Estimating timing chain wear
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Over time the timing chain and gears will wear, retarding the cam and ignition timing in the process. This will hurt performance and economy. Estimating the amount of wear with a reasonable degree of accuracy is not hard to do.
Crankshaft degrees vs. camshaft degrees
When measuring the timing set wear, keep in mind that the amount of wear seen at the crankshaft is twice that seen at the distributor. In other words, if the wear measured at the damper is 20 degrees, this is equal to 10 degrees at the distributor. This is because the crank rotates two revolutions for each single revolution of the camshaft.
Measuring timing set wear
You can get an idea of how much the timing set is worn by first removing the spark plugs, then by hand rotate the engine backwards (CCW as viewed from in front of the car) until you feel the slack has been taken out of the chain. On engines having a distributor, watching the rotor with the distributor cap removed will help- the instant the rotor moves, the slack is gone. Mark the damper at any easy to see stationary point on the engine (the TDC line of the timing tab works well).
Then rotate the engine CW and stop as soon as you feel the added resistance (and see the rotor begin to move) when the slack in the timing chain has been taken up. Mark the damper a second time using the same reference point. The distance between the two lines is how much wear there is. You can do this several times to get a feel for it.
The wear in degrees can be estimated by comparing the distance between the lines to the timing scale, similar to how distance is measured using a map mileage legend. If the timing scale shows that 10 degrees = about 0.7" (which is what it will be if the damper is 8" in diameter), use that to calculate how many degrees of wear there is in the timing set.
If the damper diameter is something other than 8", you may do this:
- Measure the damper diameter, multiply diameter by 3.14 to get the circumference.
- Divide the result into 360 to get the distance for 1 degree.
- Divide the amount of wear measured earlier by the distance equal to 1 degree and the result is the total amount of wear in the timing set.
Using a timing tape to estimate wear
You can also easily make a temporary timing tape that will make the wear easier to measure once the tape is on the damper. Simply mark or write down where the slack is removed, rotate the engine CW until the slack is again removed and the difference is the amount of slop in the timing set in degrees.
The timing tape is also useful for setting the total timing and for setting up a performance timing curve for the ignition, without needing a dial back timing light.
Correcting the wear
As mentioned earlier, wear to the timing set retards the cam and ignition timing. The ignition timing can be corrected by adjusting the distributor using a timing light, but the cam timing error cannot be corrected without disassembly. At that point it would be much better to simply replace the timing set instead of correcting for the wear by using bushings or different keyways, etc.
Gear drives have become cheap to buy- in some cases less than a timing set with a chain. This entices some into using them, thinking they are superior to a chain. But what has been found is that a gear drive can transmit harmonics from the crank to the cam. In some cases this can lead to unexpected failures of valve train components. The noise a timing gear set makes may be desired by some and disliked by others. There are "quiet" gear drives, but this is done w/helical cut gears that can cause the cam to "walk". This in turn can wear the thrust surface of the block or cause the gear to move out towards the timing cover, necessitating a cam button to position the cam correctly.
Unless a belt drive is used, or (like in the case of some reverse drive and inline engines) a gear drive is standard equipment, the best choice for the majority of engines/applications is to use a true roller chain and gears. The chain doesn't transmit the harmonics like a gear drive, and has been found to give good service at a reasonable cost.
Adjusting the cam
Many timing sets have multiple keyways or other forms of adjustability that allow changes to the cam phasing (the relationship of the cam to the position of the piston in reference to TDC). In the majority of cases, installing the cam "straight up" (without any advance/retard added by the installer) is the correct way to install it. Many cam makers have already advanced (in most cases) the cam timing when the cam was made to suit the application. Unless the builder has access to an engine dynamometer or has advanced knowledge of the dynamics involved, advancing/retarding a cam is nothing but a shot in the dark as to whether it will result in any benefits at all. And there's the chance the engine output could be adversely affected by indiscriminately changing the phasing of the cam.