This is a hardening treatment that even though it is not widely used as the chrome coating, it has several benefits and is becoming very popular, mainly for European and Japanese applications.
The microhardness is higher than the stainless steel base material, keeping good ductility beneath the hard nitrided layer (microhardness is 800HV minimum).
Surface finish is smoother than with chromed stems, having less friction between stem and guide.
Valve seat surface is harder with the nitrided layer, lasting longer with lead free fuels, alcohols, nitro or other "explosive" mixes. Matches any kind of seats (nodular iron seats, steel powder metal seats, hard aluminum-copper seats or beryllium copper seats).
Higher rpm's engines take full advantage of all previous features. The higher the engine speed the bigger the power loss due to internal engine friction! The black nitride layer reduces this power loss due to less valve-guide friction.
The nitrided layer is adhered to the base material at a microscopic level, so does not "flake" or break when the valves bend due to impacts with the pistons.
The Black Nitride applies to the whole valve while the chrome coating is only applied to the stem.
All valves are swirl polished, have stem undercut and a hardened tip.
Head Diameter 32.5mm +0.5mm
Stem Diameter 5.96mm