Hi Mike,
The 21cc Mac was the engine on the right in my post above. It is a Echo style engine with dual counter weights and built in mount. The shaft stud should be 8x1.25mm. I am converting one now from a Mac Roadrunner trimmer. The flywheel is heavy and the ports are small. I plan on triming the flywheel and advancing the port timing. It will weight about 3 and 1/2 pounds ready to fly with magnito ignition. It will swing a 15 or 16" prop close to 8,00rpm with a 9 or 10mm carb. In glow terms it will have the power of a good .75ci with the weight of a 1.8ci. You can figure a gas engine willproduce about 2/3 the power of a glow engine at about double or more the weight.


http://www.rcuniverse.com/forum/m_31...tm.htm#3229361

Dave