Drill and tap diameters for a 5/16 18 bolt

[Tom in TN]

I twisted off a 5/16 18 bolt that holds the plunger shield onto the plunger on my NH 273 baler. I'd like to try to drill out the broken bolt and refresh the threads with a tap, or conversely, I'd like to drill and tap a slightly larger hole for a slightly larger bolt. I've looked at a couple of charts that are supposed to show the drill and tap sizes for various sized bolts, but I simply don't know how to read the charts. Can you "learn" me how to determine what size drill and tap I need to try to fix this problem?

 

[Jim.ME]

I twisted off a 5/16 18 bolt that holds the plunger shield onto the plunger on my NH 273 baler. I'd like to try to drill out the broken bolt and refresh the threads with a tap, or conversely, I'd like to drill and tap a slightly larger hole for a slightly larger bolt. I've looked at a couple of charts that are supposed to show the drill and tap sizes for various sized bolts, but I simply don't know how to read the charts. Can you "learn" me how to determine what size drill and tap I need to try to fix this problem?

Here is a clip from a Starrett chart with the info for 5/16-18 circled and a pdf of the whole chart and a pdf of the entire chart. The correct tap drill for 5/16-18 is a letter F drill. 1/4" will work, best to use a cutting oil. For 3/8-16 just look to the right of the tap size you want and there is decimal size of the drill and next column left is the fractional letter or number size of that drill as appropriate.

 

[timcasbolt]

The tap drill for 5/16-18 is letter F, but in the real world G is a lot easier to tap, and 1/4" if you don't have letter drills.

 

[tom upton]

And if you dont have a tap drill chart handy subtract the pitch of the thread size you are tapping and it will get you very close to tap drill size! For example 5/16-18. .312 - (1 divided by 18) = .257. (same as f drill!) works every time.

 

[Tom in TN]

Here is a clip from a Starrett chart with the info for 5/16-18 circled and a pdf of the whole chart and a pdf of the entire chart. The correct tap drill for 5/16-18 is a letter F drill. 1/4" will work, best to use a cutting oil. For 3/8-16 just look to the right of the tap size you want and there is decimal size of the drill and next column left is the fractional letter or number size of that drill as appropriate.

View attachment 77101

Thanks Jim, I do have lettered drills (although I'm not absolutely sure that I have an F size, but I think that I probably do). I hope to give this a try tomorrow afternoon after church but otherwise I'll try on Monday. Thanks again for your help.

 

[Tom in TN]

And if you dont have a tap drill chart handy subtract the pitch of the thread size you are tapping and it will get you very close to tap drill size! For example 5/16-18. .312 - (1 divided by 18) = .257. (same as f drill!) works every time.

Thanks Tom, As I mentioned to Tim, I do have lettered drills so I hope to give this a try tomorrow or Monday. Thanks again for your help.

 

[Tom in TN]

The tap drill for 5/16-18 is letter F, but in the real world G is a lot easier to tap, and 1/4" if you don't have letter drills.

Thanks Tim, I hope to be able to try this tomorrow after church, but I might not get back to it until Monday. No matter what, thank you for the help.

 

[Mike(NEOhio)]

15/64 is a substitute for an F drill. A tad larger and makes for easier tapping. If you decide to go to a larger screw, 3/8-16 use a 5/16 drill.

 

[timcasbolt]

Maybe you mean 17/64.

 

[Mike(NEOhio)]

Maybe you mean 17/64.

Yeah, I've been away from the machine shops too long.

 

[4wdtom]

If you have a deep hole, maybe not a perfect sharp tap, tough material, etc. a tap drill on the large size will help. For a shallow hole or thin material smaller will be OK and provide a deeper thread, which will only increase the holding strength a small amount. Sometimes with stripped threads in a casting you can tap it a little deeper with plug and bottoming taps and use a longer screw and not have to drill and tap oversize. The factory often doesn't thread as deep as the hole is drilled. Do this with care as you don't want to break off a tap deep in the hole. Sharp tap. And select or custom cut the length of the new screw to not bottom out but to use nearly all of the new threads.

 

[Mike(NEOhio)]

The real trick with drilling out and retapping is getting the drill dead on center of the old bolt. Start with 1/8 and work your way up. When you start to see the threads on one side stop. I usually go with a small center punch to collapse the bolt away from the hole. Heat and penetrant will help along with patience.

 

[4wdtom]

The real trick with drilling out and retapping is getting the drill dead on center of the old bolt. Start with 1/8 and work your way up. When you start to see the threads on one side stop. I usually go with a small center punch to collapse the bolt away from the hole. Heat and penetrant will help along with patience.

I have done many as you describe. Sometimes when the thread starts to show after drilling I will saw through the thread with a thin/narrow saber saw or jig saw blade to make the thread collapse easier. Sure it takes away from the thread that is left but if the depth of the thread is more than the diameter of the thread it won't matter.

 

[M-MAN]

The little freebie charts are too hard for old eyes to decipher. I bought one of the 24 x 36 wall charts to make life easier.

 

[mechanicfred]

F drill, I use a 1/4"

 

[Mad Farmer]

Here you go. Machine screws, pipe taps, spark plugs, UNC, UNF, NPTF, NPSF, BSPT, Britsh std and fine (BSF BFW), Metric fine, Metric coarse (ISO)

The charts are handy as they have drill sizes for soft materials like aluminum/plastic vs steels/iron. Also fractional and decimal equivalents.

Drill sizes by: number, letter, fractional, decimal, and millimeters.

 

[Tom in TN]

I appear to have successfully retapped the threads on the plunger on my 273 baler. I have ordered two new 5/16 18 by 1/2 inch bolts to attach the shield to the plunger. I will pick up the bolts tomorrow and will hopefully be finished with this challenge.

Thank you to all of the responses that I have received to my original post. I could not have successfully completed this task without the help that all of you guys provided to me. Yesterday's Tractor is the best.

 

[Tom in TN]

Here you go. Machine screws, pipe taps, spark plugs, UNC, UNF, NPTF, NPSF, BSPT, Britsh std and fine (BSF BFW), Metric fine, Metric coarse (ISO)

The charts are handy as they have drill sizes for soft materials like aluminum/plastic vs steels/iron. Also fractional and decimal equivalents.

Drill sizes by: number, letter, fractional, decimal, and millimeters.

Mad Farmer, Thank you for these charts. I don't know what I'm doing wrong, but I can't successfully print them on my printer. Can you tell me what site you got the charts from and maybe I'll be able to print them from the source. In any event, thank you for your help.

 

[timcasbolt]

Congratulations!

 

[rustred]

I twisted off a 5/16 18 bolt that holds the plunger shield onto the plunger on my NH 273 baler. I'd like to try to drill out the broken bolt and refresh the threads with a tap, or conversely, I'd like to drill and tap a slightly larger hole for a slightly larger bolt. I've looked at a couple of charts that are supposed to show the drill and tap sizes for various sized bolts, but I simply don't know how to read the charts. Can you "learn" me how to determine what size drill and tap I need to try to fix this problem?

A simple thing to remember is to go 3 sizes smaller than the bolt. U can figure it out.