5/16" or 11/32"?


What is the correct hole size for tapping for 1/8"NPT threads?


I searched prior posts on this, and I have found one post recommending one size, and another the other. The salesmen at Sears couldn't help me,http://www.dieselplace.com/forum/smileys/Confused.gif but I know someone here can...http://www.dieselplace.com/forum/smileys/Big Smile.gif





I know everyone will be glad when I finish installing these things...I feel like a pest continually asking questions!http://www.dieselplace.com/forum/smileys/Embarrased.gif








Thanks,


Kevinhttp://www.dieselplace.com/forum/smileys/Geek.gif

Fireman,


I did a "test" drill and tap( 1/8" NPT) into a piece of 1/8 thick carbon steel in my basement. The 5/16 (.3125") drill was to small to tap even with all my bodyweight onto the tap, I couldn't get it started. Redrilled using an 11/32 (.3437") drill and was able to get it tapped. You have to figure in being in a tight spot when doing the actual job, so you don't want it to be any harder than it has to be. I drove the tap in about 3/4 of the way in(about 5 threads showing on the tap) and the fitting screwed in kinda hard by hand which I thought was just right. After searching the internet, it looks like the "TEXT BOOK" drill size is "R", which is .339" - Harder to find though. I will be doing the real task on monday and will be alot more nervous than I was on the test run. Hope this helps & good luck.

<TABLE cols=1 width="100%" border=1>
<T>
<TR>
<TD>
<CENTER>National Pipe Tapered Tap Chart
(Recommended tap drill to use for 75% thread depth)</CENTER></TD></TR></T></TABLE>
<CENTER>
<TABLE cellSpacing=0 cellPadding=0 width="77%" border=1>
<T>
<TR>
<TD align=middle width="42%" bgColor=#ffffff>To Tap This Size Pipe:</TD>
<TD align=middle width="26%" bgColor=#ffffff>Use This Drill Bit:</TD>
<TD align=middle width="33%" bgColor=#ffffff>(Closest Fractional:)</TD>
<TD align=middle width="40%" bgColor=#ffffff>Decimal Inches:</TD></TR>
<TR>
<TD align=middle width="42%" bgColor=#ffffff>1/8-27 NPT</TD>
<TD align=middle width="26%" bgColor=#ffffff>Ltr.R</TD>
<TD align=middle width="33%" bgColor=#ffffff>-</TD>
<TD align=middle width="40%" bgColor=#ffffff>.3390</TD></TR>
<TR>
<TD align=middle width="42%" bgColor=#ffffff>1/4-18 NPT</TD>
<TD align=middle width="26%" bgColor=#ffffff>7/16"</TD>
<TD align=middle width="33%" bgColor=#ffffff>7/16"</TD>
<TD align=middle width="40%" bgColor=#ffffff>.4375</TD></TR>
<TR>
<TD align=middle width="42%" bgColor=#ffffff>3/8-18 NPT</TD>
<TD align=middle width="26%" bgColor=#ffffff>37/64"</TD>
<TD align=middle width="33%" bgColor=#ffffff>67/64"</TD>
<TD align=middle width="40%" bgColor=#ffffff>.5781</TD></TR>
<TR>
<TD align=middle width="42%" bgColor=#ffffff>1/2-14 NPT</TD>
<TD align=middle width="26%" bgColor=#ffffff>45/64"</TD>
<TD align=middle width="33%" bgColor=#ffffff>45/64"</TD>
<TD align=middle width="40%" bgColor=#ffffff>.7031</TD></TR>
<TR>
<TD align=middle width="42%" bgColor=#ffffff>3/4-14 NPT</TD>
<TD align=middle width="26%" bgColor=#ffffff>59/64"</TD>
<TD align=middle width="33%" bgColor=#ffffff>59/64"</TD>
<TD align=middle width="40%" bgColor=#ffffff>.9219</TD></TR>
<TR>
<TD align=middle width="42%" bgColor=#ffffff>1-11 1/2 NPT</TD>
<TD align=middle width="26%" bgColor=#ffffff>1 5/32"</TD>
<TD align=middle width="33%" bgColor=#ffffff>1 5/32"</TD>
<TD align=middle width="40%" bgColor=#ffffff>1.1562</TD></TR>
<TR>
<TD align=middle width="42%" bgColor=#ffffff>1 1/4-11 1/2 NPT</TD>
<TD align=middle width="26%" bgColor=#ffffff>1 1/2"</TD>
<TD align=middle width="33%" bgColor=#ffffff>1 1/2"</TD>
<TD align=middle width="40%" bgColor=#ffffff>1.5000</TD></TR>
<TR>
<TD align=middle width="42%" bgColor=#ffffff>1 1/2-11 1/2 NPT</TD>
<TD align=middle width="26%" bgColor=#ffffff>1 47/64"</TD>
<TD align=middle width="33%" bgColor=#ffffff>1 47/64"</TD>
<TD align=middle width="40%" bgColor=#ffffff>1.7344</TD></TR>
<TR>
<TD align=middle width="42%" bgColor=#ffffff>2-11 1/2 NPT</TD>
<TD align=middle width="26%" bgColor=#ffffff>2 7/32"</TD>
<TD align=middle width="33%" bgColor=#ffffff>2 7/32"</TD>
<TD align=middle width="40%" bgColor=#ffffff>2.2188</TD></TR></T></TABLE></CENTER>
<CENTER>I believe the rule of thumb is go to next size up so 11/32 is I believe the one ...</CENTER>
<CENTER> </CENTER>
<CENTER>Mac </CENTER>

