Timing Chain

Modern timing chains can be expected to last the life of the engines, although some engines such as the M119 have reported failures and bent valves mostly due to plastic guide failure. This is uncommon, but if you have any abnormal engines noise you should have it checked.  Broken oil tubes on the M119 can also cause loud lifter noise. If your engine has more than 150,000 miles on it and you have done all the scheduled maintenance chances are the timing chain is fine.  If it's an older engine from the 1970's or 1980's or has a single roller chain it may be needed. The bottom line is the only way to know for sure is to check it, if it’s fine leave it alone. It would be rare to break without wearing out first. To check it, with the valve covers off rotate the engine at the crank bolt manually clockwise to TDC, either the cam timing marks line up or they don’t. Don’t rotate counterclockwise if you miss TDC, that’s when plastic rails snap. Regardless, you need to take up the chain slack to get a valid measurement so always only rotate in the direction of normal engine rotation.
 
If the timing marks don’t line up you have chain stretch. Installing an offset woodruff is one solution for small corrections, but for anything more than a several degrees you are inviting disaster. On an interference engine loss of the chain is loss of the engine in many cases, unless it occurs at low engine speeds. Even then valves will be bent on several cylinders.  If you replace the chain check the sprockets, and if blunted or rounded they should also be replaced considering a worn sprocket will quickly wear out a new chain. Consider chain and sprocket wear fitted parts, and if they either are worn replace them both. Realize that the labor to pull the front cover and replace all the sprockets is considerably more than feeding a new chain in, but it won’t last long if your sprockets are worn. 

If you remove the timing chain then you removed the chain tensioner. Don't forget to reset it as the ratcheting hydraulic type or you may end up with a chain that's far too tight. You'll hear the chain noise if you missed this step and risk snapping a camshaft or braking the chain if driven like that. You’ll need to dismantle the chain tensioner. To do this, remove the allen nut then the spring and spacer. Pull the pin out fully, because it only goes one way. Then insert the pin in through the back so it not extended. Install the tensioner with the 27mm nut. With the allen nut installed, the chain should be properly installed and timed. Then insert the spring and spacer, compress the spring, and install the allen nut. Replace the aluminum seals if they look at all compressed or damaged. Installing the allen nut while compressing the spring is most easily done with an air ratchet, if you use air tools to start the job make sure to finish with a torque wrench.