chain stretch?

[mburman]

How do I measure timing chain stretch?

[jerryb]

Pull the valve covers, you should also adjust your valves.
Set it on T.D.C. then look on the back side of the cam towers, there is a mark on the tower and a grove in the washer next to it, if they all line up, your great, but they wont. You then turn the engine until the cam marks line up and read the dif. on the harmonic balancer, should be less than 5 degrees.

jerryb is very close on method, however, most original installations never line up exactly to begin with. To be more accurate in measurement, line up cam mark with tower mark while maintaining constant pressure on harmonic balancer bolt in direction of engine travel. Note the timing mark. Then put opposite direction pressure on the crankshaft until the cam begins to move and note the timing mark again. The difference is a rough guesstimate of the stretch.
(I should say stretch beyond tensioner ability to moderate, in some models). Of course, as Einstein said, everything is relative.