Technical 228 gmc distributer hold down bolt

guys anyone know the size of the bolt that holds down the distributer in a 1951 gmc 228? i'm not at home and need to stop and grab one, thinking it might be a 1/4"? i should know this but... getting old and foggy! thanks.

 

There's quite a bit more to the original advance holddown plate than a bolt, but you can "get by" with a couple flat washers & a 5/16'' NC bolt

 

I just looked one my blocks to make sure and it's a 3/8"-16 if you are talking about the cap screw that is holding the plate attached to the block. There is a smaller cap screw that has a nut that you can loosen and advance or retard your timing which is attached to the distibutor. There are many different styles some of which have an elongated slot for advancing and retarding also.

 

thanks and it's got a new bolt in place.