Here's how I handled helm/ets in 3e (suitably modified for Conan). . .
Helmet: +2 DV versus Threat Rolls
Helm: +4 DV versus Threat Rolls
Visored Helm: +2 / +4 DV versus Threat Rolls
-- This is in lieu of having them add to the armour rating of armour worn.
-- This means that it is worthwhile wearing a helm/et even if no armour is worn (a fairly common occurence).
-- So, as they protect the most vital portion of a targets body, they will make it less likey that a critical hit is scored--but no less likely for the target to be hit in general.
-- For simplicity, if the DV bonus makes the difference between taking a critical and not, you can assume that the helm/et is dented--either ruined completely, or you can reduce its DV bonus by 1 (perhaps by 2 vs a x3 strike, and by 3 vs a x4 strike--depends on how complicated you want to get).
Helmet: +2 DV versus Threat Rolls
Helm: +4 DV versus Threat Rolls
Visored Helm: +2 / +4 DV versus Threat Rolls
-- This is in lieu of having them add to the armour rating of armour worn.
-- This means that it is worthwhile wearing a helm/et even if no armour is worn (a fairly common occurence).
-- So, as they protect the most vital portion of a targets body, they will make it less likey that a critical hit is scored--but no less likely for the target to be hit in general.
-- For simplicity, if the DV bonus makes the difference between taking a critical and not, you can assume that the helm/et is dented--either ruined completely, or you can reduce its DV bonus by 1 (perhaps by 2 vs a x3 strike, and by 3 vs a x4 strike--depends on how complicated you want to get).