Boat Deck Filler Cap – 38 mm Thread

⚓ Deck Filler Cap – 38 mm Thread 
For Fuel / Water / Waste Tank Filler Necks

This is a 3D-printable replacement cap for boat deck filler fittings with a 38 mm thread. 
It can be used for diesel, water, or black water / waste tank filler necks, depending on your fitting and chosen material.

I designed and modeled this cap from scratch for my own boat, after being inspired by a similar 47 mm deck filler cap design.

🧰 Features

• Designed for deck filler fittings with a 38 mm thread
• Suitable for fuel, water, or waste deck fillers, if the dimensions match
• Winch-handle socket/recess for opening and closing the cap
• Uses an O-ring for a tighter, splash-resistant seal
• Small hole at the bottom for attaching a retaining chain or cord, so the cap does not fall overboard
• Simple printable replacement part for older or hard-to-find marine filler caps

🧵 O-ring

I use a 38 mm FKM O-ring with 3 mm thickness.

A 38 × 2.5 mm O-ring should also work, depending on the exact deck filler fitting and how tight you want the cap to close.

For diesel or fuel applications, I recommend using FKM / Viton rather than a generic rubber O-ring.

🖨️ Print Settings

Recommended material: ASA 
PETG may also work for temporary or less demanding use, but ASA is better for long-term outdoor marine use because of its UV and temperature resistance.

Recommended settings:

• Layer height for threaded area: 0.08 mm
• Print orientation: upside down
• Infill: 20–40%
• Walls/perimeters: at least 4
• Top/bottom layers: enough to make the part solid and watertight
• Supports: Tree Slim Support
• Support threshold: 15%
• Brim: inner brim recommended for better build plate adhesion

The cap should be printed upside down. 
It has a winch-handle socket/recess, and I use Tree Slim Support with a 15% threshold to prevent the bottom of this socket from sagging during printing.

I also use an inner brim to improve adhesion to the build plate, especially when printing in ASA.

⚠️ Notes

Please measure your own deck filler before printing. Marine filler caps and threads are not always standardized, even when the nominal diameter looks the same.

This is a DIY 3D-printed replacement part, not a certified marine safety component. 
Before regular use, test the fit carefully, check that the O-ring seals correctly, and make sure the cap cannot loosen by vibration or movement.

PLA is not recommended for outdoor, marine, or fuel-related use. It is only suitable for quick prototypes or test prints.

Perfect for boat owners who need a practical printable replacement cap for a 38 mm deck filler without immediately buying a new fitting.