I am getting a lot of mixed advice on when to change the timing chain. Some say 100K, others 120K all say it is hard to predict when the guide rails will break. What do you suggest?

MILEAGE ISNT THE ONLY FACTOR TO CONSIDER WHEN REPLACING THE TIMING CHAIN. FIRST OF ALL THE TIMING CHAIN TENSIONER IS THE KEY TO LONG LIFE OF THE CHAIN. SECOND IS FREQUENCY OF OIL CHANGES. BUT THE BEST WAY TO DETERMINE IF THE CHAIN NEEDS REPLACEMENT IS TO REMOVE THE VALVE COVERS & ALIGN TIMING MARKS ON CAMSHAFTS THEN LOOK AT CRANKSHAFT POSITION TO DETERMINE STRETCH IN DEGREES . NORMALLY 125K. MILES IS A STARTING POINT!

If you wait too long you will be looking at hefty repair bills! The left t-chain guide rail usually will break if the engine is ever over heated or service neglected. Have your shop pull both valve covers and check the t-chain stretch at the right cam. It should not exceed 8 degrees (roughly).