1. TAPPING PLATE
This serves as the entry and exit point for oil. Small holes around the edge facilitate the free flow of oil into the container. The threaded center hole is where the oil flows out and also how the container attaches to the engine.
2. ANTIDRAINBACK VALVE
Because the oil filter is typically located toward the middle or bottom of an engine, this rubber valve has a flap that blocks oil from draining back into the filter when the engine is off.
3. FILTER MEDIUM
The porous filter medium consists primarily of microscopic cellulose fibers along with synthetic fibers such as glass and polyester, which increase filtering efficiency and durability. The medium is also saturated with resin to give it strength and stiffness. Higher-grade filters have more synthetic fibers.
Folding the filter medium creates a greater total surface area. The number of pleats depends on the medium's thickness.
5. CENTER STEEL TUBE
The center tube provides structure and lets filtered oil return to the engine. The number, size, and position of the holes is key to ensuring that the oil flow is not restricted.
6. RELIEF VALVE
On a cold startup, oil can be too thick to filter. To prevent the engine from starving, the relief valve opens when the pressure builds enough to force the calibrated spring downward, allowing unfiltered oil into the center tube through the top.
7. END DISC
To keep unfiltered oil from leaking into the center tube, a fiber or metal end disc is bonded to each end of the filter medium. Some brands forgo discs and use a sealant to create solid ends.
A thin bit of metal that acts as a leaf spring, the retainer keeps the filter medium and end disc tight against the tapping plate.