durabol wrote:
The two main methods for homebuilt construction would be casting vs.
machining or some combination of both (probably the best option).
Forging probably isn't appropriate for home construction (either the
piston or connecting rod).

One method I have thought of is to produce a wax model of the engine
with molds (to a fairly high tolerance to minimize machining) which is
then cast using lost wax casting techniques. I planned to have an
integral cylinder head/cylinder/half the crank case (this is for an
opposed style engine). The only bolts would be to bolt the two halves
together.

A completely machined engine would need a large block of aluminium to
start with which I'm not sure how practical that would be. Perhaps
lost foam casting could be used as a general model of the engine was
made in foam and then cast and the resulting casting could be
machined.

Don't sneer at sand castings, particularly for aluminum parts. Rods and
pistons can be made from billet, or can be purchased from racing parts
suppliers -- racers often customize these parts, so if you can find an
engine with e.g. a similar rod you can order one from Carillo or whoever
"yea, I want a Honda XYZ rod, but make it 5" center to center and make
the big end take a bearing from a Kumatsu 30HP diesel".

For an absolutely "I'm only gonna build one ever" engine you could
fabricate the rough parts by welding or brazing, then stress relieve,
then machine. This is, I believe, how railway locomotive and ship
engines are often built, and I believe that there were automotive
engines built this way, too.

You're building an aero engine, right? And it's going to be air cooled,
right? If so, the advantages of separate cylinder assemblies probably
outweighs the disadvantages of trying to build a monoblock engine.
Trash a cylinder on a monoblock engine and you have to do a complete
rebuild. Trash one cylinder on an assembled engine and you just have to
replace one part.

Another of the advantages of separate cylinders is that you can build a
one-cylinder "research engine" to get the myriad of details worked out.
Then when you like the porting and combustion chamber shape and
cooling fin layout and piston design and ring lands and cylinder finish,
and etc., you can build the multi-cylinder engine of your choice.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com