1-5/8 Inch Fill Cap with Bolt-On Bung

1-1/4-12 Straight Thread
Cap Weight: 1.06 oz.
4 on 2 in. Dia. Bolt Circle
9/16 in. Bung Height
(Cap Replacement O-Ring is C73-749 & Bung Replacement O-Ring is C73-748)

2 Inch Fill Cap with Bolt-On Bung

1-1/2-12 Straight Thread
Cap Weight: 1.62 oz.
4 on 2.25 in. Dia. Bolt Circle
9/16 in. Bung Height
(Cap Replacement O-Ring is C73-769 & Bung Replacement O-Ring is C73-768)

2-3/4 Inch Fill Cap with Bolt-On Bung

2-1/4-10 Straight Thread
Cap Weight: 3.32 oz.
5 on 3.125 in. Dia. Bolt Circle
11/16 in. Bung Height
(Cap Replacement O-Ring is C73-788-A & Bung Replacement O-Ring is C73-789)


The lightweight, non-vented cap features positive grip finger notches that are easy to get a hold of. Prime 6061-T6 billet aluminum is precision CNC machined into a lightweight, attractively designed cap; featuring quick-start coarse threads. Each screw on Cap is fitted with a Buna O-ring; positive sealing and "radially squeezed" to hold the cap secure.

Bolt-On Bung and Filler Cap Assembly

The Pro-Werks Bolt-on filler cap assemblies require no welding; this produces a very clean finished installation every time. The base of the Silver anodized aluminum bung is machined to accept the provided Buna O-ring for a positive seal, mounting hardware is included.

There are many very useful applications; ideal for composites when welding isn't an option and thin gauge materials that are too difficult to weld, or any installation that already has a sensitive protective finish applied.

These are also a favorite of engine builders for filling oil in a valve cover.

Leak-Proof O-Ring Seal

Great Caps - Great Colors

To compliment the color coordination of your project, the caps are finished off in Pro-Werks Brilliance anodized colors in your choice of Black, Red, Blue, or beautifully Micro-Polished aluminum – the finishes are impeccable!

The Right Size Matters

Easy Turn Fill Caps come in many sizes and are packaged as complete assemblies with your choice of one of the many versatile mounting options, or each component is also available separately.

Proudly Made in the USA

Both the cap and bung are precision CNC machined light weight billet aluminum, right here in the USA.

Be sure to check out the entire line up - Billet aluminum Easy Turn Fill Caps are a Pro-werks specialty.

You can always count on Pro-Werks to do it right!

Customer Comments:

"I use your bolt on caps and bungs on my carbon fiber Cummins common rail valve covers. Love the quality. Thank you."
– Randy Davis, Owner of OldSkoolFab, Inc.

"I upgraded my fuel cell to an Easy Turn fill cap assembly. It wasn't long until all my friends did the same. Great product!"
– As shared with us at the PRI Show

Frequently Asked Questions:

Q: How much do I need to tighten a fill cap?
A: It is not necessary to bottom the fill cap into the bung−just adequately seat the O-ring. The radial squeeze action on the O-ring creates a positive seal and helps prevent the cap from coming loose.

Q: What should I use to lubricate the Buna O-ring?
A: Applied periodically, a light coating of Dielectric grease has proven to work well as an O-ring lubricant.

Q: Is Buna rubber compatible with my fluid?
A: Buna is a synthetic rubber compound suitable for use with many fluids. Buna components may need to be replaced by a material compatible with the specific chemistry of some fluids.

PRO FABRICATION TIP: How to Calculate Tank Capacity

You can calculate how many gallons of fluid a tank will hold by dividing the total cubic inch volume by 231. As an example a basic rectangular shaped tank measuring 12"(L) x 12"(W) x 10"(H) = 1440 cubic inches. The next step is to divide the total volume of 1440 cubic inches by 231 = 6.23 gallons.

If your tank is an irregular shape, or has unique features, calculate the cubic inch volume of those areas also, then add or subtract the values to arrive at the total cubic inch volume of your custom fabricated tank.