What are the symptoms of serious engine wear? At what stage do these indicate the need for an engine overhaul? Joseph
Your engine is wearing out—to an infinitesimally small degree—every time you use it, from day one. On a car of good quality, treated with diligence and respect, the wear will be progressive but so slight that the engine will perform adequately for tens of thousands—even hundreds of thousands—of kilometres before an overhaul is needed.
An older engine will gradually have slightly reduced power—especially at lower revs, where “torque” is strongest, and less so at top revs, where horsepower peaks. That marginal loss of power is normal.
Wear becomes serious if either the oil needs to be topped up quite often between services or the exhaust emits blue smoke (burning oil, as opposed to black smoke which is unburnt diesel) when the engine is under load, and/or the power loss is severe. The benchmark for “acceptable” oil consumption is up to half a litre between services.
If more than that is needed, or blue smoke is copious or constant, the power loss will be increasingly significant and it’s time for the workshop.
Engine overhauls vary considerably in their scope—usually involving new piston rings. Assuming your question is not entirely academic, in the early stages of excessive wear, the piston rings probably need to be replaced (because they are too worn, weak, or cracked).
The replacement parts are not expensive, but removing and dismantling the engine is a palaver, so while it is out and open it makes sense to renew or remedy several other things that might be nearing the end of their best service life – things like bearings, seals, parts of the valve gear, the timing belt, clearing carbon deposits and so on.
Damaged rings might have scratched the piston skirts and cylinder walls and require sleeves and re-engineered sizing.
The longer the engine has been used in substandard conditions, the greater the consequential damage will be. So, if your car consumes oil, emits blue smoke, loses significant power, or makes a different noise, the sooner you get the problem diagnosed and fixed, the cheaper it will be. Delay will only increase the rate of wear and add (by multiples!) to the remedial cost.