Finally getting a chance to get back and do a little work on the jeep.

I am replacing the temp gauge with mechanical one. I had to drill out a brass fitting so I can make the mechanical gauge fit. I didn't completely drill it out so as not to destroy the threads in the block.

I need to chase the threads to clean out the last little bit of brass, but I don't know what tap I should use. How do I go about figuring that out? I've not done this before so I really don't want to monkey-up the block.

Here's a pic from an earlier thread pointing out the brass fitting I drilled out.