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If the purpose of the rebuild is to bring the engine back (or as close as feasibly possible) to its original performance, what are some things that should always be replaced?

This obviously depends on the condition of the engine, so let's say a 300,000 km engine which has had regular oil changes its whole life.

Gaskets, seals, belts, bolts, etc. yes you should replace, but how about components such as valves, valve springs / hydraulics, piston heads, connecting rods, camshafts, piston rings to name a few?

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  • In addition to the answers already given, I usually choose to get the block and head ultrasone washed. This is the best and most effective(maybe even only) way to get rid of deposits in the cooling and oil circuit, and it looks nice after. The heat transfer within the block benefits greatly of such a treatment.
    – Bart
    Commented Nov 23, 2016 at 12:19

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If the intent is strictly to rebuild, here is a list of items which should always be replaced:

  • Rings
    • Compression rings
    • Oil control rings
  • Bearings
    • Main bearings
    • Rod bearings
    • Cam bearings (not always needed, but good choice if equipped)
  • Freeze plugs in block
  • Head bolts (if torque to yield type)
  • Gaskets
  • Seals

Absolutely needs done

  • Block
    • Clean all threads (chase with tap)
    • Brush all oil galleys
    • Hot tank
    • If not boring, hone cylinders to break the glaze (allows for proper ring seating)
  • Clean all surfaces to remove gaskets and any other materials

Parts to consider reconditioning or replacement

  • Block
    • Bore/hone cylinders (ring ridge, out of round, tapered, scoring)
    • Deck (check for flatness, machine if needed)
    • Main bores (align hone will ensure no crank binding)
  • Crankshaft
    • Rod journals (check for out of round, machine if necessary; polish at a minimum)
    • Crank journals (check for out of round, machine if necessary; polish at a minimum)
    • Check for warpage; replace if necessary
  • Head
    • Check for flatness; resurface if necessary
    • Check valves for sealing; valve job and lap if necessary
  • Tins (oil pan, valve covers, etc)
    • Check all to avoid leaks when resealing
  • Pistons
    • Clean if not boring cylinders
    • Replace if boring cylinders

You can also look at things like injectors for reconditioning, but those would be further down the list.

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    I'd add plug wires, distributor cap and rotor if the engine is so equipped, as well as the (probably) obvious stuff like filters and fluids. Also, I'd probably not replace the freeze plug unless there was some sign of leakage there.
    – Edward
    Commented Nov 22, 2016 at 12:49
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    @Edward - Freeze plugs are a given, especially if you hot tank the block. It's just good, cheap insurance from leakage in the future to replace. They aren't hard to replace. They usually come in a rebuild kit, so there really is no down side to replacement. Agree with your other points.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Nov 22, 2016 at 13:41
  • If not boring, ridge removal is a must to avoid breaking the new rings.
    – CharlieRB
    Commented Nov 22, 2016 at 14:05
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    @CharlieRB - If it exists, but it is done before piston removal to keep from breaking the original piston ring lands, not to deal with the new rings. If you have a ring ridge, boring is almost a necessity (if you want things done right). It falls under the reconditioning portion of my list.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Nov 22, 2016 at 15:03
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all parts should be washed and checked for cracking or fatigue

The list of items you don't need to replace is much shorter:

  • the block but you may need to get the deck resurfaced and you will need to bore and hone your cylinders to get them round
  • the heads but they may need resurfacing
  • valve covers
  • oil pan and plug
  • oil pump pickup tube
  • oil fill cap
  • upper part of thermostat housing
  • intake manifold
  • exhaust manifold but needs to be checked for warping
    • -

Things you can check and reuse if within spec and the engine is not a high performance engine.

  • all the studs for stretching
  • crank
  • connecting rods
  • sensors

I am falling asleep I will finish this tomorrow but there is a start

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Water pump, timing belt or chain, air filter, oil filter, ALL wiring, rubber engine mounts, all other belts, radiator fluid, whatever makes the AC go, insulation on the underside of the hood, battery, oil, transmission fluid (I know "fluids" has been said but it can't hurt to spell it out).

And you're never going to have a better opportunity to clean the engine compartment. There are very few jobs where the satisfaction to value-added quotient is so high!

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Assuming that the goal is a fully functional engine, not a short or long block, I'd add renewing the fuel delivery system – rebuild (or at least test) the injection pump and injectors (for a diesel), new or rebuilt injectors for gasoline engine, and a new or rebuilt carburetor if you've got a classic. Ditto for the turbocharger if you have one.

In general any component that will be hard to get at after the rebuild would be a reasonable candidate for being rebuilt or replaced (water pump, oil pump, EGR valve, sensors).

Before long, you have a very extensive (and expensive) list of components to check, rebuild, or replace. The deciding line is either your budget, your need for a problem free engine, or your willingness to gamble on old parts. When I worked rebuilding large (like a Detroit 149, not a Sulzer) marine engines we replaced everything that we didn't rebuild (or send out for rebuilding) as there was no tolerance at all for an engine failure, despite insurance.

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I'm gonna say it is very wise to replace the oil pump while you have the engine out, it's much easier and relatively inexpensive.

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