SOHC V6 timing chain comments
1. Ford did an injustice to its customers by producing an engine with a camshaft timing chain in the rear that requires engine removal for replacement of the camshaft chain, guide assembly or jackshaft rear sprocket. The magnitude of the injustice was increased by utilizing (and not improving) a plastic guide assembly with no metal reinforcement on the traction side that fractures from fatigue.
2. The SOHC spring/hydraulic camshaft timing chain tensioner is an inferior design compared to the OHV ratchet spring/hydraulic camshaft timing chain tensioner. The spring pressure is inadequate to prevent timing chain slip when oil pressure is low, when the engine backfires, or when the engine is manually rotated counter-clockwise. I recommend replacing the hydraulic tensioners every 75,000 miles or when the cassettes are replaced.
3. The primary (crankshaft to jackshaft) tensioner was an inferior design subject to failure at 50,000 miles. The upgraded primary tensioner is significantly improved and should provide a reasonable service life.
4. The improved camshaft timing chain guide assemblies are still subject to fatigue failure because the metal reinforcement is not bonded to the plastic.
5. Using high quality engine oil with replacement of oil and filter at recommended interval will reduce chain to guide assembly contact wear but will not delay fracture failure. The use of a pre-oiler/oil accumulator will reduce the chance of chain slippage by preventing low oil pressure when the engine is running (start up and hard cornering/braking).
6. The SOHC is an interference engine. When the timing chain slips enough the valves will collide with the pistons. The most frequent occurrence is the exhaust valves dinging the pistons requiring removal of the head and replacement of the valves.
7. The guide assembly fails much sooner than the associated chain or sprockets. It is acceptable to replace the guide assembly without replacing the associated chain or sprocket. After the engine runs for a few hours wear patterns result on the chain and sprockets. The chain and associated sprockets should be replaced as an assembly. Individual cassette components (guides, chains, sprockets) are no longer available from Ford.
8. Aftermarket guide assemblies are frequently inferior to those available from Ford. They may be warped or soon warp. They may wear much faster. They may fracture sooner. According to a Cloyes engineer I spoke with the guide assemblies that come with their timing chain kits are from the same manufacturer that makes them for Ford.
Here are links to threads containing more information relating to the SOHC V6 timing chain system:
Rear Timing Chain Tensioner Replacement
SOHC V6 Camshaft Timing
Starting my 00M12 Installation
Main timing chain - OHV vs SOHC
Timing chain rattle resolution process - SOHC V6
SOHC V6 Timing Chain Saga