When I was working as an auto mechanic, I had a work order that said replace timing chain on a Datsun 2000 roadster. When I opened the engine, I found the chain to be in perfect condition, but the guide rails were worn, one rail broken, and the sprockets badly worn. So I just replaced the bad parts.

The timing chain on these engines slapped on the timing rails until the tensioner got oil pressure. Neither the customer or the incompetent service manager knew that this rattle was a characteristic of this engine. The later L16 and L20 engines which were in the new cars being sold did not have the timing chain rattle.

So the customer was unhappy and the service manager had me install a new chain. Of course it still rattled even though it was running good as new.

When performing an overhaul, I would replace all belts, hoses, and thermostat. Normally, those were the only parts that I would routinely replace even if they were still serviceable.