18 thou is ring gap for 4.5 inch bore.
put a ring 2 or so inches down the bore to measure.
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Piston gap is something else ENTIRELY.
Piston is oval when cold, so where do you measure?
What you do is measure the bore for ovality
wrap the piston in a sheet of A4 100 gsm paper, it should still slide down the bore easy, when you get to thicker gasket paper it should not slide, piston / barrel gap is something you need to learn from experience
You aren't running genuine lister are you, so YMMV
no chrome bores, so if you hone the cylinder you need to wash it out THOROUGHLY, on a lister(oid) that means this process is done with the barrel removed and away from the engine, lint free cloth and then old newspaper, both wetted in paraffin / kerosene are good for wiping bores.
Make sure piston / bores / rings etc are surgically clean before assembly, and then oil everything with fresh oil on assembly, if you don't oil rings etc liberally you'll have no compression when you try to start, so it will blue smoke like a bitch for 5 minutes, so what
Make sure bore isn't too "lipped" at the top, or ovaled (that's where the torque goes inside the engine) on not perpendicular to the crankshaft
Make sure rings can rotate and gudgeon / wrist pin retainers are WELL seated
No point having the head off and not stripping and cleaning valves / guides / springs / collets / seats / comet too
Never forget to clean ALL fasteners with a brass brush and run a correct #3 tap and die over all threads or your torque settings will be wrong
Grease alll gaskets, never use stick shit
pull and clean injectors and check spray pattern when head is off too, anneal copper rings before using again
(dowty washers are good for leaky banjo fittings, get em from hydraulic supply shop)
this is as dirty as a bore should be before assembly before oiling
This piston, apart from being damaged, is WAAAY too dirty to put back in, piston should match bore visually
How to check ring gap