The HANS device will work with your old helmet. It comes with the hardware you need as well as measurements you might need to drill the holes in your helmet should it not already have them. I just purchased a new Bell helmet and it had the holes drilled already. You just remove the rubber plugs and install the hardware provided.
The hard part is getting everything in place in a reasonable period of time. You will need to practice putting it all on to determine the easiest order to put on helmet, HANS device, seatbelts and window nets. If you have a funny car style roll cage, it becomes even more of a challenge to put everything in place.
I will tell you, I feel much safer with all of the safety equipment in place. It's a bit claustrophobic at first however.
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