I picked up a pair of u joints from the local NAPA for my F150. When I went to install them they were way too small. Did NAPA make a mistake, or are there different sizes based on model and packages?

They are for the rear shaft, both front and rear joint.

 

no...there are different sizes. you probably got the 1-1/16" size.
and your needing the 1-3/16"?

it most likely depends on the size of the rear end..not sure of the specific qualifiers myself. I do know the 8.8 rear end is smaller/lighter duty then the 9.75. you might be wise to decipher the axle code...listed on your door jam lable/sticker on the drivers side.

https://www.ford-trucks.com/forums/1...axle-code.html

 

Thanks for that. I also learned that in the parts book the u joints are listed according to wheel base. The longer wheel base takes the larger u joint.

Live and learn.