While I was shopping for a helmet at Modern Motorcycle the other day, I overheard an older gentleman (the store owner, perhaps?) telling a customer that Modern can do custom helmet fitting, and have been doing so for the longest time. They can either make the inside of a bigger helmet smaller by adding additional foam pieces on the inside of the helmet, or they can make a smaller helmet bigger by using a rubber mallet / hammer to pound on the inside of the shock absorbing foam. The older gentleman insists that both practices are safe, and that they have been doing this for the longest time.
From several helmet-testing articles that I have read before, it seems that in most crashes, shock absorbing foam rarely ever gets totally compressed, meaning that they still have extra capacity to absorb additional energy. But I can't shake off the feeling that tampering with a helmet is not the safest practice around.
What opinions do you guys have on this issue?