I have a 2T8780SP which should be a d4400 engine, the pistons I believe my be a little different as there is a top groove that doesn’t have a piston ring I have attached a few photos anyone know what the top groove does/ purpose? They are not a two piece piston either.

I had the pistons out of a 2T I used to own and i can’t remember that top groove in them. It was more than 15 years ago i had it apart though so old timers disease might be kicking in. I am sure there were 3 compression rings and 2 oil rings i fitted to the pistons.

Well there is 3 compression rings and 2 wiper rings, the top groove doesn’t look big enough to take a ring. I have checked in the parts manual with no luck showing the groove.

That must be the design of the piston. Not sure why they would do that but the 3 compression rings are sufficient for proper engine operation. The boffins would have had a reason they designed them that way back in that era.

RD4 pistons had 4 compression rings and 2 oil rings, same engine, just earlier version, quite possible a previous owner swapped in RD4 pistons after machining the ring lands under #1 ring land larger, to take larger rings from a later model D4400, and they decided 3 compression rings were enough, because by 1939, and the model D4 7J, Cat too had decided 3 compression rings were enough for that engine, they were learning all the time about their new diesel engines.

Just a theory, I'm usually wrong, but weird things were done back in the day by bush Mechanics to get a tractor running again ASAP.