you have to check them. even good machine shops make errors. for the time it takes to check clearance's vs. rebuilding an engine because of an error it would be foolish not to. you really don't have to spend allot on fancy stuff either. you really don't need micrometers , bore gages ect simply checking piston side clearance, ring end gap and plastigage the rod to crank . a set of long feeler gauges and plastigage is all that's needed to catch a machining error at a cost of $15 for both . even crankshaft and cam endplay can be checked with feeler gages if you don't have a dial indicator. hard to get by without a torque wrench. think about it this way; the guy putting the engine together is the last chance you have to fix something that could go horribly wrong. you are the quality check. over the years I've noticed allot of guys blaming the machine shop for their engine failure because this wasn't right or that wasn't right. bottom line you have to check stuff as you assemble.
its difficult for people that want to build their one engine so the can say I did it. I get that, small engines are relatively simple , cheap and easy work on. you mess it up you're only out a few hundred dollars. but a few hundred dollars still buys allot of donuts, Coke or beer.
I have all my cylinders rough bored. I finish hone the bore and fit the piston myself. so I don't have issues with that. I did have one block bored where they went an 1/8 in to big. they told me about it and made it right
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