At last I got back to this project this afternoon. I have borrow a large honer and it certainly has cleaned up the rusty surface very well but after a fairly long time I still cannot get the piston with no rings to slide up the bore. The top goes in with no problems but the further I slide it the tighter it gets and will only slide in as far as the bottom of the piston skirt.
This got me thinking, yep hard to believe, that maybe the piston is out of shape and not the bore as I was thinking. I had the piston off the conrod and put it in the lathe and using a dial indicator got some out of round numbers on the OD of the piston but from top to bottom it changed a lot and was messing me up.
So I engaged the grey matter again and used my digit calipers to measure the OD as far as it could reach over the skirt measuring from the side. and did this at 3 different points, top, middle and bottom.
Not sure if my calipers reached over the complete OD of the piston but with the calipers pushed all the way on then closed the gap and locked the calipers it gave the following readings.
Top - 87.77mm
Middle - 88.21mm
Bottom - 88.40mm
This meant that the piston got wider the further down I measured it and by a total of 0.63mm and this to me would indicate that the piston and not the bore is at fault.
Happy to hear any and all comments on this tricky bastard.