a complete engine overhaul involves replacing piston rings, reboring or honning cylinder walls, replacing pistons (if the cylinders are rebored), replacing main and connecting rod bearings, regriding the crankshaft to restore the journals (if needed), and restoring the valves. Rebuilding parts like the starter, distributor, carburetor and alternator while the engine is being overhauled is common to ensure that the entire runs like new but isn't always necessary.
When overhauling an engine, there are four basic choices for the kind of overhaul you want:
Choice 1: Factory Remanufactured Engine. This strictly speaking isn't an overhaul, as it results in a zero-timed engine. You send your run out engine "core" to the factory, either Continental or Lycoming, and you get back an engine that is built to new engine standards. Some parts are re-used such as crankshafts and cases, but the top end parts are generally replaced with new parts. Since the factory does the work, they can certify the engine as "zero time" and it counts as a brand new engine as far as the logbook times are concerned.
Choice 2: Factory Overhauled Engine. The factory overhauls the engine, but the engine is not zero timed. In theory a modern factory overhaul, where the factories have begun using more new parts, will result in exactly the same engine as a reman.
Choice 3: "Name Brand" Overhaul Shop. There are a number of "name brand" overhaul shops that have enough volume to justify national advertising for their services. These shops are specialists in engine overhauls and should be the best experts you can find outside of the factory
Choice 4: Field Overhaul. Any certified Powerplant mechanic with Inspection Authority can perform and sign off an engine overhaul. The mechanic may or may not be associated with a FBO, although most are. A field overhaul is often performed locally and so you have the most knowledge of the mechanic, and potentially the most control over the schedule and overhaul options ranging from a completely vanilla service-limits overhaul up to a completely tricked out custom built and finished engine. It is also potentially the most work for you as owner and is probably the least valued by the marketplace for resale value because buyers of your aircraft usually will not have the same knowledge of and trust in your mechanic that you do.A field overhauler will almost never have direct access to specialized overhaul tooling and equipment such as cylinder boring and honing machines or valve seat grinders.
In practice this is not an issue because with there are specialty parts overhauler shops that make a business ofdoing this kind of work and your mechanic will simply send the parts out.The advantages of a field overhaul are price and flexibility
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