the spline diamiter is the how thick the spline is measured across,,,, just like the clutch plate is

think of is as a drilled hole,, 25.4 is gonna be looser so there is a good chance that it will only engage the ends of the teeth and chip them off,,,,, and then no power transmitted,, but it will be alot easier to line the box up.... well the first time as you will be taking it back out again to fit the right plate