A tap is not the best choice to clean threads. It will often cut more than clean.
If you have a Dremel or air powered cutoff wheel and an old bolt, you are in business. Use the cutoff disk to cut 3 or 4 slots starting at the end of the bolt to the end of the threads. Make the slots parallel to the bolt shank. When you are done, it will look a lot like a tap. The good thing is that the slots will have sharp edges that will clean the holes as they are threaded into them. The steel is not hard enough to cut new threads, so you are safe in that area. Turn the chaser in a turn and then back up 1/2 turn. That will be the most effective cleaning process, since both sides of the slots will clean the crud. Use lots of WD40 when you do it and you will get excellent results.
You really are a backyard mechanic jrichker! haha... Good cheap tip tho.
Buy youself a set of THREAD CHASERS. You can get them from Summit or Jegs and they are MADE for the job you are looking for. It comes in a set so you have them for all those other holes you need to clean out when assembling your motor.