Piston theory

piet..thank you the pictures..I must say I've not seen that. Probably all the old tractor parts I have laying around.

It looks like the is a notch (V) at 12 o'clock just inside the gudgeon pin. It may just be the optics. Is there a V cut there too?
 
Rusty,

The pin bore is perfectly smooth save for the clip groove and the 2 pockets on each side.  There is a small notch on each side to enable the clip to be prised out, that's it.

piet
 
way I see this groove is that it is the lubrication provision for the wrist pin
the oil rings scrape/gather the oil from the cylinder bore
oil goes through the rings and then travels through the oil drillings in the piston oil ring groove
oil is now on the inside of the piston, above the wrist pin, these grooves gain their lubrication from the oil rings

I have also noticed that the piston manufacturers seem to have increased their clearance in the piston/pin area over the last number of years
compare the pin fit on a set of new J&E, Ross or Keith Black pistons against the old Mondial tractor piston and there is a huge difference.
I'd presume that this change has come about through a better understanding of what happens in the combustion process where the piston is under massive forces as Piet pointed out
 
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