Coned & Threaded Hydraulic Fittings – a Primer

Dynex hydraulic pumps and valves have some of the highest pressure ratings in the industry. When operating at pressures above 8000 psi (550 bar) it’s important to select the appropriate outlet port configuration and use a fitting that’s suitable for your high pressure requirements. We created this overview to help answer some of our most frequently asked questions. 

Before we cover the distinctive features of coned & threaded connections, here’s a quick overview of the most commonly used connections.

  • SAE Straight Thread Connection — The port consists of a machined spotface surface, a tapered seal cavity, and a straight thread port. The fitting forms a seal by compressing the o-ring into the seal cavity. 
    — Dynex does not recommend S.A.E. straight thread ports for operation above 8000 psi (550 bar). 
  • British Standard Pipe Parallel (BSPP) Connection — The fitting forms a seal by compressing a flat elastomer o-ring onto a machined surface. There are several sealing methods for these ports.
    — Dynex does not recommend B.S.P.P. ports for operation above 10000 psi (700 bar).
  • SAE 4-Bolt Flange Connection — Flange connections are often used for higher flows requiring larger diameter tubing. It consists of an unthreaded port with four bolt holes on a machined face. A flanged head is bolted to this surface. A seal is formed by an o-ring in a groove on the underside of the flange head. 
    — Pressure capability is determined by the fitting used in the flange.



Commonly found under brand names such as Autoclave Medium Pressure, Butech M/P, or HIP-High Pressure Equipment Co., this type of fitting is capable of withstanding extremely high pressures, and is an excellent choice for components with pressure requirements above 8000 psi (550 bar).

These fittings use a metal-to-metal seal with an interference fit. They do not not require an o-ring. In this design, a tube is forced into a coned surface. A collar is threaded onto the tube and a gland nut holds the sleeve and tubing against the coned surface.



Coned & Threaded Port

A unique feature of this fitting is its use of a weep hole.

Visible on the outside of the pump cover (or fitting), this small hole acts as an indicator of any abnormal leakage caused by system conditions (i.e., excessive pressure) or improper installation. 

Users of these fittings who observe leakage coming from the weep hole should repair the connection or replace the fitting immediately.



Although Dynex components can be specified with a wide variety of ports, we do not supply the fittings, so it’s important that you consult the fitting manufacturer to ensure your selected fittings are rated for use with the anticipated maximum system operating pressure.

By having a full understanding of the possible port choices and selecting appropriate fittings, you can avoid potential pitfalls and design a robust system that will perform for years.



For a more detailed explanation of the distinctive features of the coned & threaded connections as well as a demonstration of installation of this type of fitting, please see our video: 


We hope that you found this primer helpful, and of course we would be happy to help you with any further question or concerns. Feel free to contact us today.

Tip Ninja

Applying his legendary skills toward answering tough questions, defeating difficult hydraulic related problems, and sharing his vast knowledge of Dynex hydraulic products with the world.