Orbit,
I have to respectfully disagree with your statement
The TD 049 and 051 were designed to have exactly the same power output... The Cox TeeDee .051 turns a prop about 1500 to 2000 rpms faster than its smaller TeeDee .049 brother (This are numbers based on my own experience, using cox 5X3 2 blade props with 25% Nitro fuel).

muzzlemagot, you can pm me and I will see If I can find a NIB set of Piston/Rod/Cylinder I might be able to sell you.