I think the .77 was an adapted or shared marine engine. It was a rear disc rotor induction unlike the the .65VRDF and .91VRDFs which were drum rotor. I know you could use a .77Marine case with the .77DF internals as the only difference was the bosses around the exhaust port were drilled and tapped for water cooling. They did have weak rods. An RPM rod was the rod to have when the .77s were popular. The VRDF series would idle much lower when setup properly. They also didnt need 30% nitro to make the power which further leads me to believe the .,77s started life as marine engines. Those boys LOVE their nitro.