All you need to do to keep your NHK helmet clean is you should never let your helmet gather dirt. To keep it clean all the time with proper mild soap in warm water for the inner components and cleans the liner and visor without harming any of the plastic parts.
When you have finished clean the helmet, turn the helmet upside down so that any excess water can drain out of the air vents and leave it to dry at room temperature and well away from any sources of heat (examples direct sunlight).
For further information, see NHK Helmet’s manual book
To ensure maximum safety, the helmet should fit perfectly and feel comfortable.
To determine the correct helmet size, the first thing to do is to measure your head by passing a tape measure around it a couple of centimeters above the eyebrows and ears. ( You can see the size chart table below).
The next step, is Based on the helmet size that you have just determined, you must try it on to make sure that it fits properly and feels comfortable.
The last step is fasten the chinstrap as tightly as possible but without hurting and with your head bent forward, try to remove the helmet by pulling it from the back to the front.
To maintain full eficiency of the helmet, please do not make any modifications including applying any paint or any other solvent because the solvents in the paints can negatively affect the safety structure (bubbles and polystyrene) and the plastic parts (mechanisms, visor, etc).