Two spin-on type filters are used on the Series 60 engine. The first in the fuel flow is a strainer, and the second is a filter. Although they are not marked as such, the threaded sleeves that accept the cartridges are different sizes to prevent mismatching. The primary filter has a 1 in. x 12 thread. The secondary filter has a 13/16 in. x 12 thread. The word “Primary” or “Secondary” is cast onto the top of the respective adaptor.