To enable an engine to be reconditioned (and last well) good general practice must be taken, cleanliness is the overriding maxim. The block and head should be chemically dip cleaned twice, the core plugs and oil gallery plugs should be removed prior to this.
Cleaning should be done once before machine work and once after to clear out any metallic production swarf. The cleaning of the block before machine work might seem odd but it is difficult to align an engine block squarely on a milling table if its covered in 20 years of grit clogged oil. If the company in question doesn't mind putting horribly oily and mucky engines in their machines you should also be asking yourself how much care they are putting into the rebuild.

        Compressed air is often used to blow through oil ways to ensure clear passage for the lubricant, I would not attempt it without an in line air filter and a dedicated blow gun which is kept spotless. The chemical dip will be sufficient provided all engine core plugs etc. are removed before hand.

        All components (even brand new out the box) must be cleaned in paraffin, never soapy water as most soaps contains salt (and it doesn't work as well as paraffin!). The crankshaft oil ways must be carefully cleaned with small brushes and paraffin (ones meant for cleaning test tubes are perfect).

        Grit or dirt acts like an abrasive paste when in oil, slowly grinding up the internals of your engine. Oil can cope with minute metallic particles by keeping them in suspension. Bigger garbage will be fatal to your new engine.

        When inserting the bearings you must maintain an absolutely spotless working environment, even one tiny bit of grit or metal will badly score a crankshaft journal and grit behind a thin bearing shell will make it assume an oval shape once assembled which will kill your bearings very efficiently.

Magnaflux or Similar:

        Magnaflux is one technique used for crack detection in engine parts, it is 100% essential to have this operation on your crankshaft, con-rods and block.

        I polished up the con-rods on my spit engine, when I got to a smooth finish I noticed small hairline fractures on one rod.
I had not had them crack tested.

        Had I not spotted it the con-rod would almost certainly have wrecked £1000 worth of new engine. The crack test would perhaps have cost less than one tenth of that. Even then I only spotted the crack when I polished the metal. The moral of the story is: GET THE ENGINE CRACK TESTED!

        If you find ANY cracks no matter how small in any engine components put them straight into the scrap bin.
I heard of a bloke with a Spitfire who had the engine block welded up after a crack appeared, I don't need to tell you what happened after he started it up again do I...

Finding a good machine shop

        The first Spit engine I had done was delivered to me in an appalling state from the machine shop. It was very badly wrapped and dirt and grit had got into the packaging, the block had not been dipped and they had not even replaced the timing chain tensioner. This was from a machine shop on an airfield that did aero engines! It goes to show always to be very suspicious.

        The classic warning signs are things like dirty oily workshops, old engines lying around outside rusting and unknown companies who don't have a reputation to keep up. A really good engine rebuild outfit will have clean workshops, clean looking well maintained machinery and will probably give the impression of taking an almost surgical approach to their work.

No chemical dip = A dirty unreliable engine

        Some chemical dips like Caustic solutions actually dissolve Aluminium. For alloy components they are usually steam cleaned with a very powerful jet pump washer. Anything up to several hours are required with a block to ensure its nice and clean throughout. If you put an Aluminium block in a Caustic dip overnight you will find it quite empty in the morning!

Crack testing should be a mandatory procedure there too.

        The best outfits will charge you perhaps 50% more than the `bloke round the corner you met in the pub last week`. By the time you have either had it done properly or worn the engine out in 20,000 miles the total cost is very similar. The inconvenience and stress from having to get it done again properly is considerable.

        I recommend you go to a reputable machine shop who have experience with your engine,
(or at least 4 cylinder engine work). A well known company will not give you a duff engine when they have spent 20 years building a reputation for excellence. Motorsport oriented companies in particular depend upon their reputation for business.

        CES Power in the UK build race winning engines, they have been used by some of the most illustrious teams in racing yet they will happily machine you the man on the street an engine for much less than you might imagine. Putting it all together once the machine work is done will save you about 20% on the cost of an in house build. Certainly CES Power can make a Spit engine better than new for less than the cost of two cheap and sloppy rebuilds.

With engine rebuilding you really do, without exception get what you pay for.

        The same goes for cylinder head gas flow work, Blysenstein engineering in the UK do heads for many top race teams but will do a gas flow on a spit head for around £300 ($420 USD). Buying quality saves in the long term, where head flow work is concerned this will guarantee no horrid disappointments when you fire up the "new" engine to discover it is not any more powerful than before. Or if you are very unlucky even worse than before.