Helmets do two things, regarding game mechanics; they protect the head, and due to their nature reduce perception, expressed as a penalty to Spot and Listen Checks. They also increase the weight of armor worn.
Under my system, most helments will offer a single point to AC, and have an additional chance of stopping a critical hit, by adding to the AC for the confirmation roll only, for our purposes, this will be called a critical threshold. First, however, we'll look at the types.
Cap: This most basic of all head protection is a padded, leather, or even steel skullcap that is that is worn much like a hat. It offers no vision penalty, and a small -1 penalty to listen checks. Unlike other headgear, it does not add to AC, but adds 1 point to AC when rolling to confirm a critical hit. Usually, it is worn with lightler armors such as Padded, Leather, Studded Leather, etc... Note: This also includes arming caps, which are worn under heavier helmets.
Coif: Basically, a padded chainmail hood, fitting closely around the neck, and pretty closely around the head, leaving the face from chin to forehead uncovered. It adds a +1 to AC, has no penalty to Spot Checks, and has -1 penalty to Hearing checks. The coif offers an additional +1 to AC when rolling to confirm a crit. Often a heavily armored knight will wear a coif under his normal helm. This offers no increased bonus, except the knight can remove the helmet and still enjoy some measure of protection.
Open Faced Helm: This is the most commonly encountered form of head protection for the average soldier who can afford it, or the adventurer who desires maximum head protection with a minimum of penalties to perception. Examples of this type of helment are the nasal helm, the pot helm, the kettle helm, Roman Legionary helms, Morions, and many others. Regardless of type, they feature an iron or steel covering that back, sides, and top of the head, leaving most of the face open. It grants the wearer +1 AC, -1 to Spot checks, -2 to listen checks, and an additional +2 to AC when rolling to confirm a crit.
Closed Helms: Much like an Open-Faced Helm, save for the presence of a piece of armored plate (often a visor) protecting the face. Types of this helm include the Greek Corinthian Helm, the medieval Sallet (when worn with a Bevior) and the Armet, the typical helm worn with suit armor. It grants +1 to AC, -2 to Spot checks, and -3 to Listen checks, adding +2 to AC (above and beyond the normal bonus) when rolling to confirm a crit. Masterwork versions of these helmets are special, which will be explained later on.
Great Helms: Massive Helms which cover the entire head, leaving only slits for the eyes and holes for breathing, these offer unsurpassed protection, at the cost of being able to percieve the world around you. Great helms offer +1 AC, -3 to Spot checks, and -4 to listen checks. They also offer +3 (above and beyond the normal bonus) to AC when rolling to confirm a critical hit.
Masterwork Helmets cost an additional 75 gold pieces, and feature additional reinforcement, so have their critical Threshold increased by +1. Masterwork Chain mail is assumed to come with a masterwork Coif, making it an exception, as the coif is already part of the armor. Masterwork Close Helms and Masterwork Great Helms offer an additional bonus: they provide a +2 to saves vs spells that require vision to work, such as Color Spray, and Blindness, and even such effects as a Basklisk's gaze. When purchasing a suit of Field or Full Plate, an Masterwork Close Helm is included in the price of the armor.
The following is a list of helmets and their statistics, starting with AC, bonus, Spot Check penalty, Listen Check Penalty, Critical Threshold, and, lastly, weight. Remember with the critical threshold, that the additional Bonus to AC is in addition to the normal AC bonus provided by the helm.
Cap: 0/0/-1/+1/2lbs
Coif: +1/0/-1/+1/3lbs
Open-Faced Helm: +1/-1/-2/+2/5lbs
Close Helms: +1/-2/-3/+2/6lbs
Great Helms: +1/-3/-4/+3/10lbs
Cap: 1 gp
Coif: included in armor cost, otherwise, 5gp
Open helm: 5 gp
Close Helm: 10 gp
Great Helm: 25 gp
Note on Critical thresholds: I refuse to follow the 3.5 Edition Rule that the Keen enchantment and the Improved Critical feat do not stack. One represents a sharper, more dangerous weapon, while the other represents increased skill with a given weapon. While this leads to characters becoming "Crit Whores" (ie getting Keen and taking Improved Crit, Scimatar) The critical Threshold makes this tactic less desirable. If you are not allowing keen and Improved Crit to stack, I recommend that you reduce the critical thresholds shown here. My rationalle with this is to provide more detail with the armor system, add more realism, and improve the overall protective qualities of the AC system.
Regards Enchantments, even though the helm adds an AC bonus, I consider them when enchanted to be wonderous items, not armor. Therefore, they cannot hold armor/shield enchantments, but can hold deflection bonus enchantments and other kinds of enchantment. As I allow all deflection bonuses to stack, this isn't too bad, but still takes up a "head slot"
Integrated into my entire overhaul, a character with a 12 Dex, wearing a suit of masterwork Half Plate armor, carrying a large shield with a normal Kettle helm would have the following qualities: AC of 24/21/21 vs Slashing/Piercing/Bludgeoning weapons, 4 points of Damage Reduction, 1 point of max Dex bounus (already figured in), an armor check penalty of -8 (-6 masterwork half plate armor, -2 for the shield) an -1 to spot checks, a -2 to listen checks, and an AC of 25/23/23 vs Critical hit confirmation rolls from slashing/piercing/bludgeoning weapons. A formidable opponent. Balance in the AC system has been restored.
lol.