Chains "stretch" because wear occurs in the individual link pivots. This is why clean oil at the proper level is important. An engine with poor oil maintenance will have stretched chains a lot quicker than those that don't. I'm over 315K mi on original chain and tensioners. When I was doing my lifter job months ago, they still seemed okay. That said, I've never had to pull the chains, but from the video, you have to pull the chains anyway to get to the guides. If you're doing that, it's prime time for replacing. There are guys here that have done loads of them, so they can chime in if I'm wrong.

 

Dogevity is spot on. Just be careful to double check that the timing chain kit has the correct "tone" ring for the passenger side camshaft. I found out the hard way when after replacing the heads and timing set the engine would crank and not start...even on starting fluid. Do a search for my posts, I put pictures on what I found when dealing with the timing chains and sensors. Like has been said before, MOPAR sensors for crank and cam shaft sensors are the "only" ones these 4.7L engines like...you MIGHT get lucky with another brand but these engines are picky. Make sure to set the crank pulley to TDC mark on the timing cover....before disassembly and don't move it until you have replaced the timing set.

If you have specific questions about it feel free to ask away..might be able to save you a headache Also don't forget to download the manual from the DIY section...will save you in the long run

Steve