I have 1987 V6 GT with 140,000 miles that has a bad water pump. I was considering replacing the timing chain, given that the water pump has to come out. However, I did a quick 'slack test' on the timing chain, where you measure the rotation of the crank between the first movement of the distributor and when the distributor moves again when you reverse the direction of the crank. There's only about a 1/2" of slack, which I believe is acceptable. How reliable is this type of test? Should I replace the timing chain anyway?

My feeling is, that's very little slack ( depends on where you're measuring it at) - I respectfully question what you're doing here

The other part of me says, with that many miles on a known wear item, go ahead and replace the chain - for the experience, if nothing else.

I just completed a timing gear change in the car on friend's 1986 V6. This car had around 130-140K.

It just would not start one morning. After noting that there was very low compression, no immediate history of overheating, and no other know problems, I became suspect of the chain.

After pulling the cover off, I found that the chain was just hanging over the lower gear and making little to no contact.

I am sure there are a variety of opinions here, but I found an exact replacement chain at PEP BOYS sold in a seperate set (chain, crank gear, cam gear). I bought a cam gear and chain. Considering the use and age of the vehicle I was not too worried about going through the trouble of pulling and replacing the crank gear. Plus, when you are doing this in the car there is very little room to get a puller in place to remove the gear.

It ended up working just fine. The car runs great now. In fact, it is much stronger than it was in the months leading up to the failure. You may be giving away plenty of power with an old slack chain.

If you have it torn down then just replace it. Especially on a car with over 100K. Old timing chains can look fine one day and be shot the next. been there done that.... wasn't fun.