Safe — Secure — Sexy

Pro-Werks Easy Turn Billet Aluminum Fill Caps & Bungs

Pro-Werks’ signature Easy Turn Filler Caps will turn heads while working on your race car or hot rod. Once you’ve added one of these jewelry-quality assemblies, you’ll want to install one at every location possible. These lightweight, non-vented fill caps feature deep finger notches for a positive and comfortable grip — another Pro-Werks innovation appreciated by everyone who uses them.

We start with American melt 6061-T6 billet aluminum, then precision CNC machine to exacting standards. This design uses a “radial squeeze” action on the Buna O-ring for a secure, leak-resistant seal. Precision machined quick-start coarse threads make using these caps a breeze. Available at The Chassis Shop.

Available in 4 Brilliance anodized colors: Black, Red, Blue, or Micro-Polished aluminum — the finishes are impeccable!

Popular uses: Custom-fabricated tanks, fuel cells, gas tanks, rear end housings, valve covers, overflow tanks, intercooler ice boxes, ballast tanks, inspection viewing ports.

Complete Fill Cap Assemblies — Cap + Bung, Ready to Install

Each complete assembly includes your choice of cap size, bung material, bung height, and cap color. The stepped shoulder at the base of the weld bung serves as a positioning locator — holds the bung in place while welding in a flat surface or tube end. Ample cross section in the weld land reduces warping during welding.

Cap sizes available across assemblies: 1-5/8", 2", 2-3/4", and 4-1/4" — with matching bung and O-ring.

• Fill Cap with Aluminum Weld Bung — 4 Cap Sizes, 3 Bung Heights, 4 Colors — Pro-Werks Easy Turn Filler Cap with Aluminum Weld Bung — 6061-T6 aluminum bung, weldable, stepped shoulder locator, Made in USA
• Fill Cap with Steel Weld Bung — 4 Cap Sizes, 3 Bung Heights, 4 Colors — Pro-Werks Easy Turn Filler Cap with Steel Weld Bung — mild steel bung, weld to steel tanks and housings, Made in USA
• Fill Cap with Stainless Steel Weld Bung — 3 Cap Sizes, 2 Bung Heights, 4 Colors — Pro-Werks Easy Turn Filler Cap with Stainless Steel Weld Bung — stainless steel bung, corrosion resistant, Made in USA
• Fill Cap with Aluminum Bolt-On Bung — 3 Cap Sizes, No Welding Required, 4 Colors — Pro-Werks Easy Turn Filler Cap with Aluminum Bolt-On Bung — ideal for composites, thin gauge materials, or finished surfaces; Buna O-ring base seal, hardware included, Made in USA

Fuel Cell Fill Cap Assemblies — Direct Replacement for Flip-Latch & Bail Caps

Say goodbye to pry bars. Pro-Werks fuel cell fill cap assemblies are a simple bolt-on replacement for the original manufacturer-provided flip-latch or bail-style filler caps on most popular racing fuel cells. Compatible with aluminum, plastic, and steel fabricated projects. Includes stainless mounting hardware and Buna flange gasket. The cap features a starter nose to lead the cap into position accurately — a very nice feature when you’re taking care of business in the dark. Built-in lanyard boss accepts the optional lanyard chain kit.

Compatible fuel cell brands include: Jaz, RCI, Fuel Safe, ATL, Summit Racing, Rhodes Race Cars, Tim McAmis, Jegs, Speedway Motors, Raceworks, Pyrotect, Aeromotive, RJS, and others (verify dimensions for final compatibility).

• 6-Bolt Fuel Cell Bung with 2-3/4" Fill Cap — 2-1/4-10 Thread, 6 on 4.438" Bolt Circle, 3/4" Bung Height, 4 Colors — Pro-Werks Easy Turn Fuel Cell Filler Cap with 6-Bolt Aluminum Bung — with lanyard boss, bung in Silver or Brilliance Black, Made in USA
• 12-Bolt Fuel Cell Bung with 4-1/4" Fill Cap — 3-3/4-8 Thread, 12 on 5.343" Bolt Circle, 7/8" Bung Height, 4 Colors — Pro-Werks Easy Turn Fuel Cell Filler Cap with 12-Bolt Aluminum Bung — with lanyard boss, bung in Silver or Brilliance Black, Made in USA

Remote Mount Fill Cap Assemblies — Flush Panel Filling Point with Hose to Tank

Remote fluid filling made easy. These unique assemblies provide an attractive flush mount solution for locating a filling point in a panel, allowing fluid transfer through a hose to the tank. The integrated hose nipple features a retention barb for positive sealing of the hose connection. A work of art for any show-quality installation.

