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Misthreaded 14-20 hole needs to be tapped. How do I determine the next size up?

Misthreaded 14-20 hole needs to be tapped. How do I determine the next size up?


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Old 07-03-22, 10:17 AM
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Misthreaded 14-20 hole needs to be tapped. How do I determine the next size up?

I have a misthreaded/rethreaded hole hence both the bold and hole are now useless so I bought a replacement bolt along with a Harbor Freight Pittsburgh 60 piece tap and die kit.
It appears I'd need to drill through the hole cleanly first to eliminate the existing threading before jamming the tap in there and start twisting away.
In that case I would have already eliminated the existing 14-20 threads so I couldn't use the 14-20 tap or replacement bolt. How do I determine the next size up bolt-wise so I know which size tap to use?
Also I don't have any specialty tapping oil, what would be better between WD-40, PB Blaster, and regular motor oil?
 
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Old 07-03-22, 10:26 AM
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In other words, let's say a 3/8" bit eliminated the existing threading in the hole.
Then would I use a 1/2" tap to create the new threading so now that 1/4" bolt needs to be replaced with a 1/2" bolt?
 
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Old 07-03-22, 11:35 AM
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There are a lot of parameters that go into thread and tap size.
What are you doing.... where is this ?

Of the choices.... use motor oil for tapping.

 
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Old 07-03-22, 12:06 PM
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Does a 1/4" bolt mean it's a 1/4" diameter inclusive of the threads?
If so then for that 1/4" hole that the bolt goes through I could use a 1/4" drill bit to get it smooth/remove the existing threading.
Then in that case I could just go up one size to a 5/16" tap to make a hole that will accept a 5/16" bolt, is my assumption correct?
 
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Old 07-03-22, 01:56 PM
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Next size up is a 5/16-18.

Motor oil is good.

Hope you get more than a single tap/die from that HF junk!

For every tap size there is a relative drill size. For a 5/16-18 the drill size is 1/4"


 
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Old 07-04-22, 06:00 AM
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You can chase the existing threads by simply running a tap properly (square, thread started properly) through the hole. It will clean up the threads so a bolt can go in the hole again but cross threading has already damaged and removed part of the threads so it's holding power is reduced. There is really no reason to re-tap a hole from coarse to fine threads or vice versa.

For maximum holding strength you should go up to the next bigger fastener size. You've used almost used both metric (14x2.0) and English nomenclature 5/16"-18. Since you are essentially creating a brand new hole you can use either. You need to use the proper size tap drill for the size of threads you want to create. Usually it's a range represented by a percentage. A higher number is stronger but can be harder to tap in difficult materials while a lower percentage is easier to tap. For a 5/6-18 tap a tap drill size of F (.257") is a good all round choice.

If taping steel I would use motor oil as a lubricant. If tapping aluminum I'd use diesel or jet fuel if you have them or motor oil will also work. For taping a plastic you don't need any lube. Coat the threads of your tap with oil and insert the tap into the hole. Make sure it is square and not crooked. The harder the material you are tapping the slower you turn the tap and the harder you push. I often use a cordless drill to turn the tap when tapping plastic and aluminum, brass... but when tapping steel or stainless I'll use the manual handle for more control. In hard materials like steel about every 1/2 to 3/4 turn of the tap handle turn it backwards for 1/4 turn. 3/4 forward, 1/4 backwards. When you go backwards it will turn easily at first then you'll feel it get harder to turn. That is the tap hitting the metal you're cutting. You want to back up through the difficult part which cuts the chip free so it can fall out of the cutting area and not clog things.

Most common taps have a long taper so the threads at the tip are smaller and easier to start. You don't get the full size threads until about one diameter up the tap. So for a 5/16 tap run the tap in about 5/16" deeper than you think you need to. When you've gone far enough turning the tap will get easier as it's no longer cutting metal.
 
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Old 08-13-22, 05:20 PM
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Thread-repair inserts are another option

I know I'm late to this party, but for the benefit of the next guy let me also encourage you to consider the thread repair kits which are readily available in both SAE and metric versions at most auto parts stores (Heli-Coil is perhaps the best known brand name). They will recommend the proper size bit to prepare the hole (normally you purchase the drill bit separately) and will include a tap to thread the hole once it's drilled. Then, using an included tool, you thread the insert into the hole; your bolt will go into the threads of this insert. Once the insert is in place you break off the "tang" you used to thread it in and then install your bolt or stud. This way you can go back with the original size fastener and not have an "oddball" if there are multiple fasteners in the piece.
 
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Old 08-14-22, 03:27 AM
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If the torque on the bolt is not large and the hole is not blind , I would try tapping original hole with a 1/4 x 28 tap and trying a 1/4 x28 bolt of the needed length. A bottoming tap is used for a blind hole because a regular tap reaches the bottom of the hole minimizing full threads. The next size up is 5/16 x 18 or 5/16 x 24.
 
 

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