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Forum topic by Elvin posted 08-28-2011 04:11 AM 1287 views 0 times favorited 9 replies Add to Favorites Watch
View Elvin's profile


72 posts in 3422 days

08-28-2011 04:11 AM

Don’t know if this is the right forum but I need some help. I am going to tap some new threads in an old iron oil pump to make a table lamp. Now the tap size I am going to use is 3/8 -24 teeth. Now I am trying to determine the size of the drill bit for this hole. I have looked on the internet at the charts they have available and they state to use a “Q” bit. I am unable to determine what exactly a “Q” bit is. If anyone has any knowledge on this subject I would appreciate any help available.

-- Elvin, Southern California, "How great would life be if we lived a little of it everyday"

9 replies so far

View Mickey Cassiba's profile

Mickey Cassiba

312 posts in 3054 days

#1 posted 08-28-2011 04:20 AM

Hi Elvin…here’s a link to a drill size chart. The Q is .332”

-- One of these hammers oughta fix that...

View JJohnston's profile


1622 posts in 3314 days

#2 posted 08-28-2011 04:27 AM

Try McMaster-Carr ( Search for “drill bit”, then narrow it down to size “Q” using the pulldown menus. They’re also in SoCal, so you might even be able to go there and pick it up.

-- "A man may conduct himself well in both adversity and good fortune, but if you want to test his character, give him power." - Abraham Lincoln

View Grandpa's profile


3259 posts in 2698 days

#3 posted 08-28-2011 04:31 AM

There are fractional drills, numbered drills and letter drills. Most of the number and letter drills are used for making holes to tap or something very precise. They are not expensive. For a lamp you might be able to substitute the next fractional size up. Do a test hole first. If this were something of strength then the letter drill would be used for sure.

View Arlin Eastman's profile

Arlin Eastman

4230 posts in 2584 days

#4 posted 08-28-2011 04:43 AM

+1 on what grandpa said, however, going up one size depends on the size going to. I would only go up afew thousands. You have to leave enough metal to thread with.

-- It is always the right time, to do the right thing.

View Grandpa's profile


3259 posts in 2698 days

#5 posted 08-29-2011 08:37 PM

That was what I was trying to say Arlin. I will cheat a little with a larger drill only if the dia. is close and the strength required is not great. Otherwise, go to the store and make the purchase. You should have it when the job is completed.

View crank49's profile


4032 posts in 2994 days

#6 posted 08-29-2011 09:11 PM

For what it’s worth, a 21/64” bit is .3281”, just a tad smaller than the “Q” bit. Might bind up on the tap.

And the 11/32” bit is 0.3437”, just a tad over size. This is what I’d use if the strength is not an issue. Your finished 3/8 threaded hole will be 0.375”, so you will still have some meat in the threads.

View Dallas's profile


3599 posts in 2510 days

#7 posted 08-29-2011 09:56 PM

It’s for an oil lamp…. 3/8-24 would use a 5/16 bit. It’s not a precise fit or hardened steel. If you have a 21/64” bit, use it.

Make sure you tighten in a turn, back out a turn, tighten in a turn, back out a turn…. Keep the cutting edges clean as you go.

It’s not rocket surgery, so get as close to the size with the closest undersize that you own.

Good Luck, If they don’t find ya handsome at least they can find ya handy!

-- Improvise.... Adapt...... Overcome!

View Flyin636's profile


57 posts in 2516 days

#8 posted 08-31-2011 10:59 AM

Agree,not rocket science.Reason for post was that improper storage eats up more taps than actual usage.Throwin them in a drawer to bang around into each other will guarantee a dulled tap.Use compressed air to blow chips off tap,and put it back either in its box or;grab a nice block of wood(we use highly figure “drops)and drill a hole in top for taps along with their mated drills.Flyin636

View Elvin's profile


72 posts in 3422 days

#9 posted 09-07-2011 11:13 PM

Thanks everyone. I couldn’t find a Q bit around town so I tried the 11/32” drill bit and it worked. I have a secure fit for the light and harp. Now I just have to finish the table. I will keep you posted.
Again thanks for taking the time to help.

-- Elvin, Southern California, "How great would life be if we lived a little of it everyday"

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