Both of the swing cylinders on my 580SL backhoe were leaking out at the rod.I got one of them apart and i am confused.The piston has a cast iron ring that does not fit tight in the ring grove.I can get a 0.67 feeler guage in between the ring and the grove.I do not think any one has ever been in the cylinder nor do i think the grove in the piston is worn.My new rebuild kit has a new cast iron ring that is the same thickness as the old one so it is not worn either.You would think it would fit tighter than this would'nt you?
Don't panic, the clearance is by design. That's a high pressure ring to control shock load to prevent damage to the piston seal from high pressure shock loads such as abruptly stopping swing. It works the same way the rings work on an engine piston. The "clearance gap" allow oil to pass between ring and piston groove, then the oil goes "under" the ring. Since the oil pressure has forced the ring against the other side of the ring groove, the oil now under the ring can't escape. This pressure forces the ring to "expand outward" tightly against the cylinder barrel, which keeps all this high pressure oil from encountering the piston seal which is on the back side of the iron ring seal, saving the piston seal from high pressure shock load.
Oh, I forgot to tell you, be careful working with that iron ring. It will break easily and it's edges are sharp. That thing will cut your finger in a hurry.
Thanks Atco,Do i need to hone the bore with a bead hone?It does not have any scratches at all.
I don't think that's necessary. Piston seals are happy with smooth bores in cylinder cans.
Thanks ATCO for the reply.I understand a little.
The cylinder rebuild job and bushing renewal went real well.Thanks for the replies.