While I was rebuilding my engine I sent the heads out to be reworked. When the heads came back they had painted them with a high temp rust inhibiting paint. This was all well and good, but part of the problem was that the threaded holes for the rockers, in addition to other mounting points were now gummed up with paint.
Regardless of whether the shop should have plugged the holes prior to painting, I found myself in a situation where cleaning the threads was needed.
I set about to remedy this by getting a proper sized bolt, a vice and my 4 inch angle grinder.
So first step is to find the proper bolt for your threaded hole.
Step 1: Prepare to Grind
So, now that you have the proper sized bolt for your application. Lock it down securely in a table vice. Now grab your grinder. In my case at the time I didn't have a dremel or other small hand held grinder so I used an angle grinder. It was a lot more difficult to get an exact grind but completely doable. I suggest that if you have a small hand held precision grinding tool such as a dremel, you should definitely use that.
Start out by making a tapered grind along the length of the bolt on the threads. The taper should be a fatter/wider grind at the bottom of the threads and thinner at the top.
Step 2: Attention to Detail
As you make your lengthwise grind you also need to ensure that you make a taper across the threads as well. For standard right hand threaded bolts if you were to look at the bottom of the bolt your grind would be mildest at the 10 o'clock position, and heavier at the 12 o'clock position. The 12 o'clock position should be cut deep and angled as close to 90 degrees to the threads on the bolt.
What this kind of cut will do is allow you to easily start the bolt into the threaded hole. As your bolt progresses into the opening debris will collect and fall away from the 90 degree cut and into the hole via the ground down opening in the 11 o'clock position. The further you put this chaser into the threaded hole the more clean the threaded hole will get, since the taper of the bolt grows smaller and allow less tolerance between the threads.
Now, I know some people would suggest just using a tap from a tap and die set. But taps are meant to cut metal. We're only looking to remove, debris whether it's paint, dirt, or rust and other corrosion. The tap could actually shave your threads in the hole making them thinner, which would allow for slop in the bolt or make the threads weaker, depending on how fine or course the thread set is.
Once you're done with this throw it in your tool box. You might be surprise to see how often you might need it.