Hi GB,
Yes, there are several considerations to be taken into account. First is draft angle (I use 2 *) otherwise the pattern will not extract from the sand. And that applies to all parts of the mould. Shrinkage allowance depends on several factors - type of alloy used, pouring temperature, effect of 'risers' placed by the casters etc.
And, as always. I have two of everything made. If I make a cock-up when machining or the engine goes bang later on, I don't then have to go through the process of getting more castings made. Making two of everything (or in the case of barrels, three) is more convenient and cheaper to have done 'whilst they're at it' than the alternative. And that way one can be sure they'll never be needed!
I'm not a pattern-maker by trade and glad I'm not. It's something that might fascinate the sort of people who spend years making fantastic model trains, but I don't really enjoy it. It's the fiddly bits, like trying to get a perfectly smooth radius (and draft angle!) at the base of the lugs you see inside the timing cover that test my patience. And these are relatively simple, one piece patterns. Imagine making a pattern for a cylinder head where several inter-locking parts are required - all able to be extracted from the sand. And that's without worrying about cores for ports etc.
As it is, I have to make a core box for the new cylinder barrels, but at least that's straightforward. Even the previous engine wasn't too bad, where the cylinder barrels needed a core for the timing chain tunnel as well.
