I don't have a real life experience interchanging the pistons, but looking at Partzilla in the Related Fitment section it doesn't show KZ and Voyager compatibility.
I think there is no problem with the bore, or perhaps even where the piston pin is. Looking at the parts, the connecting rods are compatible. My wild guess would be these pistons would go deeper inside the combustion chamber and would hit the valves, and perhaps don't seal the cylinders all the way to the top edge. Could be a combination of both, or if just the second part. That would damage the cylinders over time because the lining would be directly exposed and overall would have much smaller compression (larger chamber).
I had to remove my installed rings from my pistons and did it without a problem, if you do it gently.
...well... certainly the cut-out at the bottom is very different, but despite the different profile at the top, they are exactly the same height, from top to bottom as well as from top to centre of the pin.
is a Voyager cylinder head very different from the KZ, particularly in the combustion chamber area?
[sigh]... maybe I should just install the ones that came with the block, with five oil holes...
Now I see the photos.
If the pins are exactly at the same level from the bottom of the piston, then I'd say the piston would hit the cylinder head, and probably the valves as well. Besides that, the compression would be totally different, the engine couldn't be tuned right even if they would fit. Also, with a different shape the gases would behave differently inside the combustion chamber. If the piston pins are higher, then you'd also have that cylinder wall exposure problem.
I suggest removing the rings very gently, then either wait for correct pistons with the extra holes (I'd definitely do that), or of course you can always put the original pistons back in.
If I was the seller I would have no problem taking them back. A good thing is you are both in the UK. I will start asking around. I wouldn't worry, it's really not a a set back but a small inconvenience in the larger picture. Can be taken care of in a few minutes at the table
Let me know how it went. I hope he will confirm being one of the 'good guys'.
A thought, it's a gamble though. Before removing those rings, perhaps you could advertise selling them as a set with the rings? At the cost of what you have paid.
Just trying to think how to avoid removing those rings. It will be easier and straight forward though if you do, and he takes the pistons back.
If you are not on facebook I could post around about it for you.
Well, he came back to say he will take them back, however, he assures me they came out of a KZ, he is sending me the frame and engine numbers tomorrow, that will tell us for sure.
In the meantime I did a little test just to convince myself. I turned the engine until cylinders 3 and 4 were at top position and taped the other rods with tape so as not to damage the liners. Then I attached an original piston and a new one - without the piston rings! - to rods 3 and 4 and slid the tub on using lots of lubricant.
The result is to be seen here:
As you can see there is no discernible difference and the larger cut-outs on top of the new pistons are there to 'accommodate' the valve heads, just like the smaller ones are on the old ones. Then I put some green sticky assembly oil on the tops of the pistons and then slid the cylinder head on. I let it sit for a few seconds and then took it off. The combustion chamber with the original piston had nothing clinging to it, the chamber with the new one had a slight ring of oil around INSIDE the chamber edge where the bevelled edge of the piston was close enough to the head to leave an oil deposit.
It is not easy to see (if at all) in the picture, but in addition to the oil on the head 'face' that you can see easily, there was an oil ring INSIDE the combustion chamber, roughly same size as the piston bevelled edge. There was nothing on the valve faces, so obviously they (in closed position) did not touch.
Not sure if the piston bevelled edge was actually touching the inside of the combustion chamber or not and there was no head gasket involved in the set up, so still not entirely sure if it would actually hit or not, but for now I cannot see any exposed liner or the new pistons hitting the valves...