Stork the piston clearance on your mates large ship engines is not a good example to use because the pistons and cylinders on them are made of materials with matched rates of thermal expansion, so there is no requirement to allow for expansion from cold to hot. The Bultaco motor has cast iron sleeve and aluminium alloy piston which have quite different rates of expansion.
Gday feetup, most bike engines are like that as you know. A quick example - Honda XR 400 max piston to bore clearance = 0.004 inch (0.10 mm). Similar engine in that it is air cooled, Cast Sleeve in an alloy barrell. Most pistons in use these days use struts cast in to control expansion, and are cam ground so the piston expands into a perfect (reasonably anyway) circle. God knows what boat engines are made from...
Bogwheel - Also, I just remembered, it is important to check your ring end clearances as well before fitting up. Rule of thumb is 4 thou per inch of bore size. Use the piston crown to set the ring square into the bore and watch out for the ports. Too much is better than not enough. You can carefully file the end as needed but be gentle as rings can break easily. I set them in a vice with soft jaws and go slowly.