Creating and updating numbered variables (flash8)

So what I’m trying to do, is allow users to create a bike type game. There are four different parts to it that they can choose (frame, material, tires, and handelbars), each that change one of the four different statistics (weight, durability, speed, and maneuverability) and I’d like it to update these global statistic variables each time a part is changed so that the statistic bars can update.

I want to have the bike frame, when selected, to set a base for all 4 variables, and then each other part will add to it. This is what I did so far. . .

on the framescript I just have:
_global.durability = (_global.framedur + _global.materialdur);
_global.weight = (_global.frameweight + _global.materialweight);
_global.speed = (_global.framespeed + _global.tirespeed);
_global.man = (_global.frameman + _global.handelbarman);

then on the bike frame changing buttons i have:
on (release) {
_global.framedur = 8;
_global.frameweight = 5;
_global.framespeed = 12;
_global.frameman = 5;

trace (_global.durability);

}

and on one of the other parts (material) I have this:
on (release) {
_global.materialdur = 1
_global.materialweight = 1
}

when the _global.durability variable gets traced it just output “NaN”, so obviously I’m missing something that would do something like refresh that framescript. What would be needed to throw in there to do this? I don’t know much about actionscript so I was just guessing on how to do a lot of this haha. Any help would be mucho appreciato.