It depends. From the 1948 parts book (section 1.064), the bolt circle is either 2-1/4" with 1" shaft hole, or 2-1/16" bolt circle with 1-1/8" shaft. The 2-1/4" and 1" shaft seems to be the most common, the 2-1/16" with 1-1/8" seems to be an optional low speed operation fan. My 1937 SD has a measured 2-1/4" bolt circle and a measured 31/32" shaft hole. It was tight on the water pump pulley, which also measures 31/32".
