Remove drain plug, Cessna Part Number AN814-3DL, from the fuel selector valve, and replace with CAV-180 (MT-1072-77) or CAV-180H-4 (CM-172-77) (*Model: C-172N Serial Nos.: 17261445, 17267585 through 17269309 (1977 Model), 17261578, 17269310 through 17271034 (1978 Model), \t(Except 17270050), 17271035 through 17272884 (1979 Model), 17270050, 17272885 through 17274009 (1980 Model)) View PDF on drs.faa.gov ↗

The STC PDF is authoritative: installation instructions, ICA requirements and limitations.