Kind of doubt the "all 13 spline props are equal" thing. Must be shaft diameter differences between motors (running the gamut from 2 to 350 hp.)
Simple way, of course, is to choose a prop designed to fit your motor. Most online suppliers have a prop selector applet to help you do that.
If you're buying used, I would test fit it at the very least (including tightening the prop nut), and measure everything out if you want to be anal about it. The above mentioned selector may help here, too, if you know the manufacturer and part number of the prop in question (unlikely, though - they tend not to be marked very well.)
13 spline yamaha, mercury and tohatsu props will fit.
the splines for a 13 spline johnson/evidrude match up but the hub is slightly too large, so unless you're running an overhub cleaver/chopper they won't work.
I'm not sure about suzuki although I've heard they will also fit mercury, but don't take my word for it.
You can mix and match mercury and yamaha props (tohatsu don't make so many props) depending on you're application. All the best props are made by them anyway
Merc DO make good props I run Rev4s on my Etecs, superb prop.
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Seutrek
BLACK PIG
Piglet
I run Rev4s on my Etecs, superb prop.
