What Size Hole For A #10x32 Tap?

LumberJack

LumberJack

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I bought a Boyds Thumbhole for my Marlin model 25, Boyds does not make a stock for this rifle so I got one for a model 925. the only difference inleting wise is that the 25 has only one action screw and a bedded barrel. The barrel channel on the Boyds (designed to free float) is too deep and causes the gun to tilt forward and be loose in the stock.

My plan is to add the second action screw to my model 25 by drilling and tapping the reciever and opening up the hole in my trigger guard.

What size drill bit do I need to use for the hole and what are some of the problems I could run into?

Im no gunsmith but I do a fair amount of tinkering with my guns and others as well.(measure 10 times drill/cut once)
 
#21 drill bit i believe, as for problems you may run into getting it straight and centered is a must. Not familiar with the marlin 25 so any specific problems i can't say.
 
.159" drill (number 21) for 10 -32.

http://bobmay.astronomy.net/misc/drillchart.htm

For anyone interested, you can easily calculate tap drill size for fractional national series and metric too. Both are 60 degree thread forms. In both cases, subtract the pitch from the major diameter to get the tap drill size.

Example: 1/2-13 NC
1 divided by 13 = .0769
.500" - .0769" = .423" drill size

Example: M4 x 0.5
4 - 0.5 = 3.5 mm tap drill
3.5 divided 25.4 (the constant for converting millimeters to inches) = .138" drill size

Hope this may help.
 
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Metric is easy just subtract pitch from size and you are correct every time.

4mm x0.5thread is 3.5mm drill 4 subtract .5 is 3.5 mm .
Not that easy with sae. you need a chart for quick reference.
You can do metric in your head without a chart.

R


R
 
Nice chart, but now to find one that crosses the lettered into fractions and the mm in fractions and
the numbered into fractions....

You will look forever for that no such animal let me know when you find it LOL

R
 
A 3/32 drill is only .005" small than the #21 and has worked fine for me. Also, if you buy your 10-32 tap and Can Tire, it will come packaged with a #21 drill so problem solved for less than $10.
 
ronnie01 said:
Metric is easy just subtract pitch from size and you are correct every time.

4mm x0.5thread is 3.5mm drill 4 subtract .5 is 3.5 mm .
Not that easy with sae. you need a chart for quick reference.
You can do metric in your head without a chart.

R


R
Click to expand...

Yeah.

Nominal diameter, minus the pitch of the thread, generally comes pretty good on the tap drill size.

You need to know how to convert fractions and decimals back and forth, as well as needing to know where the numbering system is based, for the knowledge to do you any good, but diameter minus pitch equals tap drill is a good rule of thumb.

In this case its .190 = #10 aka .060"+(10 x .013") minus 1/32nd of an inch or .03125", equals .15875", or close enough to .159", that you wouldn't know the difference.
.060" is # Zero, each step up is at .013" intervals.
Letter sizes are a soup sandwich of intervals that were used to cover needed sizes that were not available in other standard sets (fractions, wire numbers sizes) and Metric is the reasonable one, until you try to stock every size in .1mm increments up to the maximum size you might need...

And 'Looky, yep, any machinery supplier or tool outfit likely gives away thousands of wall charts and pocket reference card that show the inch, number, letter and metric standard drill sizes (.1mm steps typically) and the cross equivalent sizes in the 'other' systems. Dead bloody handy when you need that next size up or down, and need to start rooting through the bins to find it.
If yu don't want to trust to luck,Busy Bee, KBC Tools, Starrett, and a bunch of drill and tap companies like Dormer, all sell the charts too.


Cheers
Trev
 
Ol' Flinter said:
A 3/32 drill is only .005" small than the #21 and has worked fine for me. Also, if you buy your 10-32 tap and Can Tire, it will come packaged with a #21 drill so problem solved for less than $10.
Click to expand...

I think you meant to say 5/32 as mentioned before. 3/32 is only .093.

Kamloopy and anyone else. There are OODLES of tap drill charts or drill size charts to be found online with an easy search. Check a few out and print out the one you like best.

The problem with these small size taps is that even a couple of thou difference makes a BIG difference on the pressure required on the tap handle. If you're cutting threads in some tough correctly heat treated steel of a receiver it can easily be the difference between an easy job and a busted tap.

Anyone doing much of this sort of work owes it to themselves to invest in a set of number drills. Or even better is a complete index of fractional, number and letter drills. The discount tool stores such as KMS or Busy Bee sell such sets of import drill bits for around $80 when they go on sale and around $100 at regular pricing.
 
Be careful with those charts, I've seen lots of them come from the tooling companies with typos that will ruin your day if you follow them. Reference more than one chart if you can.
 
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