The FPO-1 oil filter differs from FPO-3 with it's internal and external construction. From the outside the three threaded rods used to fix both ends can be seen. Internally it's construction is simple - on the air intake side a spring with a pressure disk is placed, underneath the two layers of felt insert interleaved with a metal grate are located - their task is to capture the air droplets and particles, as well as to keep the activated carbon granules inside which fills the filter's body. On the other side such inserts are placed as well.