace the chain and sprockets as a matched set.  The procedure is outlined in the appropriate year factory service manual available for download from the General Information and Maintenance section of our Technical Forum.  Before removing the old chain and sprockets, bar the engine over by hand until the timing marks on the sprockets are aligned (Fig. 5).  Pre-aligning the timing marks makes the repair process much easier and more straightforward.  Note that the timing marks are aligned when the engine is on TDCC of cylinder #6.

Figure 5 - Schematic illustration of the factory timing marks aligned for proper camshaft timing with regard to the crankshaft.  When the timing marks are aligned as shown, the engine is on TDCC of cylinder #6 and TDCE(E) of cylinder #1.

A  OE - original equipment; factory installed
B  pitch - the spacing or linear separation between adjacent pins of a "new" chain
C  TDC - top dead center
D  TDCC - top dead center compression stroke
E  TDCE - top dead center exhaust stroke
« Last Edit: February 24, 2018, 04:14:25 PM by bd »
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