Morris Minor Owners Club

Working on my 1098 silver seal engine ,with new bearings

You will need to use plus 20 thou pistons.
The pistons and rings are matched.
The pistons are marked because either you have overheated the engine and the pistons have picked up on the cylinder walls and/or lack of lubrication.
You need to check the condition of the bores as they may well be marked - re-bore?

I second that, the marks on the pistons are a classic sign of them nipping up in the bore either through over heating (but that is unusual in a minor unless low on water) or lack of oil. Re-bore and over size piston and rings needed I'm afraid

could I put all the +20 parts on a std piston engine .change all the +20

The bores are likely standard or oversize already? Pistons must fit the bore and rings must match the bores and pistons. 20 thou oversized pistons woud only fit a rediculously worn standard bore! (And the bore would not, by then, be perfectly or even close to round!).

Btw, if the crank has been reground, it will be udersized, so bearings would also be smaller eg minus 20 thou. They would only be oversize if the block had needed to be line bored for some reason. They are usually line bored, ay +15 thou,so they are not the same measurement as an undersize internal dimension.

RAB

The bores must be 20 thou oversize if +20 pistons were in there. They have partially seized in the bores. So the bore will AT LEAST need honing to remove any damage, and new +20 pistons and rings are needed. If honing doesn't removev the marks then a rebore to +30 is needed - with +30 pistons and rings.