Report on ¼ x 28 Special Taper Grease Fittings

Kingfisher obtained 3 competitors’ samples of ¼ x 28 special taper grease fittings. These fittings appear to have been cold forged (as opposed to machined) and as such have rounded corners on the hexagon and a concave ball check face.

As the corners of the hexagon were rounded and not formed properly, it was difficult to measure the hexagon dimensions. The rounded corners may prove detrimental when using a socket to screw in the fitting. Most sockets and spanners rely on the corner part of the hexagon to screw them in.

Report on 1 4 x 28 Special Taper Grease Fittings
Rounded corners on grease fitting

The threads are rolled (not cut) which produces a slight undercut at the top of the thread. The fittings supplied had a shank diameter of 0.248″-0.249″.

Picture showing concave ball check face, also there is a score mark on side of the head which appears to have been caused during manufacture as the plating is undamaged. The ball check face should be flat to reduce the build up of contaminants on this surface which may get introduced into the fitting during greasing.

Also of note is the yellow colour of the plating has rubbed off on exposed surfaces.

The fittings have been hardened to 85-88 HR15N, but the lip appears to have been subsequently softened to make it easier to fold over to retain the spring. (The end of the fitting is discouloured suggesting heat has been applied) This could also soften the first threads which could affect the fittings’ ability to start forming a thread in a hole.

The fittings were grease tested, the opening pressures were 60, 70 and 240psi which is a wide range indicating inconsistent manufacture and 60/70psi is rather low for a hydraulic fitting.

The fittings passed the leakage test specified in SAE J534.
When the fittings were high-pressure grease tested at 8,000psi (which is the pressure that Kingfisher test at) both springs collapsed. This is a critical failure.

The other fitting was high-pressure tested to 5,000psi as per SAE J534. The spring also collapsed.

Concave ball poor manufacture
High pressure test results
High pressure test result failure

Picture of the grease fittings after high pressure testing showing the that the balls haven’t re-seated, the 2 fittings on the left were tested at 8000psi and the one on the right was tested at 5000psi.