please help

hi guys, i have a 73-77 merc 402 that runs awesome but it wont let me shift into reverse,,, so i bought a 80-81 merc 40 to switch lower units,, after getting both off i noticed that the smaller shaft the drives the water pump is bigger and has splines on the 80-81 the 402 lower unit only has a square top on it,, is thier any way i can make the 80-81 lower unit work on my 402

Yup. You need to change the upper shaft so it's the same. (I have one if you need it. $10 plus postage.) You'll have to drive out the roll pin with a thin punch, drop the shaft out the bottom, and slide the new back in.

That said, how do you know the old LU is bad? Sound to me like it was installed off one spline.

Try this: Take some Vise Grips and carefully rotate the shifter in the CCW direction while trying to turn the prop shaft. You will go through neutral and then into reverse IF it's still working. Turn very slowly since it doesn't have to turn far--about 45 degrees total.

Jeff