The author advises the alteration of the styrofoam inside of a helmet to attain a better fit...."Use the back side of that spoon and compress the Styrofoam in the part of the helmet causing the marks," and also "To cure any problems you will have to pull away the liner and use a knife and cut away part of the Styrofoam around the ear wells within the helmet."

In the event that these statements aren't just oversight, let me explain.  A helmet is designed to work in two ways.  The first is to self destruct upon impact to absorb the impact.  The outer shell will delaminate/crack to absorb much of the shock and the inner styrofoam shall compress to slow the movement of the head.  The second way a helmet protects the head is by distributing the impact to all areas of the head and thus lowering the maximum force with which the head is hit.

It may seem like a benign little alteration to cut away/compress a little part of the styrofoam to make room for ears/forehead, but the danger in it is great.  It probably doesn't affect the first functionality of a helmet that much, correct?  After all, it is just a little spot and if you can depress it with little force with a spoon what good would it do in an accident, right?  Wrong. 

First of all the spoon is not the size of one's head.  A little push with a spoon could have been many pounds of impact force absorbed because the head has much greater area and thus it takes much more force to depress the styrofoam with it than with a spoon.  Cutting away at the ear has a similar drawback.

The second is an even greater danger than the first.  Because of the helmet distributes impact to as much area of the head as possible, and because the structure of the helmet is what enables it to do so, damaging ANY part of the helmet will GREATLY reduce it's function as a whole.  The shock of impact cannot travel through the helmet if there is no helmet in that spot.  So if the impact is on the back of the head, instead of traveling efficiently across the whole helmet it shall remain concentrated in the back of the head.  This is not simply a function of how much has been cut away, but also everything *after* the area that has been cut away.

Little tiny alterations to a helmet mean a great deal.  Any damage done to a helmet is automatic grounds for NOT using that helmet anymore and getting a new one immediately.  It's fine to cut away from the liner  when comfort is an issue because the liner is there purely for comfort.  But NEVER the Styrofoam.  If it really doesn't fit, get a different helmet.

Also, while on this subject, let me bring up the issue of old helmets.  I would not personally use a helmet that is more than two years old.  I do not know for sure what sort of Styrofoams are used in motorcycle helmets and what their lifespans are so I can't set this down as a firm rule, but one thing for sure is that the Styrofoam can get crusty over time and not absorb impact with full (or any!) effectiveness.  Excessive heat can bake the styrofoam into brittleness, so can excessive exposure to the sun's rays (if that's possible).  I wouldn't leave a helmet anywhere it may be exposed to such elements. 

I don't know what the motorcycle helmet makers say about this, probobally nothing.  That's what bicycle helmet makers said about this (nothing) until it became clear that an alarming number of people left helmets to bake under the back window of their cars.  I've seen plastic toys *melt* in the back window on hot days, imagine what could happen to the safety of a helmet.  Now, only a poser would do something like that <g>, but there are other implications that 'bikers should consider.  Mostly it's that an old helmet or a used hand-me-down are *very* suspect and shouldn't even be considered for use.  Also at fault are certain FAQs I've read that suggest saving up old gear (helmets) to give out to girlfriends and others who ride pillion.  This is wrong.  If you are going to have two helmets, make them two *new* helmets. 

Sincerely,
Hendrix Fan

PS: If you're going to give a loved one an old helmet, do something respectable and wear it yourself instead and give them your new one so that their safety does not rest upon your soul.