If you haven't seen the magnetic set-up for my kid's faceplate, you can check out my youtube video. You certainly won't need magnets this big if you were just talking about the back section of your helmet, but I'll pass along the exact magnet types I used on the faceplate anyway... just in case that might help out someone looking to do the same type of thing.
I buy my neodymium magnets from K&J Magnets
, so I should plug them as well. In general the number after the 'N' in the name represents the strength of the magnet, so something in the N50's is going to be stronger than something in the N30's range.
For my build (which is smaller/lighter than what you might be working on) I used the following magnets at each of the position discussed in the video:
The heaviest magnets I used were the ones on the inside top of the helmet (for use when the faceplate is in the open position). These have to effectively hold the faceplate in the open position through the thickness of the top of the helmet (since they are inside the helmet and the faceplate is on the outside). Magnets lose effective holding power FAST as you move away from them, so I went with these bad boys for my build... 1/2" dia. x 3/8" thick - Grade N42 - Nickel Plated neodymium magnets (~$2 each). <I used 2 of these>
The lightest (thinnest) magnets I used were on the chin of the faceplate (and under the metal electrical connector plate on each side of the top of the faceplate). Those were 10mm dia. x 2mm thick - Grade N35 - Nickel Plated neodymium magnets (pack of 25 for ~$5). <I used 3 of these>
The magnets at the eye-hole level of the faceplate and the ones underneath the metal plates on the main body of the helmet (the metal plates that connect to the faceplate connectors) were12.6mm dia x 5mm thick - Grade N35 - Nickel plated neodymium magnets (10 pack for $5). <I used 4 of these>
So, that's NINE total magnets for the faceplate.
P.S. Keep your helmet away from pacemakers and magnetic computer discs! :darnkids
P.P.S. You probably don't need to use all of these if you don't also wire up the eye's through the connections.