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

The N5B in the Sierra and Escort RS Cosworth is a forced-induction engine: its 8 exhaust valves send to the turbo gases whose temperature rises with boost. Their head metal goes past 1,000 °C there, where a naturally aspirated unit would stay around 760 to 870 °C. The seat drains about 75 % of it, whereas 20 % goes via the guide. EV8-family steel, used as the reference, offers 550 MPa at 600 °C and then only 300 MPa at 800 °C: the head then fatigues, loses its shape and lets gases leak.

FERREA makes both valves in its 6000 series, in EV8 stainless steel, a grade designed to withstand exhaust temperatures over time. The original size and the enlarged version share the 7.95 mm stem and 105.85 mm length of the Cosworth layout.

In the +1 mm version, the head diameter moves from 31.15 to 32.15 mm: each added millimetre frees about 10 to 12 cfm, the exhaust clears sooner and the next fresh charge enters better. The trade-off: all 8 exhaust seats are cut to 32.15 mm.

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 Racing Components takes its name from Horacio Ferrea, who set it up in 1978 alongside Daniel Urrutia. From Florida and Argentina, it machines stainless steel, titanium and nickel-rich alloys into valves that compete in NASCAR, drag and endurance racing on several continents.

How to choose

On the N5B, FERREA offers two EV8 stainless valves from the same 6000 series, with a 7.95 mm stem and equal length. Only the head size sets them apart, decided by seat condition and the cutting work intended:

When reinforced valves become necessary

• Boost pressure turned up to gain power
• Taller cam profile or stiffer valve springs
• Historic rallying, hillclimb events or track sessions
• Larger turbo, new intercooler or revised engine map
• Exhaust valves found worn when overhauling the head

Vehicle compatibility

OEM part number (exhaust valve):

Don't forget the inlet valves

An N5B head on the surface plate is the ideal opportunity to fit new inlet valves, with a fresh set of seals on every stem. The part for this Cosworth: