Actually a Savery engine (the type of engine the OP's post starts off with) doesn't have a piston at all. And the Newcomen atmospheric engine didn't require a very precise fit to function either. It wasn't until Watt started the drive for more efficient engines that accurately boring the cylinder became important.
Exactly! Part III or IV will end up talking about boring techniques of the 1650s (long before the famous John Wilkinson methods), but that is precisely the thing with Savery engines - and indeed with Newcomen engines, which worked with pistons long before Wilkinson too!
