It is possible to come across car builds that have u joints of different sizes. U-joint sizes are classified by a series number. Each series has a very specific cap diameter and overall joint width. Normally the larger the U-Joint the stronger it is.
The 1310 to 1350 U-Joint Conversion is a mix of two different series specifications on the same u-joint so that that different sized yokes and driveshafts can be connected. The strength of the u-joint depends on several factors, material, design, the presence of a grease fitting, cap size, etc. Take note that the u-joint is only as strong as the smallest series number.
The 1310 to 1350 U-Joint Cross Width is 3.625 (3 5/8”) on the 1350 side and 3.250 (3 1/4”) on the 1310 side. The Cap diameters are different as well. 1 1/16″ on the 1310 side and 1 3/16″ on the 1350 side.