This set is the correct diameter and thread pitch to match the original. These are for the screw on style armrests seen in 210,150, or the rear of your Bel Air sedans. These are not to be used with the Long Bel Air style that uses a bracket. We have another listing for those styles. 
This screw set will work on OEM original arm rests. If you are using NEW arm rests from other suppliers, you will need to drill out the holes in the armrests, in order for these to work. This can easily be done using a 15/64 bit, then chasing it with a 17/64. Our new armrests are set up to accept these screws (we include them). The reason for this product improvement, was due to our finding that on some cars, the smaller diameter screws that come with the new 210/150 style arm rests, do not grip well enough in the door or rear quarter panel. This is becasue the original holes had the larger diameter screws, such as the ones included in this listing. If you are using an original set of armrests, these will work perfectly, without modification.