While wrenching your car occasionally, you will come across a stubborn bolt. A complicated or difficult bolt is one that provides a compelling resistance while trying to tighten it, though you are completely confident that it is not cross-threaded. The reason this normally appears is because of corrosion accumulation on both threads and the bolts of the hole that you are screwing into. You can either try to push the bolt forcefully into it or you can clean up the threads which will help the bolt to slide easily. This can happen on a stud and nut as well. The most common areas to see this type of problem is the area that faces a high heat cycling as your header exhaust bolts, brink bolts etc. It is also seen on valve cover bolts, oil pan bolts and any other bolt which is threaded into your engine block or head directly.
A tap and die set is a perfect tool for cleaning the bad threads. A tap and die set normally includes taps which are made to create or clean up the threads and dies are created to clean up male threads. Additionally, there is normally a tap handle which you can change taps in and out of just like swapping screwdriver into multi-bit screwdriver and a die handle as well which you can exchange dies in and out. You should make sure that you use some cutting oil while chasing the threads with a tap and diet to keep the things lubricated and stop yourself from completely damaging the component. Cozyhousetoday.com is the best place to know more about tap and die sets.