FTA-C-722L Locking Diesel Fuel Cap
Safety chain and crossbar included. Designed for use only on vehicles equipped with properly vented fuel systems. Non-Vented Cap, 3.0"-8 UNNS Threads. Measures 2-7/8" across threads.
This is the cap used on Mack and Volvos with the Low Sulfur PNG Style Tank mfg # 90-04776-01
Checking your original non-locking cap you will see one of these numbers on the inside if it is a PNG tank: 21118646, 16MF333M or 90-03948-01.
For Mack and Volvo trucks with 3-3/8" fill neck see FTA-C-69
Use the key to unlock the cap (the key won't come out when it is unlocked) then screw the cap onto the filler neck and turn the key to lock it. When you turn the key the cam will hit the side wall of the fill neck and bite in. It is very important not to over tighten the cap... Do not over tighten the cap when you screw it on.
