Why are some filter cartridges 9-3/4” in length and others are 1/8” longer at 9-7/8” in length?

The filter cartridge length, 9-3/4” vs. 9- 7/8”, depends on the type of filter cartridge. The nominal 10” standard filter housings are designed for 9-3/4” long replacement cartridges, the distance between the top and bottom knife seats in the filter head and filter bowl. The reasoning for the two lengths of cartridges is explained below.

9-3/4” Long Cartridges

The 9-3/4” long cartridges are designed so that the smooth plastic end caps or gaskets on each cartridge end press against the bottom and top knife seat in the filter bowl and head when the filter bowl is tightened. This makes a seal which prevents by-pass. 9-3/4” cartridges are provided in the following types:

NOTE: These rigid / non-compressible types of cartridges, if they were made to the longer 9-7/8” length, would prevent the filter bowl from being tightened up to seal the O-ring between the filter bowl and the filter housing. That is why the 9-7/8” length is limited to two types of cartridges.

9-7/8" Long Cartridges

The 9-7/8" long cartridges are only supplied in two types, string wound and spun poly (a.k.a. depth cartridges or melt blown cartridges).  These two types of cartridges are purposely made 1/8” longer than the standard 9-3/4” distance between the knife seats in the filter head and filter bowl.

WHY? These cartridges are softer, and the “V” shaped knife seats will easily press into the string or poly filter media 1/16” at both ends. This ensures that there is no by-pass between the end of the cartridge and the knife seat. By-pass can be a problem when these two types of cartridges are only 9-3/4” long, especially with string wound as their end surface is not flat / even.