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I have stripped the threading on this bolt and would like to clean and re-tap the bolt hole.

Damage of bolt near the tip showing some stripping.

I have read that I can use a file or hacksaw to cut some grooves into it like so.

Cutting groves into a bolt to form a bolt tap.

I assume I must first grind down the broken threads and then cut those grooves, but I'm not sure how many grooves or how deeply they should be cut.

I'm also unsure how the tapping process will feel. Will I need to really force it? What should I expect. Should I clean the bolt hole first? Lubricant?

This question is paired with another titled "Courses of action after having started to strip the threading on a bolt".

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    Lube, Yes. several flutes are best. Cut to the bottom of the thread is all that is needed. Better to use a proper tap for the hole.
    – Moab
    Sep 2 '20 at 17:42
  • I've never used taps before, how do you use several? Start small? Only a single cut to the bottom of the thread? How deep? What kind of lube?
    – Karoh
    Sep 2 '20 at 17:53
  • Bolt only looks damaged on starter threads. cant see the hole. A matched die ought to clean the threads with slow back and forth rotary motion. Sep 3 '20 at 16:13
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To make what you are describing, first grind down the tip of the bolt you are going to use so it is completely flat and goes up into the threads. This ensures the threads are at the tip of what is left and that there is no taper there. This will allow you to get to the bottom of the hole (if this is a through hole, you don't really need to grind the tip and might work better with the tip on it). Next, when grinding the flutes, you'll want several on there. Your flutes should be wide enough so you have equal amounts of flute to what's left of the bolt threads. Four would probably be good, but three (or even two) would work if the bolt is too thin for more flutes. Make sure these go up the bolt quite a ways. You want to make sure there is plenty of meat left at the center of the bolt so as to support itself. Last thing you want is for it to crank around itself and get stuck in the hole. The flutes are there to pick up any ash/trash, not to cut new threads. When you run it down the hole, it will be slightly precarious, so you have to be careful you don't cross thread it.

While a tap would work for this operation, you really run the risk of cutting new threads with it if you don't do it right. A bolt cut with flutes should be quite a bit softer, but be hard enough to straighten out threads and remove ash/trash without too much of a hassle. If you've not used a tap before, I believe using this would most likely be a better (read: safer) solution.

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  • Any advice on how deep to cut the flutes? The image I showed above shows the process with very shallow flutes. Would you use lube? Fortunately, it is a through hole.
    – Karoh
    Sep 2 '20 at 22:12
  • If you use any lube, it would be something light like WD40. But remember, you're not cutting threads, just trying to straighten them out. The flutes can be cut up the bolt about as much as you like. I'd make them more on the long side then on the short (like what you see in the image you posted). Sep 2 '20 at 22:27
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Good start , if the bolt is surprisingly hard and hole material is unusually soft , it could work. A normal tap is hard (Rockwell 60 to 65) : If your bolt is high strength ( such as Grade 8) it is about R c 30 , if it is a regular hardware store bolt ,it is about R c 20. So ,chances are it will not work. You might get lucky and it will clean some relatively loose metal debris out of the hole. A tap must be hard to cut metal ( steel). If you want to experiment : 1, heat the bolt to red/orange ( 1500 to 1600 F) . 2. Plunge into cold salt water ( quench). Heat for as short a time as possible because the bolt will be loosing carbon. I wouldn't bother trying to temper . Try and see if it cuts. It will be difficult to get plain carbon steel hard enough to cut. That is why high strength bolts are low alloy like chrome moly.

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  • "File hard" is about Rockwell C 58. So, if you can cut it with a file ,it will not cut steel effectively. Sep 2 '20 at 20:26
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    Its not for cutting but cleaning existing threads.
    – Moab
    Sep 2 '20 at 20:37
  • Sure it could clean out relatively loose debris. Probably worth getting a price on an inexpensive set of taps and dies that will last a hobbyist a lifetime. I got a set from Sears 40 years ago , probably have not used 70 % of them . But those I did use made it worthwhile. Sep 2 '20 at 23:50
  • Done over 500 threaded holes this way, 99 percent effective. 40 year old Sears is better than any consumer sets today, hang on to it.
    – Moab
    Sep 3 '20 at 0:15

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