The Stovebolt Site Forums The Shop Area The Tool Chest 1/2"-14 Tapered Drill bit?

I need to install a 1/2"-14 bushing (reducer bushing 1/2" to 1/8") into a hole in a GMC engine block. I purchased the recommended 45/64" drill for the 1/2" NPT tap. The bushing is tapered - and I can thread it into the tapped hole about (barely) 3/32" to 1/4" in. However, it protrudes too far from the block surface and I would like to turn it in further for a closer but not flush fit.

(the tap only makes it into the hole about 3/8" at best possibly less)

This bushing is placed on the block forward of the fuel pump, and it serves as the drain hole for the oil filter mounted above there. If I can thread it deeper into the block I can have a straighter line from the bottom of the oil filter to the reducer and eliminate a 90 degree bend or reduce a possible constriction for the oil to drain into the block.

The issue I have with the 45/64" drill bit is that it easily slides through the existing hole and therefore the diameter is not large enough to turn the tap or the bushing deeper into the hole. I will need a slightly larger diameter drill bit (or a tapered hole) rather than 45/64 - and wondering if a 23/32" tapered bit would be the preferred option. Or, is this risky? and I should try to come at this oil drain problem from a different approach - and problem solve the drain line and live with a bushing that is proud of the block 3/8 to 1/2"

Questions:

Is there a tapered drill bit that can account for the taper on the tap and bushing so I can thread it further into the block?

Or, is there a un-tapered 1/2"-14 bushing that would thread further into the block? I plan to use a thread seal on this.

Is this what I need to accomplish the task?

https://www.ebay.com/itm/45-64-HSS-Taper-Length-Drill-Bit-DWDTL45-64/133299922924?_

Last question: Are these tapered bits 45/64" at the point or do they taper from a smaller diameter at the point to 45/64" ?

Need advice from the experts about these things. Appreciate the help.

Tom

Of course, the other option is to modify the block for a full flow adapter and screw on oil filter. But I am not confident enough to do that job. Or do I have a place nearby here to fabricate the oil lines.

You should be able to use your 45/64's straight drill and then carefully run your pipe tap in.
Tapping cast iron is not too hard, keep it as straight as you can and when it gets hard to turn, back it off, clean and then back in 1/4 to 1/2 half turn deeper.
Use some kind of cutting oil also. Cooking oil will do in a pinch.
Rule of thumb is tap till you have about four threads still showing on the tap.
Be careful, it is real easy to go too deep and then your fitting will not seal.

Another trick is to magnetize the tap so that most of the metal chips stay on it.
Rake the shank of the tap over a good magnet and not the cutting edges.
Clean the tap after every half turn once it's starts getting hard to turn and then gage the depth with your fitting.
All fittings are also not all the same thread length, so you'll have to go slow until the feel is right and where you want it.
I also prefer pipe dope over teflon tape.
If you lived closer, I'd come over and show you how to do it.

Hope this helps and let us know how it worked out.
Don

Hi. Thank you for responding. I understand - but the hole is already 45/64" (or miniscule amount greater than 45/64) and the new45/64 drill bit just drops through the hole.

The need is to create a tapered hole for a tapered tap and bushing to thread deeper into the hole.


[*]Is that advisable and is there a tapered bit with the point of the bit at 45/64 gradually getting larger in diameter up the drill bit shank as the bit is drilled deeper into the block.

Yes, caution is key when I do this.

As a 35 year career machinist I have tapped hundreds of holes in cast iron. I have never used a tapered bit for any of them. Tapping a 1/2” NPT thread in cast iron is going to take a bit of force. I use a T-handle on the tap to give me more power. You can use a socket and ratchet but the will require more force and is harder to keep it straight. I tap about an 1/8 of a turn at a time, then back it out 1/4 turn to break the edge. About every 1 turn, I back it out completely to clean out the hole and check the thread.

As 2-Ton said, go slow so you don’t go too far. One more word of caution, when you tighten the bushing, it will go in one or two threads more after it’s hand tight before it gets tight.

I also prefer pipe dope over teflon tape. Good luck and keep us post on your results.

Got it - my inexperience and trepidation again, keeps me from believing. I went out to the shop, put my largest crescent wrench on the tap, and a 30" pipe for fulcrum, and gradually came to understand that the tap won't snap, and a lot of force is needed to do this.

I kept thinking, when is the tap going to break off? but with quarter turns, backing out, re-oiling with cutting oil, and going again, it slowly worked its way down the hole. It takes patience -my initial reaction is to keep reaming on it to get it done...but...nope, wrong temperament.

I appreciate this tutoring... always learning - and doing is the best way to learn.

Oh, yeah, time for a beer...oh wait...its only 9:20 AM.

Thank you, Two-ton and Phak1.

Tom

Good to hear Tom, glad it worked out.
It's amazing on the knowledge you can get off of the folks on this site!

Phil, I am also a career Machinist Until I retired 6+ years ago at the age of 58.
Started when I was 16 at a Tech High School.

Don

Also glad it worked out for you Tom. Always great to hear success stories.

Don, if I recall correctly, we both worked in Nuc plants also. Small world!

When I need a pipe tap to go deeper than usual into a hole I grind a few threads off the end of the tap. Start the hole with the normal full-thread tap, then switch to the shortened tap to make the hole bigger where the reducer fits. We used that trick frequently when tapping the back of small block V8 oil passages for flow restrictors when running solid lifter camshafts.
Jerry

A homemade bottoming tap?

A bottoming tap is straight threaded and meant for blind holes. What HR is talking about is making the threads a bit wider so you have more thread engagement on pipe fittings. Useful if the mating fitting or pipe threads don’t have enough thread or not made deep enough.

If you grind a starting (straight threaded, ex. SAE) tap down you can make it into a bottoming tap.

Read the original post- - - - -he's working with a 1/2" NPT TAPRED PIPE THREAD TAP! The bushing he's trying to install isn't threading into the hole he's tapped far enough, and apparently it's bottoming out against something. Grinding off a few of the starting threads of ther tap (THE NARROW END OF THE TAPER!) will allow the TAPER THREADED BUSHING to screw in deeper!
Jerry

Easy now. I found some NPT bottoming taps, believe it or don't. Otherwise I wouldn't have mentioned it. At least that's what they were labeled as. Nothing as small as 1/2", but they do exist. I understand what bottoming taps are and I understand what he is trying to accomplish. And I understand what you are explaining to him. No need to go all red font on me!

Nobody ever dare redfont me, by golly, or I'll threaten to make a foot stool out of their teakettle toot sweet!

Pretty sad nomenclature here...

Cast iron is very forgiving but in difficult materials tapering the hole will prevent tearing of the threads. This tool is not a tapered drill bit but is called a pipe tap reamer like this:

https://www.mcmaster.com/pipe-reamers/

In shallow bores there are no tricks required. One simply requests a short projection pipe tap like this:

https://www.mcmaster.com/pipe-taps/pipe-and-conduit-thread-taps-for-shallow-holes/


How deep to go with the tap? To gauge NPT threads insert the fitting/plug hand tight and make a mark or dot. Turn counterclockwise and count turns until it lets go. 3 full turns plus/minus 1/2 turn is what you want.

I've spent every day of the last 40+ years metalcutting...kinda short of apprentices lately so there have been days I have run all these machines simultaneously.

Almost looks like a blower pulley.

Today I have a part that requires a 1/2" NPT port for a transducer so I'll share the tooling and method I use. This tool is an indexable carbide threadmill blade which is moved in a helical motion around the bore. I rough the thread in the first pass and then take a light finish cut to insure consistent gauging. Total cutting time = under 5 seconds.