Usually the larger piston in your pump.
The piston is an essential component in a compressor pump. Whether it’s an oil-free or oil-bath design, it creates compression on each stroke. In an oil-free design, a Teflon-coated compression ring provides the seal between it and the cylinder sleeve, thereby creating compression; in an oil-bath design, there are compression rings and an oil-ring, like in an internal combustion engine. Replace if the top of the piston is pitted, cracked or warped.
This item fits the following models:
15-pc Thunderbolt Black Oxide Drill Bit Set
1/4" x 50 Ft Noodle Air Hose with Coupler and Plug