Hi rollmyown,
I don't have prop hubs for my Toro single carb or Makita 24.5cc yet, so they havn't bee run. The Makita takes a 6mm. I don't know the pitch for the Makita yet, it is the same as the G23 Zenoah. It would be nice if you could post some pictures. My 25cc weedeater has a four bolt aluminum back plate. I have a 18cc with a plastic four bolt backplate. I trimm them and sand the back plate flat for the mount. I don't have the equipment to turn down the cylinders for compression increase. You really got good performanec from those 21cc weedies. If the 32cc Mac carbs are the same as the 28cc they would be a good choice for the 21cc. Mine is a 10mm Walbro. I planned to use it on my Echo 21cc and put a 11mm on the Mac 28cc. Using a 5/16x24 bolt on a metric prop hub is a good idea. That is how my Echo 44cc is setup. I planned on doing this with all metic shafts and I use a standard bolt pattern for all of my mounts. I hope that I anwsered all of your questions.

Dave