OK first of all I'm not getting a 66cc. I have 6000+ miles on my current 49cc motor and want to stick with that size. I ran a 66cc for 500 miles and the only good thing was the power. I couldn't stay on the bike for more than a few miles because of the vibrations, the bearings went, my kickstand fell off at 25mph, etc. I am going to start building a couple of bikes for a friend to ride and to sell. I have 2 options: 49cc slant head or flat head. price difference literally 1 cent. I want more power and know that the slant head gives that, but does it wear the bearings significantly faster, how much more power does it give, any other pros or cons that I should know about? vibration? excess heat? both motors are from BGF, I'm sticking with him, he's been great, even replaced my piston, cylinder and muffler for free when the muffler cap blew during break in and seized the motor. models are z50 and z50f. I have the flat head version now.