• Remote Mount Bung — 2" Cap / 2" Hose or 2-3/4" Cap / 2" Hose, 4-Bolt, 4 Colors — Pro-Werks Easy Turn Filler Cap with Aluminum Remote Mount Bung — silver anodized bung, hardware included, Made in USA
• Recessed Remote Mount Bung — 2-3/4" Cap / 2" Hose, 6-Bolt on 5.125" Bolt Circle, 4 Colors — Pro-Werks Easy Turn Filler Cap with Aluminum Recessed Remote Mount Bung — deeper recess for flush panel installations, Made in USA

View fill cap sizes and order the cap only with O-ring

Fill Cap Only — Cap with O-Ring, No Bung

Purchase the lightweight filler cap separately to pair with any Pro-Werks bung or to replace an existing cap. Available in 4 sizes and 4 colors. The 2-3/4" and 4-1/4" caps include a built-in lanyard boss.

• 1-5/8" Cap — 1-1/4-12 Thread, 1.06 oz., No Lanyard Boss
• 2" Cap — 1-1/2-12 Thread, 1.62 oz., No Lanyard Boss
• 2-3/4" Cap — 2-1/4-10 Thread, 3.32 oz., With Lanyard Boss
• 4-1/4" Cap — 3-3/4-8 Thread, 7.86 oz., With Lanyard Boss

Pro-Werks Easy Turn Filler Cap with O-Ring — 4 sizes, 4 colors, Buna O-ring included, Made in USA

Bungs Only — Weld-In & Bolt-On, Sold Separately

Need just the bung? Pro-Werks offers every bung style separately — in aluminum, steel, and stainless steel — weld-in and bolt-on. All bungs are precision CNC machined and sized for the “radially squeezed” O-ring standard of the Pro-Werks Easy Turn Fill Cap.

• Weld-On Fill Cap Bung — 4 Cap Sizes, Aluminum / Steel / Stainless, Multiple Heights — Pro-Werks Weld-On Fill Cap Bung — stepped shoulder positioning locator, ample weld land cross section, Made in USA
• Bolt-On Aluminum Filler Cap Bung — 3 Sizes (1-5/8", 2", 2-3/4"), No Welding Required — Pro-Werks Bolt-On Aluminum Filler Cap Bung — silver anodized, Buna O-ring base seal, hardware included, Made in USA
• Bolt-On Fuel Cell Aluminum Filler Cap Bung — 6-Bolt & 12-Bolt Fuel Cell Patterns — Pro-Werks Bolt-On Fuel Cell Aluminum Filler Cap Bung — direct replacement for fuel cell flip-latch bungs, Made in USA

Click image to learn more about Pro-Werks

Proudly Made in the USA. You can always count on Pro-Werks to do it right!

PRO Tips & FAQs

How tight should I tighten a fill cap? It is not necessary to bottom the fill cap into the bung — just adequately seat the O-ring. The radial squeeze action creates a positive seal and helps prevent the cap from coming loose.

O-ring lubricant: A light coating of Dielectric grease applied periodically works well as an O-ring lubricant.

Welding your bung: Apply several small, equally spaced tacks and preheat before welding. Keep the weld small to minimize distortion. If threads are tight after welding, apply a small amount of fine grit lapping compound to the threads and turn the cap back and forth until the desired fit is achieved. Thoroughly clean both sets of threads before use.

PRO TIP — How to Calculate Tank Capacity: Divide the total cubic inch volume by 231. Example: a rectangular tank 12"(L) × 12"(W) × 10"(H) = 1,440 cubic inches ÷ 231 = 6.23 gallons. For irregular shapes, calculate each section separately and add or subtract to arrive at the total.

Complete Your Tank Build

• Weld Plumbing Bungs — AN & NPT thread weld bungs in aluminum, steel & stainless for fuel and dry sump plumbing
• Bar, Bottle & Universal Mounts — tank and bottle mounting solutions for your chassis build
• Access Door Kits — aeronautical-style aluminum access doors for quick access to tanks and fill points behind body panels

Note: Welding should always be performed by a qualified welder.