So here is part duex, where I keep some things and re-write others. Keep in mind I am smitten by Swords & Wizardry and use it almost exclusively, and have taken the typical rule of Xin6 chance up to an equal Xin12 chance:
- Effect on Armor Class: Not wearing a helm for the appropriate suit of arms is a +1 [-1] penalty to that Armor Class (Referred to as AC from here on out). So wearing a helm (the default) gives you the listed AC. Note: There is NO penalty for not wearing a helm when 'unarmored'. unarmored is not an AC as far as I am concerned and it retains its 9  AC, and wearing a helm when 'unarmored' does not improve your AC.
- Surprise: by default, characters are surprised 4in12 (2in6), were default assumes wearing a helm. Removing the helm, or being unarmored, should improve this. Un-helmed and unarmored characters are therefore only surprised 3in12 of the time. Great Helms, discussed further below, do provide better protection but increase the chance of being surprised to 5in12 when worn. Characters removing a Great Helm become surprise as an un-helmed or unarmored character (3in12), but it does take one round to put any helm back on.
- Initiative: as with surprise, by default assumes characters with armor are wearing a helm. Initiative is typically decided by the higher roll of a d12. Unarmored and un-helmed characters should have a slight advantage, perhaps a +1, to initiative as they would be able to perceive the actions of others better and react accordingly. Note: An un-helmed character will almost certainly take the advantage of the first round to don their headgear. If not treat them as un-helmed until they do (see Combat Effects below).
- Hear Noise: dwarves, elves, and halflings, by default have a 4in12 chance to hear noise and humans a 2in12 chance. Thieves, if used, also start at the same chance as demi-humans to hear noise, but increase as they advance (Hopefully oneday I will get thief rules wrote up). Again, the default assumes that armored characters are wearing a helm. A character, for whatever reason, devoid of a helm should increase the chance for hearing noise by one.
- Basic Guideline: by default assume that an Armored character has their helm on unless they say otherwise. If the proposed action can conceivably have better results when not wearing a helm, give Un-helmed character a +1 to their Xin12 chance.
- Combat Effects to Attack: when attacking; on a modified To Hit roll of 19+, the attacker may call the head of a helmed opponent as their target, and on a modified To Hit roll of 17+ the attacker may call the head of an un-helmed opponent as their target.
- Combat Effects to Damage: striking a helmed head does regular damage, but the target must make a Saving Throw with a +2. Passing the Saving Throw causes the defender to be dazed in addition to the damage received, and will be at -1 for their next round's actions. Failing the Saving Throw causes the defender to be knocked prone and be dazed (-1 for their next round's actions). Striking an un-helmed head does full damage, no roll required, and causes a Saving Throw. Passing the Saving Throw causes the defender to be dazed in addition to the full damaged received for 1d4+1 rounds. Failing the Saving Throw causes the defender to be knocked unconsious for 1d6+2 rounds.
- Great Helms: will provide an additional -1 [+1] to AC, as do shields, but this type of helm will also add an additional +1 to the Xin12 formula, for helms mentioned earlier, for Surprise and an additional -1 to formulas for Initiative, Hear Noise, etc. (See Base Guideline rule above for guidance. Basically a Great Helm provides an additional +1 when its size and protection is beneficial (Saving Throw vs called head shot and AC) and an additional -1 when its size and protection is hindering.