In 1942, during World War II, the United States Army introduced the M1 Army Helmet. The M1 helmet offered a steel shell with alloy seams and edging, which prevented the helmet from rusting around the edges and seams. The M1 Army Helmet offered a single chinstrap, which was later replaced by a three-point chinstrap helmet that was made to fit more securely and provide better protection. If you have an M1 Army Helmet, adjusting and fitting the helmet on your head will require it to have functioning straps. If your helmet offers this, then you can adjust the helmet to fit securely.
Locate both straps that hang down from the inside of your M1 helmet and make sure they are unfastened.
Place the helmet on your head with the visor toward the front and one strap on each side of your head.
Locate the buckle on one of the helmet's straps.
Guide the strap that does not have a buckle from the opposite side of the helmet and insert one end of the strap into the first slot in the buckle.
Guide the strap through the first slot, pull it out on the opposite side of the buckle, and then guide the strap back through the second slot on the buckle. The strap should now be wove between the two slots in the buckle.
Pull the free end of the strap that you inserted into the buckle until the helmet feels snug against your chin.
Test the comfort and tightness of the helmet by placing two fingers under your chin or open your mouth as if you were letting out a yawn. You should be able to fit two fingers under the strap with ease or be able to open your mouth without the helmet shifting.