A problem that arises when tightening a hydraulic fitting is need for a washer that does not crush but seals under pressure and remains sealed at high temperatures. That’s what Dowty washers do. It’s a bonded design that uses a rubber face to compress on the port and metal ring to take the torque load.
Choose the right size and seal lasts. Choose the wrong size and you’ll have problems or destroyed threads as you battle leaks. There are three primary thread families found in everyday applications, and their sizes is listed in this chart. For mobile and most industrial equipment, find the BSP sizes here. For manifold blocks and other European machinery, they’re metric. And finally, brake and fuel line assemblies often uses AN sizes.
How to Use Dowty Washers Correctly
The columns show the maximum pressure rating, then the ID (inner diameter) of the thread to clear, and the OD (outer diameter). Why do these matter? Your 700-bar-rated seal won’t hold up under pressure of a bigger thread where 200 bar would contain it.
Diameter is one thing, rubber material selection are another. Nitrile is flexible as low as -40 degrees and is compatible with most hydraulic oils. EPDM will not swell from brake fluid, steam, or water. Viton is resistant to harsh chemicals and heat up to 150+ degrees. Elastomers are matched by thread sizes (see chart). However, you should of base your decision more on the temperature and type of fluid in your system than what the chart says.
Torque is where most leaks start. Too little torque means you won’t seat the rubber. Too much will make the rubber squeeze out beyond metal ring. Torque specs goes up as thread size goes up because there’s more sealing surface in bigger fittings. You can lube the threads lightly with some oil before torquing them down. This keeps them from galling and helps you get an accurate torque reading.
If there is no flat spot on the washer, no cut through of the rubber bond, then Dowty washers can be reused. That means they save money compared to one-time use copper crush washer over time as long as you check them prior to reuse. Store extra washers in closed bags away from direct sunlight and ozone so they can still form a good seal many years later.
The real advantage appears once you stop thinking of these seals as just another washer. You can make any threaded port into a repeatable, dependable face seal with no additional tools or fluids required. Just match the rubber to the fluid, match size to the thread, and torque it to spec. Do those three things and the fitting will remain dry.
