Reinforced exhaust valves Ford Sierra Escort RS Cosworth N5B

Ford engines covered

The N5B is the Cosworth 2 litre, 16-valve, twin overhead camshaft four of the Sierra RS Cosworth and Escort RS Cosworth.

Its 8 exhaust valves, with a 7.95 mm stem, work in the hottest gases of the engine; tuning loads them further, in temperature as well as in stress.

FERREA reinforced exhaust valves

On a turbocharged engine, the metal of the exhaust head goes beyond 1,000 °C, against 760 to 870 °C on a naturally aspirated engine. The seat removes about 75 % of the heat and the guide about 20 %. An EV8 family steel holds 550 MPa at 600 °C and only 300 MPa at 800 °C: beyond that, the head fatigues, distorts and loses its seal.

FERREA offers its 6000 series, in EV8 stainless steel, at standard size and +1 mm.

With the +1 mm oversize, the head goes from 31.15 to 32.15 mm: burnt gases leave faster, with a quoted gain of 10 to 12 cfm per millimetre of head, and the 8 exhaust seats are machined to the new diameter.

Exhaust valve technical specifications

The F6746 is standard size, the F6748 +1 mm; both belong to the 6000 series.

Valve fitting

Before refitting the springs, check that the collets suit the groove of the chosen valves.

At 31.15 mm the seats stay as they are; with a 32.15 mm head the 8 exhaust seats are machined to the new size.

Both part numbers have a 7.95 mm stem and a 105.85 mm length.

Technical guide

Our brand

FERREA, founded in 1978, makes its valves in Florida and Argentina. For the N5B it offers its 6000 series at standard size and +1 mm.

How to choose

Size separates the two versions, both in EV8 stainless steel:

When reinforced valves become necessary

• Engine tuning with more power
• More aggressive camshafts
• Historic rally and track
• Cylinder head rebuild with worn exhaust valves

Vehicle compatibility

OEM part number (exhaust valve):

Don't forget the inlet valves

An N5B cylinder head on the bench is the right time to replace the inlet valves and the valve stem seals too.