1-5/8 in. BLACK FILL CAP WITH O-RING1-5/8 in. BLACK FILL CAP WITH O-RINGCap Installed with Weld-On BungCap Installed with Bolt-On BungProudly Made In The USA
Quantity Your Price
1 $40.99



1-1/4-12 Straight Thread
Includes O-Ring

Made in the USA

(Cap Replacement O-Ring is C73-749)

These lightweight, signature Pro-werk non-vented Fill Caps feature deep finger notches for a positive and comfortable grip, this design is another Pro-werks innovation, appreciated by everyone who uses them. The 6061-T6 American melt billet aluminum is CNC machined for accuracy, and is available in your choice of either brilliance anodized black, red, blue, or silver, as well as micro-polished aluminum - all of these finishes are impeccable.

Great caps. Great colors.

This design uses a radial squeeze action on a Buna O-ring for a secure seal. Precision machined quick-start coarse threads make using these caps a breeze.

The large selection of Pro-werks’ mounting bungs are crafted from steel, aluminum or stainless steel and available in both weld-in and bolt-on styles.

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.

Popular uses for Easy Turn Fill Caps include:

  • Custom-fabricated tanks
  • Fuel cells
  • Rear End housings
  • Valve covers
  • Overflow tanks
  • Ice boxes for intercoolers
  • Ballast tanks
  • Inspection viewing ports

Be sure to check out the entire line up - Billet aluminum Easy Turn Fill Caps are a Pro-werks specialty.   Watch the Video below to learn more !

Proudly made here in the USA.

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


How much do I need to tighten a fill cap?


It is not necessary to bottom the fill cap into the receiving bung. Adequately seating the O-ring to make a seal is all that is required. The radial squeeze action on the O-ring creates a strong seal and helps to keep the cap from coming loose.



What should I use to lubricate a Buna O-ring?


A light coating of Dielectric grease works very well as O-ring lubricant.



What is Buna and is it compatible with my fluid?


Buna is a universal composition synthetic rubber, a standard of plumbing related products because it is compatible with many fluids. Due to the complex chemistry of modern day performance oils and fuels, you may find it necessary to replace Buna components with a material that is compatible with the unique chemistry of your specific fluids.


1-5/8 in. Fill Cap: C73-740
Inside Grip Diameter: 1.625 in.
Outside Grip Diameter: 1.970 in.
Height: 0.50 in.
Height: 0.312 in.
Thread: 1-1/4-12 Thread