Fireman,


I did a "test" drill and tap( 1/8" NPT) into a piece of 1/8 thick carbon steel in my basement. The 5/16 (.3125") drill was to small to tap even with all my bodyweight onto the tap, I couldn't get it started. Redrilled using an 11/32 (.3437") drill and was able to get it tapped. You have to figure in being in a tight spot when doing the actual job, so you don't want it to be any harder than it has to be. I drove the tap in about 3/4 of the way in(about 5 threads showing on the tap) and the fitting screwed in kinda hard by hand which I thought was just right. After searching the internet, it looks like the "TEXT BOOK" drill size is "R", which is .339" - Harder to find though. I will be doing the real task on monday and will be alot more nervous than I was on the test run. Hope this helps &amp; good luck.





Thanks Rayzi,


I thought it may have been difficult to cut the threads if I used only a 5/16", but I sure don't want to over-bore the hole either!


I'll use the 11/32".








Thanks,


Kevin

<TABLE cols=1 width="100%" border=1><T>
<T>
<TR>
<TD>
<CENTER>National Pipe Tapered Tap Chart
(Recommended tap drill to use for 75% thread depth)</CENTER></TD></TR></T></T></TABLE>
<CENTER>
<TABLE cellSpacing=0 cellPadding=0 width="77%" border=1><T>
<T>
<TR>
<TD align=middle width="42%" bgColor=#ffffff>To Tap This Size Pipe:</TD>
<TD align=middle width="26%" bgColor=#ffffff>Use This Drill Bit:</TD>
<TD align=middle width="33%" bgColor=#ffffff>(Closest Fractional:)</TD>
<TD align=middle width="40%" bgColor=#ffffff>Decimal Inches:</TD></TR>
<TR>
<TD align=middle width="42%" bgColor=#ffffff>1/8-27 NPT</TD>
<TD align=middle width="26%" bgColor=#ffffff>Ltr.R</TD>
<TD align=middle width="33%" bgColor=#ffffff>-</TD>
<TD align=middle width="40%" bgColor=#ffffff>.3390</TD></TR>
<TR>
<TD align=middle width="42%" bgColor=#ffffff>1/4-18 NPT</TD>
<TD align=middle width="26%" bgColor=#ffffff>7/16"</TD>
<TD align=middle width="33%" bgColor=#ffffff>7/16"</TD>
<TD align=middle width="40%" bgColor=#ffffff>.4375</TD></TR>
<TR>
<TD align=middle width="42%" bgColor=#ffffff>3/8-18 NPT</TD>
<TD align=middle width="26%" bgColor=#ffffff>37/64"</TD>
<TD align=middle width="33%" bgColor=#ffffff>67/64"</TD>
<TD align=middle width="40%" bgColor=#ffffff>.5781</TD></TR>
<TR>
<TD align=middle width="42%" bgColor=#ffffff>1/2-14 NPT</TD>
<TD align=middle width="26%" bgColor=#ffffff>45/64"</TD>
<TD align=middle width="33%" bgColor=#ffffff>45/64"</TD>
<TD align=middle width="40%" bgColor=#ffffff>.7031</TD></TR>
<TR>
<TD align=middle width="42%" bgColor=#ffffff>3/4-14 NPT</TD>
<TD align=middle width="26%" bgColor=#ffffff>59/64"</TD>
<TD align=middle width="33%" bgColor=#ffffff>59/64"</TD>
<TD align=middle width="40%" bgColor=#ffffff>.9219</TD></TR>
<TR>
<TD align=middle width="42%" bgColor=#ffffff>1-11 1/2 NPT</TD>
<TD align=middle width="26%" bgColor=#ffffff>1 5/32"</TD>
<TD align=middle width="33%" bgColor=#ffffff>1 5/32"</TD>
<TD align=middle width="40%" bgColor=#ffffff>1.1562</TD></TR>
<TR>
<TD align=middle width="42%" bgColor=#ffffff>1 1/4-11 1/2 NPT</TD>
<TD align=middle width="26%" bgColor=#ffffff>1 1/2"</TD>
<TD align=middle width="33%" bgColor=#ffffff>1 1/2"</TD>
<TD align=middle width="40%" bgColor=#ffffff>1.5000</TD></TR>
<TR>
<TD align=middle width="42%" bgColor=#ffffff>1 1/2-11 1/2 NPT</TD>
<TD align=middle width="26%" bgColor=#ffffff>1 47/64"</TD>
<TD align=middle width="33%" bgColor=#ffffff>1 47/64"</TD>
<TD align=middle width="40%" bgColor=#ffffff>1.7344</TD></TR>
<TR>
<TD align=middle width="42%" bgColor=#ffffff>2-11 1/2 NPT</TD>
<TD align=middle width="26%" bgColor=#ffffff>2 7/32"</TD>
<TD align=middle width="33%" bgColor=#ffffff>2 7/32"</TD>
<TD align=middle width="40%" bgColor=#ffffff>2.2188</TD></TR></T></T></TABLE></CENTER>
<CENTER>I believe the rule of thumb is go to next size up so 11/32 is I believe the one ...</CENTER>
<CENTER> </CENTER>
<CENTER>Mac </CENTER>









