I have the new version Dolphin with the OS 55 AX. The overall build went very nicely with this one; however I found the plane to be tail heavy when balancing at the 3" mark as specified in the manual (2.9" to be exact per the manual). I was bummed when I discovered I would need to add 6 - 8 oz's on the front to get it to balance. (The stack of lead on the engine in the pic is basically what it needed to balance) The airframe is solid and could definitely handle a .60 size engine. I would love to go that route as an option and put extra weight in a stronger power plant vs adding dead weight just to balance. I also have the battery up against the fuel tank to help. Unfortunately, the engine area is fairly narrow, so I'm doubtful it has enough room for a .60 or larger engine. It would be great to hear other's experiences with this plane /engine combo or any input on the plane in general would be appreciated.