Page last updated July 17, 2008



Common tools needed

Bolt Hole Rethreading / Tapping

When you strip that bolt and you get that ugly feeling inside, its time to advance your skills up a notch. This guide is based on the fact that you have already attempted to extract the bolt using a bolt extraction kit. If you broke the bolthead, it is moer than likely frozen and the extraction tool will more than likely fail. I have had no luck with these and have even had to drill a broken extraction bit out. Very hard to do and sometimes impossible if it is deep inside the bolt.

With the correct technique and some patience, you can, with practice, repair any stripped thread.

My first experience was with the Mercury 700 that was never covered and most bolts were corroded. I tried removing the exhaust cover only to find some of those bolts would not give. I will describe various methods utilized to rethread, tap, and braze certain parts.

Wow!! I have some experience dealing with tapping. All of it is due to errors on my part and impatiance.


When attempting to remove old bolts that just won't give, here are few pointers and ideas.


The metal is hot and that might be enough to strip threads, especially spark plug threads.

  1. If you find a bolt hard to remove, apply some WD-40 or Deep Creep to the bolt and give it some time. Some people give it days to soak in.
  2. If that does not work, with a butane pentorch, heat up the surrounding metal (should expand slightly) and try to see if it comes loose.
  3. You can also lightly tap the top of the bolt with a punch and this might break the corrosion loose enough for it to give.
  4. When are in the process of unscrewing a bolt and it slightly gives, be careful and make sure it is not about to break off. If there is considerable amount of friction, the heat buildup may be enough to basically melt the bolt head resulting in a break. SO WORK IT SLOWLY. Turn slowly, then stop, let it cool, turn some more, let cool, turn some more ........

It is preferable, by far, to strip threads rather than to break a bolt head. Stripped threads have various means of rethreading, to the point that the rethread will be stronger than the original.

If you so happen to break a bolt head, read on.


If you happen to strip some threads, go out to your local automotive shop and look for a tap and die set. Do you need a set, probably not but each tap and drill is expensive and is more cost effective if you purhcase a set.


Check out your local Northern Tool Company or Harbor Freight Store. They usally carry inexpensive kits. If you want quality with a warranty, Sears and Lowes have no frill return policies on broken parts from kits.

HeliCoil for stripped threads

The helicoil repair option requires that you drill a hole slightly larger than the bolt that it is replacing. So you will be removing more metal. I have found HELICOIL at Car-Quest. Each kit will have the drill, tap and a few Helicoils. This is by far, a superior repair option. A bit expensive, but worth the cost for the added strength, but again, more metal will have to be removed. Remember, if you stripped the threads, you may not be able to rethread the same size threads again since that is stripped. The Helicoil may be the only option you can try. You can try filling the stripped area with some brazing metal and try to tap new threads that way. Check out my Helicoil page for more info and pics. Otherwise, read on.



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