Sweet!


Thanks Mac!








Kevin

Use the "R" Like MAC said. It's the proper size, period.

"R" it is then.





Thanks all!








Kevinhttp://www.dieselplace.com/forum/smileys/Big Smile.gif

hoot, I know your the guru around here when it comes to this stuff but when I posted this on another board I got corrected by another member. This was his reply:


The correct size is for when using a drill press, when using a hand drill you can not bore perfect and need to be safe by using a smaller bit.


Just wanted to hear your opinion on this. http://www.dieselplace.com/forum/smileys/Confused.gif Edited by: Max Power

hoot, I know your guru around here when it comes to this stuff but when I posted this on another board I got corrected by another member. This was his reply:


The correct size is for when using a drill press, when using a hand drill you can not bore perfect and need to be safe by using a smaller bit.


Just wanted to hear your opinion on this. http://www.dieselplace.com/forum/smileys/Confused.gif

I wouldn't argue with that but i wouldn't go much smaller either. Makes it hard to tap. I used the .339 drill when I did mine and it came out good. I did pilot drill it about an 1/8" though.

Also it's important to use quality, sharp tools. Because it's a tapered thread, a slightly oversize hole because of hand drilling has no repercussions. Matter of fact, it makes it even easier to tap.Edited by: hoot

The correct size is for when using a drill press, when using a hand drill you can not bore perfect and need to be safe by using a smaller bit.





I have to throw the BS flag on that one. Typically any drill bit will make a hole 0.0025-0.004" larger than the indicated size. A precision hole can only be made with a laser, hydrolaser, or an end-mill.


In reality you can adjust the tap drill size based on what class of thread fit you're trying to achive. Hoot is right the normal size is "R", if you try to get to tight on the thread fit, specially on NPT's, hang on. It will get real rough. I've snapped a few taps in my time trying to go for the snug fit using the less is better theory. http://www.dieselplace.com/forum/smileys/Embarrased.gif


In the interest of spirited Machinist debate. Other Machinist's can weigh in with their opinion. http://www.dieselplace.com/forum/smileys/Approve.gif

Most taps have a recommended drill size either etched into the shank, or on the packaging. I typically go by this first, and if not available, call my friendly machinist. Would be easier to print the handy chart though...

The correct size is for when using a drill press, when using a hand drill you can not bore perfect and need to be safe by using a smaller bit.


I have to throw the BS flag on that one. Typically any drill bit will make a hole 0.0025-0.004" larger than the indicated size. A precision hole can only be made with a laser, hydrolaser, or an end-mill.


In reality you can adjust the tap drill size based on what class of thread fit you're trying to achive. Hoot is right the normal size is "R", if you try to get to tight on the thread fit, specially on NPT's, hang on. It will get real rough. I've snapped a few taps in my time trying to go for the snug fit using the less is better theory. http://www.dieselplace.com/forum/smileys/Embarrased.gif


In the interest of spirited Machinist debate. Other Machinist's can weigh in with their opinion. http://www.dieselplace.com/forum/smileys/Approve.gif

Accurate holes most often are produced by drilling undersize, align boring and then using a reamer. Nothing we need to worry about here.

I've been in the machining business since 1974. Not bragging cause i know people in the auto repair business longer and still don't know what they're doing.

weird, my thermo couple used 1/4 NPT

There are different sizes depending on manufacturer and sensitivity.