... Time to shop for pistons, I reckon.
Perhaps, if you have done the prior steps. The choice of piston and resulting compression ratio is primarily based on:
- The chosen fuel. After determining the primary use of the engine and operating requirements, this choice is the basis for the entire engine build.
- The chosen camshaft and timing, based on a bunch of factors, and determined by #1.
- Other compression modifiers, such as deck height, actual chamber CCs, head material, induction temperatures and pressure, etc.
While builders often swap steps, it then becomes a compromise, rather than a choice of the best part. So, plan as much as you can, by answering as much as you can, and then make your parts list based on those choices and resulting plan. For example, by knowing the fuel and chosen camshaft to meet performance goals, The heads can be chosen, and then the dynamic compression can be calculated to fit those well for best performance, which will then tell you the static compression (piston choices) you can make. Whew.
This is important, as your block machining cannot be done correctly until you know what you're putting in it, what dimensions those parts are, and the clearances and finishes they need to work right. You can't determine what exact bore diameter you need until you know what pistons you will use, what they're made from, and each of their diameters. The finish that should be used on the bores is then unknown until you know what ring material and type you are using.
What Ra finish should the block decks have? Well first the deck height itself depends on the complete rotating assembly you're using and head gasket thickness. Then the finish needs to be the correct one for the gasket in the previous step, as (for example) MLS and composite gaskets should have deck finishes specific to the type, to help avoid leakage or blown gaskets. And the list goes on, with answers necessary far in excess of grocery-getter engines, if the engine is to perform to it's full capability.
Machining is one of the last steps in a build, and based on all this stuff that goes before it. Take your time, and focus on what you need the engine to do, and the build plan will take shape.