If you’ve spent any time researching water tank installs for campervan builds, you’ve probably come across a wide mix of advice on fittings – especially around threaded connections.
One thing we’ve noticed popping up more frequently is the recommendation to use tapered thread fittings directly in plastic water tanks. It might seem like a snug, leak-proof solution at first, but it can actually cause problems over time.
Tapered threads are designed to tighten progressively as they’re screwed in, creating a seal by wedging tightly into the mating thread. That works well in materials designed to handle that pressure – like metal fittings.
However, most water tanks (especially in campervan builds) use parallel threads and are made from relatively soft plastics. When a tapered fitting is forced into a parallel threaded port, it creates outward pressure as it tightens.
Over time – or sometimes immediately – that pressure can:
Stress the threads
Cause small fractures
Lead to cracks around the tank opening
And unfortunately, once a tank is cracked it is often difficult (or impossible) to repair reliably.
Use fittings that match the tank’s thread type (usually parallel/BSPP)
Rely on appropriate seals like washers or O-rings rather than thread deformation
Avoid overtightening—tight enough to seal, not to force
We know a lot of advice online comes from personal experience, and what works in one setup might seem fine in another. But when it comes to water tanks, small differences in fittings can have big consequences down the line.
If you’re unsure, it’s always worth double-checking the thread type before installing—it can save a lot of hassle (and water damage!) later.
If you’ve come across other tips or experiences, feel free to share—it’s how we all learn and improve our builds.
