from the interwebz
Timing chains should be replaced:
1) When stretch is beyond acceptable limits.
2) Whenever you're pulling them out anyhow (typically for a cam replacement.)
Timing chain stretch should be checked every 100Kmiles. Either:
Open - Using a straightedge, there should be not more than 1/4" of total deflection in either long span of the chain.
Closed - Remove the distributor cap, grab a Sharpie(r) and a 3/4" socket and large socket wrench. Turn the engine slighly in one direction by the crank nose (doesn't matter which way) to make sure all of the slack is out of the chain. Mark the harmonic damper at a convenient point on the timing scale (more than ten degrees from both ends,) and mark the distributor housing where one edge of the rotor terminal would intersect it.
- Turn the engine in the other direction until the rotor just begins to move. Stop. Note how far you've turned the crankshaft.
Five degrees or less - Ideal.
Ten to fifteen degrees - plan on changing the timing chain soon.
Fifteen degrees or more - change the timing set now.
