1988 k1500 pulley question

I have a 1988 gmc 1500 with a 350 in it. It's a 6 rib pulley with a serpintine belt system, with a/c. Today the nut on the alternator pulley fell off. This is a prob i never expected because usually you can never get them off. My question is, does anybody know what size nut it is? I can use the pulley if i have too, but the nut i can't find.

thanks

You can get in touch with a starter/alternator rebuild shop and they can point you in the right direction or sell you a nut.