Hydraulic Fittings - Metrification

Fig. 1:

Metric fastenings are being used increasingly in motor vehicle manufacture, but the use of some UNF threaded parts must be expected. Technicians must take note of warnings that dangers exist while UNF and metric threaded hydraulic parts continue to be used together.

UNF pipe nuts should not be fitted into metric parts, nor vice-versa. Experience indicates that it is not sate to rely upon the difference in thread size. Providing permanent identification of metric parts is not easy, but recognition has been assisted by the following means:

All metric pipe nuts, hose ends, unions and bleed screws are colored black. The hexagon area of pipe nuts is indented with the letter 'M'. Metric and UNF pipe nuts are slightly different in shape.


NOTE: In the illustrations on this page, 'A' indicates the metric type and 'B' the UNF type. The metric female nut is always used with strumpet flared pipe and the metric male nut is always used with a convex flared pipe (Fig. 1).

Fig. 2:

Metric ports in brake cylinders and calipers have no counter-bores; however, a few cylinders with UNF threads also have no counterbore. Therefore, all ports with counterbores are UNF, but ports not counterbored are most likely to be metric (Fig. 2).

The color of the protective plugs fitted to cylinder ports indicate the size and the type of the threads, but the primary function of the plugs is to protect the cylinder from contamination and is not intended as a positive means of identification. As an indication, the plug colors and thread sizes are:

color Size
Red 3/8 in. x 24 UNF
Green 7/16 in. x 20 UNF
Yellow 1/2 in. x 20 UNF
Pink 5/8 in. x 18 UNF
Black 10 x 1,0 mm
Grey 12 x 1,0 mm
Brown 14 x 1,5 mm


NOTE: In Fig. 1 and Fig. 2, this page, 'A' indicates the metric type and 'B' the UNF type.

Fig. 1:

Fig. 2:

Hose ends differ slightly between metric and UNF (Fig. 1). Gaskets are not used with metric hoses; the metric hose fitting seals against the bottom of the port and consequently there is a gap between the hexagonal face of the hose fitting and the cylinder (Fig. 2A). The UNF hose fitting is sealed on the brake cylinder or caliper face by a copper gasket (Fig. 2B).

Pipe sizes (outside diameter) for UNF are 3/16 in., 1/4 in. and 5/16 in. Metric pipe sizes are 4,75 mm, 6 mm and 8 mm. The 4,75 mm pipe is exactly the same diameter as 3/16 in. pipe. The 6 mm pipe is 0.014 in. smaller than 1/4 in. pipe. The 8 mm pipe is 0.002 in. larger than 5/16 in. pipe.

Convex flares for metric brake pipes have a different form than those on UNF brake pipes. When making pipes for metric equipment, metric pipe flaring tools must be used.

The greatest danger lies with the confusion of 10 mm and 3/8 in. UNF pipe nuts used for 3/16 in. (or 4,75 mm) pipe. The 3/8 in. UNF pipe nut or hose can be screwed into a 10 mm port, but is very slack and easily stripped. The thread engagement is very weak and cannot provide an adequate seal. The opposite condition, a 10 mm nut in a 3/8 in. port, will cause difficulty during fitting. The 10 mm nut will screw in for one or two turns and then seize. It has a crossed thread 'feel' and it is impossible to force the nut far enough to seal the pipe. With female pipe nuts the position is reversed. The other combinations are so different that there is no danger of confusion.