Lost an O-Ring or Gasket Aboard? Print a TPU Stand-In That Fits
It always happens at the worst moment. You unscrew a strainer lid, a deck plate or a pump cover, and the O-ring underneath is flattened, cracked, or simply gone over the side. The chandlery is closed, the nearest marine store is two islands away, and the part number is worn off. On a boat, a missing seal turns a five-minute job into a weekend of dripping, weeping and makeshift fixes with tape.
This guide covers how to measure a seal you can't identify, how to print a flexible stand-in that actually fits, and where a printed seal makes sense aboard (and where it doesn't).
Why seals go missing, and why the right one is hard to find
O-rings and flat gaskets live hard lives at sea. UV hardens them, heat cycles take away their memory, salt crystals cut them, and every time you open a lid they get a little more flattened. Most of them are also small, black and easy to drop into the bilge.
Finding a replacement is harder than it sounds:
- Odd sizes. Older and imported equipment often uses metric, imperial or proprietary sizes that don't match the kit in your spares box.
- Non-round shapes. Square and rectangular section gaskets (lids, hatches, inspection ports) are rarely stocked as loose parts.
- Discontinued gear. The pump or filter housing may still work perfectly, but the maker no longer sells the seal kit.
That's where a parametric, printable seal earns its place: you type in the dimensions you measured, and you get a seal in exactly that size, in the time it takes to print.
The parametric O-ring / gasket file
Our free Parametric O-Ring / Gasket generates a seal from three profiles:
| Profile | What you set | Typical use aboard |
|---|---|---|
| Round | Section (cord) Ø + ring inner Ø | Classic O-ring grooves: strainer lids, filter bowls, threaded caps |
| Square | Side + ring inner Ø | Face seals under caps and lids with a flat seat |
| Rectangular | Both sides + ring inner Ø | Flat gaskets under covers, spacers, anti-rattle rings |
One detail that trips people up: in this file the "O-ring Ø" is the INNER diameter, the hole the ring sits on, not the outside. That matches how O-rings are normally specified (inner diameter × cross-section), so it's also what you'll measure.
How to measure a seal you can't identify
If you still have the old seal, even a squashed one, it's a useful clue but not a reliable template, because it will have stretched or flattened in service. Measure the groove and the part it seals against instead:
- Inner diameter. Measure the diameter of the spigot, boss or groove bottom the ring sits on. That is your inner Ø. Round to the nearest 0.5 mm.
- Groove width. Measure across the groove with the inside jaws of a caliper. The seal section should be close to this, usually a little smaller so the rubber has somewhere to go when squeezed.
- Groove depth. Use the depth rod of the caliper. For a static seal, the section is normally slightly larger than the depth, so the seal is compressed when the lid is closed. If the section is equal to or smaller than the depth, it won't seal.
- Flat gaskets. For a lid or cover with no groove, measure the inner and outer diameters of the flat seat. The difference divided by two is your rectangular width; choose a thickness that fills the gap with some squeeze.
If your first print is slightly off, that's fine. Seals are small and quick to print, so printing two or three variants 0.5 mm apart and test-fitting them is the fastest route to a good one. Our guide to measuring hoses and threads for printed fittings covers caliper technique in more detail.
Material and print settings
The product page recommends TPU 85A: the softer the filament, the better it conforms to small surface imperfections and the better it seals. PETG or TPE can be used when you want a rigid ring, for example as a spacer rather than a seal.
Flexible filaments behave differently from PETG or ASA, so a few general tips help:
- Print slowly and use a direct-drive extruder if you have one. Soft TPU buckles in long Bowden tubes.
- Print solid. A seal should be dense throughout. Enough perimeters and high infill avoid internal voids that could let water track through.
- Print flat on the bed. The seal then has continuous perimeters around its circumference, with no layer lines crossing the sealing face.
- Clean up the seam. Place the Z-seam where it won't sit on the sealing surface if your slicer allows it, and trim any blobs with a sharp blade.
For more on flexible filaments and where they work aboard, see TPU flexible prints for chafe guards, gaskets and bumpers. If you're sealing a printed part rather than printing the seal itself, our guide to PTFE tape, epoxy and TPU gaskets is the companion read.
Installation and use
- Clean the seat and groove. Old rubber residue, salt and grit are the usual cause of leaks, whatever the seal is made of.
- Lubricate lightly with a compatible grease (silicone grease is common for water systems) so the ring seats without twisting.
- Don't over-tighten. Close the lid until the seal is compressed evenly. Cranking down harder only deforms TPU and can crack a plastic housing.
- Check it after the first use. Look for weeping at the joint after a few hours of use, and again after a day in the sun.
The product page puts it plainly: a printed O-ring does not match the mechanical properties of an industrial moulded one. Treat it as a temporary or emergency seal that keeps a system running until you can fit the proper part, and put the correct size on your shopping list once you've measured it.
Parts that pair with a printed seal
Several of our parametric designs have an O-ring seat built in, and the gasket file is the natural way to fill it with a seal of exactly the right size:
- Screw-out Deck Inspection Plate: O-ring seat on the top face under the lid, never on the thread.
- Tank Cap with Winch Socket: O-ring face seal under the head, with adjustable position.
- BSP Blanking Plug / Cap: optional O-ring face seat, free to download.
You'll find these and more fittings in the Plumbing & Pumps collection, and other no-cost files in For Free. Ready to size yours? Open the O-ring / gasket customiser, enter your measurements and download the STL.
Safety note
3D-printed parts are not certified marine equipment. A printed seal is a stop-gap, not a replacement for a moulded O-ring in a critical system. Do not rely on one below the waterline (seacocks, through-hulls, raw-water strainers where failure could flood the boat), in fuel or gas systems, or anywhere a leak could put people or the boat at risk. Check material compatibility with whatever the seal will contact, inspect it regularly, and fit the proper part as soon as you can.
Questions about sizing or a seal you can't identify? Email us at info@marinelab3d.com.
