Drilling Props
 

Just bought a beautiful 26x10 Mejilik carbon prop for my 3W78 twin boxer.

What's the correct procedure for drilling the prop hub to take the 6 mounting bolts. Is there a jig available? I assume you need to do it on a drill press, but any other hints?


Thanks,

Steve

Drilling Props
 

Steve,

Yes, they do have drill fixtures for the props, I know Cactus Aviation carries them(and probably many other dealers) and they have several bolt patterns on them for various engines. They also have a centering stub on one side that fits in the props center hole so they are centered. I strongly suggest you use one unless you have access to a rotary table or CNC mill or drill to do the job. Since all the bolts must fit through the front plate, the prop,the spinner and finally screw into the hub alignment and straight holes are imparitive and yes I do think you should use a drill press with the drill fixture and make sure the drill press table is square to the spindle front to back and side to side before drilling. Check your manual but I think most of the engine manufactures recommend the prop be aligned with the piston stroke and this is easy to do by rotating the drill fixture until the pattern lines up accordingly.
Hope this helps ,
Jim ;)

Drilling Props
 

Thanks, Jim.

Couldn't quite understand "lining up with the piston stroke." Do you mean that the prop should be at TDC when the piston is?

Thanks,

Steve

Drilling Props
 

steve
The prop should be inline with the cylinder when the engine is at TDC. For a single the prop would be vertical(assuming it is mounted upright or inverted), for a twin the prop would be horizontal.

Drilling Props
 

Steve,
That is correct, the piston/pistons stroke is parallel with the prop not square or on an angle to it. The tip/s of the prop should point toward the sparkplug/s.

Drilling Props
 

I line up the prop at 3 oclock when the piston is on compression...on all engines no matter what position the engine is mounted....vertically or horizontally mounted engines. This is the best method for easy starting and keeping the prop from getting broken on a bad dead-stick landing. I can think of absolutely no reason to mount the prop inline with the piston....unless of course this makes the prop line up at 3 oclock on compression. A balanced prop cannot influence how the engine runs depending upon the mounting position of the prop. There is no mysterious force between the prop and the piston that needs alignment.