Take a bolt to a Big Box store or hardware store and use the bolt sizing display in the hardware section. The bolt sizer will tell you what tap you need.
I thought a tap is designed to tear new threads and a chaser is designed to screw into the threads and collects the junk in the threads.
Do I NEED to use it? It has (what I am assuming is) quicksilver perfect seal. In the threads. I say I assume it is that because it has something in the threads, and that is the proper stuff to use. For all I know it could be loctite or simply corrosion.
A bottle cleaning brush or shotgun cleaning brush can clean up any crap in the hole. I like to use a paper towel twirled into a cone with a little grease to get any remaining small bits of junk in the hole, especially after chasing.
Is a brush effective enough for getting the thread locking crap out? Is it suggested to use a brush over a tap? I may be over thinking this.
EDIT: I just threaded a headbolt cleaned by a wire wheel into each of the bolt holes in the block. They seem to be clean. So, just don't worry about it? I blew compressed air in there. I know the bolt holes lead to the coolant ports. There was a lot of rust blowing out.
