It took some time to get this off the machine and disassembled for inspection and evaluation. What was found inside isn't pretty, and not a sight for the squeamish. Still not sure why this occured, maybe somebody might look over the pics and get some idea. Thanks for looking.
Once carnage of that sort happens, it's difficult to say what caused it by viewing the damaged components, and there are key pieces missing in pics. One thing I see is extreme wear on the piston guide plate where it would have been riding on it's guide bearing ring (one of the missing pieces), a kind of a dome shaped ring that the piston guide plate rides on. The wear, shown by rolled kerf of steel, of that guide plate did not happen suddenly, it happened over a period of time. Once the guide plate wore enough it was no longer riding on the guide plate bearing. Once this happened, the brass piston shoes were simply floating around, rotation of shaft was banging them into the swash plate. Went downhill fast once that guide plate let go.
One time that I saw similar damage: Back about 1991 I put a new hydrostatic trans. in a large diesel powered commercial mower on an estate. It was the type w/ a variable swash plate pump in the top, and a swash plate motor on the bottom. They called me back a few weeks later and said it wouldn't move. The machine was in the shed, and when I disassembled the trans. it looked like that, only worse. The oem refused to warranty the unit, saying it had been towed without the engine running. When I got to the bottom of it, it turned out they had run it out of fuel and drug it back to the shed and restarted it the next day. The lawyer that owned the estate sued me over it, but that's another subject, and part of the story of why I quit working on other people's equipment.
Thanks for your response Willie 59. That domed ring you mentioned. I think that's the piece (what's left of it) in the right corner of the photo showing the pistons. This drive was purchased used to replace another one that had been destroyed prior to me buying this machine. The salvageable components from the first one are hopefully going to be installed in this used one that I just got. Same make and model, so in theory, it should all work. The history from the previous owner was vague as to why the drive failed in the first place. It looks as though a bomb went off inside. I'll get some pics up later. Is there something that is reocurring in this system that I should be looking for? I'm running tight on salvageable/ replacement parts, even tighter on money. I'm trying to prevent putting this back together and have another catastrophic failure. The guy that's helping me with this suggests I get it up and running again and sell it before anything else goes awry. I'm hoping to keep it for a while as I'm into it for short money. Not to mention the cost of upgrading to something newer is not in my budget. I told my wife that I'd been saving aluminum cans since I was twelve so I could buy this one!