Unused Port on a Manifold or Fitting? Print a BSP Blanking Plug or Cap That Seals
Every boat plumbing job ends up with a port that isn't used. A manifold with four outlets when you only need three. A tee left over after a hose was rerouted. A pump fitting you removed during a refit. Leave it open and you get a leak or an air path. Replace it with a brass plug and you pay for a part that costs more than the time it saves. This guide covers a simpler fix: a blanking plug or cap printed to the exact BSP size of the port.
The problem: ports that need closing
Common places you will meet an open BSP port on board:
- The spare outlet on a water manifold (see our guide to printing a BSP manifold).
- A female socket on a coupling or tee after you changed the plumbing layout.
- A male nipple or stub you want to protect over the winter.
- A test or bleed port you want sealed once the system is commissioned.
In each case the job is the same: a threaded part that screws in or on, seats against something, and stops water.
Why a printed plug or cap makes sense
Blanking plugs are low-stress parts. They carry pressure but no mechanical load, they are small, and they print in a few minutes. Because BSP sizes are standard, a parametric file lets you pick exactly the thread you need instead of hunting for an odd size at a chandlery. They also double as spares: a few in the toolbox mean you can cap a failed branch line in seconds. For longer-term winter protection, see Winterizing: 3D-Printed Caps and Plugs to Seal Your Boat.
The protagonist: the BSP Blanking Plug / Cap
The BSP Blanking Plug / Cap is a free parametric design from MarineLab3D. You choose the options in the customiser, preview the part in 3D and download the STL. It comes in two forms:
- Male plug: a solid male BSP thread with a wrench hex. It screws into a female port, such as a manifold outlet.
- Female cap: a closed hex cap with an internal thread. It screws over a male port or nipple.
Other options listed on the product page: BSP sizes from 1/8" to 1 1/2" (ISO 228) and an optional O-ring face seat.
What to measure before you customise
You only need to answer three questions:
- Is the port male or female? A female port takes the male plug; a male stub takes the female cap.
- What BSP size is it? Common sizes are 1/4", 3/8", 1/2" and 3/4". If the thread is unmarked, measure the outside diameter of a male thread with calipers and compare it with a BSP chart. Our guide to BSP vs NPT threads explains how to avoid mixing standards.
- Do you want an O-ring seat? If the mating part has a flat face, an O-ring face seat gives a second sealing surface in addition to the thread.
Material and print settings
The product page recommends ASA or PETG, with PA612-CF for fuel or higher-pressure use. For anything holding water pressure, follow the pressure guidance from the listing:
- Print in PETG (not ASA) for better layer sealing.
- Use solid walls: enough perimeters to fill the wall thickness completely, rather than relying on infill.
- Set the Z-seam to scarf joint, keep part-cooling fan low and raise flow by 2-3%.
- Orient the thread and seam axis vertically.
Most onboard plumbing runs at about 3 bar maximum. At that level these settings plus a generous amount of PTFE tape or thread sealant are enough. Always pressure-test at roughly twice working pressure before relying on the part. For more on sealing methods, see Sealing 3D Prints: PTFE Tape, Epoxy and TPU Gaskets.
Fitting and using it
Wrap the male thread with PTFE tape (or use an O-ring on the face seat), thread the plug in by hand, then snug it with a spanner on the hex. Do not over-tighten: plastic threads fail by cracking, not by slipping. For a female cap, the same rule applies. After fitting, pressurise the system slowly and check for weeping at the joint over a few minutes, then again after a day.
One practical tip: print a small batch in different sizes at once and label them in a bag. When a hose bursts at sea, you will be glad to have a cap that fits.
Related designs
Blanking plugs work best as part of a small BSP kit. These designs pair with the plug:
- BSP Manifold Body, where the spare outlets get capped.
- BSP Hex Nipple (Double Male), a double male fitting that takes a female cap.
- BSP Female Coupling / Reducing Socket, which takes a male plug.
Browse the full range in the Plumbing & Pumps collection.
Safety note
Printed parts are not certified marine equipment. Check the material and suitability before use, especially below the waterline, on fuel systems or in any critical application. Do not use printed plugs as the only closure on a through-hull or any opening where failure could sink the boat; use a certified bung or valve and treat the printed part as a secondary or non-critical closure.
Questions about a size or a custom port? Write to info@marinelab3d.com.
