[Canuck_Targa]

Be careful with the terminology, the original poster is correct in asking if a thread forming tap and a thread chasing tap are different and they are. Regular machine taps cut the threads (as we all know) where as a thread forming tap does no cutting and displaces the material to form the threads. I don't suggest that you use a thread forming tap to chase threads in a hole, the reason being if you are cleaning out crud or loctite or any other debris in the threads it will tend to try to displace it into the threads which can actually loosen the fit.

The thread chaser as shown by cgarr is made to clean the hole with no cutting edge like a tap. This is no different than as some have suggested cutting a groove in a bolt with a dremel or die grinder and running that down, works exactly the same and is way cheaper.

Running a tap into an existing (old) thread hole is dependent on the condition of the existing threads, if they are worn or deformed from torquing, the tap will re-cut the "true" thread and it will normally feel too loose after that. Not recommended on older worn out threads.

As far as quality, I have used everything from the most expensive to the cheapest and really the only difference is the quality of the steel used, the quality ones hold their edge much better which translates to better cutting and better threads. The cheap ones chip the cutting edge then bind and break much easier.

Sorry for rambling, I was a Machinist then a Tool and Die maker for almost 20 years and have tapped about 10,000 holes in my time, so I have seen a hole or two.