Frequently Asked Questions About Mack CV/GU Truck Fuel Tank Caps

What size fuel tank cap do I need for my Mack CV or GU truck?

Most Mack CV and GU trucks use standard 4-inch diameter fuel tank caps with either 8 or 11 thread pitch. To determine your exact size, measure the outside diameter of your fuel tank opening and count the number of threads per inch. The CV series typically uses 4-inch caps with 8 TPI (threads per inch), while GU models may vary between 8 and 11 TPI depending on the year and tank manufacturer.

Will these fuel caps work with both aluminum and steel fuel tanks?

Yes, our fuel tank caps are compatible with both aluminum and steel tanks. However, the key factor is matching the thread pitch and diameter rather than the tank material. Most caps feature zinc-plated steel or chrome-plated construction that resists corrosion on either tank type. For aluminum tanks, ensure the cap has a proper gasket seal to prevent galvanic corrosion at the contact points.

Do I need a vented or non-vented fuel cap for my Mack truck?

This depends on your truck's fuel system design. Trucks manufactured after 2010 typically require non-vented caps due to evaporative emission control systems. Earlier models (pre-2010) often use vented caps to prevent vacuum buildup in the tank. Check your owner's manual or look for a separate tank vent line - if present, you likely need a non-vented cap. When in doubt, match the venting style of your existing cap.

How do I prevent fuel cap theft on my Mack CV or GU?

Locking fuel caps are your best defense against theft and fuel contamination. We offer keyed-alike sets that use the same key for multiple tanks, reducing key management hassles. Look for caps with reinforced locking mechanisms and pick-resistant cylinders. Some drivers also add tethers or chains to prevent loss if someone forgets to secure the cap after fueling.

Why does my new fuel cap leak or not seal properly?

Poor sealing usually results from three issues: wrong thread pitch, damaged tank threads, or worn gaskets. First, verify you have the correct thread specification (4-inch with either 8 or 11 TPI). Inspect your tank opening for damaged or crossed threads - these may need chasing with a thread repair tool. Always replace the rubber gasket when installing a new cap, and hand-tighten until the gasket compresses evenly. Over-tightening can damage the gasket and cause leaks.