Rob - quite a long way back in this thread you said that you had been supplied with oversized pistons because the cylinders had been honed.  When I read that I assumed you meant the cylinders had been bored to an oversize and you were just speaking casually. 
But, reading your comment re the piston skirts, I wonder if the cylinder has been bored correctly to suit the oversize pistons.  Do you know?
Just checking . . .
Cheers, Mike

Hi Mike,
I had brought the engine revision shop the old piston that had little pieces broken off, and the two, scratched cylinders, and had them do what they had suggested: find a supplier of new, slightly wider pistons for this particular engine (given the type, model and engine number) and hone the cylinders accordingly.

When I later picked up the new pistons and honed cylinders, how could I expect the clearance then to be not OK? It was their suggestion to do it like that, and their job to do that right, wasn't?

I don't think I did anything wrong, with the reassembly, or it was that it didn't come up in my mind, that I should find a way to check if they'd done the job right.
I think... but correct me when I'm wrong.

You can't be that helpless, Glort! I have to do it all the time, go from mm to silly inches...
Here's a tool
https://www.rapidtables.com/convert/length/mm-to-inch.html

Hi Rob,
            I'm a newby to this forum but am an ex motor mechanic, (retired).
Did you notice any scars on the piston skirts, assuming they are alloy pistons, if so, the pistons are 'grabbing' the bores, due to insufficient clearance, if not, then it's likely the top rings are too tight, ( too little end gap), this gap should in your case be around 4 thou per inch of bore, 4x4 = 16 thou.
Hope you have a successful outcome.

Hi Matt,
thanks for the extra info. I'll check for scars at the pistons, today. But the 16 thou may not be correct, at least not for this particular engine, as earlier written by dieselgman. According to his data is has to be like double that distance. The Petter engine collector Jim Perkins says the same. He'll snail-mail me the manual today.
When pushed the rings into the cylinder for about 2 inches, the gap is only 0.3 mm, which is 0.118 inch. According to the proper manual for this engine (PetterPJ2) it should be 3 times that.

Hi guys!
This video shows a honing tool. I knew it was something like that, but would this size be OK for my bigger cylinders?
https://youtu.be/-NRUZNX0IAg?t=642
Indeed, I'm not going to buy new pistons. They were expensive: 245 euro each. But I could do some light honing, to just get that one hardly noticeable scratch away, right?

I took both cylinders off, with the finger nail I can detect just one light scratch. Other lines are visible but I can't feel them, and I'm quite a sensitive guy, really ;-)! Most of my working hours so far were spent on illustration and design.

I let the pistons sitting on their rods and the crankshaft. There's really no vertical 'play' of the pistons. All the pistons can do is move sideways, over the pins they're attached to the rods with. The rods can not tilt one bit. I see no reason to dig into the block and take it all apart.

I got all other rings off the pistons without breaking any. Did that with sliding them up over three thin metal strips, one of them being a valve clearance feeler gauge.

I'll wait for the manual to arrive in the mail (and stuff) before adjusting the gap of the piston rings. The manual I was using so far, of a one cylinder air cooled Lister LD, actually had measures of the gaps equal to the ones of the new piston rings.

I'll ad the flyer for this engine (earlier I wrote it weighed 300 lb, but that's in kilos. It weighs 611 lb).
I also ad a photo of the boat it sits in, parked in an Amsterdam canal. It was an open life boat coming from a cruise ship. I build the cabin over it. Last summer I painted the black figures at the white hull. The wooden roof part and windows can be taken all off in 2 minutes. There it also has a fire place in it, for wood fire, oh yes! So, that's why I need this engine to work again. I missed 3 months of boating fun already!

You might want to feel the Rods and make sure there is no freeplay in the bearings. If there is any detectable movement especially in the big end, You'll want to do those as well whether it's a big job or not. Otherwise you could do damage and be pulling the whole thing down again anyway pretty soon.

You're not making me happy with this... I expect horizontal movement to be not a problem (even a necessity). Vertical it would be problematic. Before I did not feel any vertical freeplay, when I checked it for that purpose.

As far as using the old rings, it would seem likley that they overbored the cylinders when they did them and the pistons are also larger. You'll need to know what size they went over.

When you rebuilt the engine, Did you do any of the bearings or just the top end? Also, did you get the specs for the head bolts and torque them down with a tension wrench in the correct order to the right specs? If not, best you do this time.

Ah, yes, of course. The engine revision guys said they'd order slightly bigger pistons, because of the honing.

I didn't do any bearings. I took the cylinders off, and removed the pistons. Nothing else.

And with the reassembly I followed instructions for how to bolt in the right order as I wrote earlier. No 'force meter', but an instruction of how the force used should feel/ I don't have all the specialists' tools.

I MAY BUY ME ONE, though, after reading about the importance of it (protecting the gaskets).

PJ1 and PJ2 ring end gap  .029 to .037 inch
bump clearance .036 to .042
bore 3.8175 to 3.8185 inch

dieselgman

Hey dieselgman, I just measured the gaps of the first pistons' rings, when taken from the piston and put into the cylinder at the top side, and that was a bit over 0,4 mm, less than 0,45 mm being around 0,0165 inches.
(Photo attached to next post)
So, then indeed, the gaps are too small. They'd need double the size.

It means that ajaffa1 was spot on with his first comment!

Now shit happened. I still broke the 4th ring, the scraper ring. Dang! Got a bit confident I guess after the first three went OK. So, I'll see if Jim Perkins has one for me, part from the manual and an oil banjo bolt (replacing a wrong one put in by the life boat's former mechanic) he'll mail to me.
I still have the old pistons with intact rings, but they're a fraction thinner. I guess, worn a bit. What you think, guys? Would that indeed be a cause a problem?

I also see the tiniest vertical scratches, all the way down, in the wall of the first cylinder I just removed. How would I know which scratch is severe enough to demand new honing? And what if a second honing means a tinier wider cylinder, for the same pistons?