I had an 855 that did the same thing. I took it apart and it looked good inside. Very little ridge. Cylinders measured good, round with almost no taper. Main and rod bearings were all good. The problem was the rings. The end gap was three or four times what it should have been. I honed the cylinders and installed new rings. I had a machine shop grind the valves and surface the head. I needed two new valves, an injector, and rings. When I was done I had about $1000 and my time in it. It ran great for the next seven years until I sold it.