What is the Proper Technique for Tapping an NPT Thread.

[Capt. Hack]

NEVER TAP CAST DRY! Use some kind of cutting fluid. Any kind is better than no kind

[Capt. Hack]

One more thing,if your chasing old threads they could BPT meaning British Pipe Thread.which is a metric

[artisanmach]

Helpful info

[georgeg305]

Just a thought/trivia on NPT threads.
NPT stands for National Pipe Thread. NPT is an "American" Standard for tapered threads. The engine on my Catalina 30 is a Universal M-25. This is really a Kubota block. The tapers on the Kubota block holes are "JIS" ( Japanese Industrial Standard ) and are different than NPT. BPT stands for British Pipe Thread. BPT is compatibile with JIS. NAPA sells BPT to NPT and NPT to BPT adapter fittings.
I discovered this when I wanted to put an oil pressure gage on my M-25 and not just be content with the pressure switch which is set around 5-7 psi. Normal pressure is around 50 psi.

[coastalexplorer]

Here I write to give a bit of historical information,

Decades ago the teaching was to turn a tap forward a turn or two and then back it off too break the chip. During the late 1970's better manufacturing methods were developed to grind clearance on taps So the instruction by Union Butterfield tools was to turn to depth or all the way through turning in the same direction. Because backing off to break the chip was no longer necessary.

Now of course in the case of Pipe threads fitting is necessary. so removal of the tap several times for testing for fit becomes the exception to the rule.

The Victorian tradesmen that I apprenticed with taught this rhyme concerning the use of cutting fluids.

Oil on wrought, nothing on brass.
kerosene on ally, nothing on cast.

*Oil on wrought (also meaning steel's)

The fact is that If a fluid is used on Cast iron the swarf turns to the consistency of mud. Cast Iron has such a high quantity of graphite in it that it acts as a dry lubricant. Thus vacuuming or blowing away the dust with an air line will give a clean thread for a more accurate fit.

[Wotname]

Originally Posted by Capt. Hack

NEVER TAP CAST DRY! Use some kind of cutting fluid. Any kind is better than no kind

Originally Posted by coastalexplorer

..............

The Victorian tradesmen that I apprenticed with taught this rhyme concerning the use of cutting fluids.

Oil on wrought, nothing on brass.
kerosene on ally, nothing on cast.

*Oil on wrought (also meaning steel's)

The fact is that If a fluid is used on Cast iron the swarf turns to the consistency of mud. Cast Iron has such a high quantity of graphite in it that it acts as a dry lubricant. Thus vacuuming or blowing away the dust with an air line will give a clean thread for a more accurate fit.

Hmm... conflicting advice
Anyone want to suggest which is best?

We are talking cast iron in this instance, not cast alloy.

[Jim Cate]

Hmm... conflicting advice

Why, fancy that! Hardly a thread goes by without that cropping up. So, in the long run the decision lies on your ability to sort wheat from chaff, Wottie... the buck stops in your capable hands.

My thought? I suspect that much of the general advice from pros is driven by wanting high production speeds. For your case, with careful approach and slow speeds it won't matter much whether you lube or go dry. I've done both with reasonable success... but I always use some sort of sealant just in case! And I doubt if there is a lot of difference in success rate between different brands. I usually use good ole Permatex #3. Shows my age, I guess!

Jim

[artisanmach]

always cut cast iron dry...

[Wotname]

Originally Posted by Jim Cate

Why, fancy that! Hardly a thread goes by without that cropping up.

Pun intended??

So, in the long run the decision lies on your ability to sort wheat from chaff, Wottie... the buck stops in your capable hands.

My thought? I suspect that much of the general advice from pros is driven by wanting high production speeds. For your case, with careful approach and slow speeds it won't matter much whether you lube or go dry. I've done both with reasonable success... but I always use some sort of sealant just in case! And I doubt if there is a lot of difference in success rate between different brands. I usually use good ole Permatex #3. Shows my age, I guess!

Jim

I just had a quick look on my sealants shelf, leaving aside the silicone stuff etc, I can choose between Loctite 222; No.3; 515; Cyberbond TM44; Stag Jointing paste; Hylomar universal jointing compound and an old tin of Flexment. One of them gotta work . At least none of these fitting will be under pressure
The Stag will probably get the nod for this job.

I see you have made Eden, looks like you are making good time, I hope it continues this way for you.

Originally Posted by Capt. Hack

NEVER TAP CAST DRY! Use some kind of cutting fluid. Any kind is better than no kind

Originally Posted by artisanmach

always cut cast iron dry...

Hey Jim, maybe I should take the one capitals, they must account for something

[Pelagic]

I'm enjoying this Thread!
(Pun intended)

[coastalexplorer]

Wotname you asked Anyone want to suggest which is best?....I have already told you. and artisanMach whom I do not know is perfectly correct.
in the machine shop all grey cast iron is machined dry. During the process of drilling the tapping sized hole you will have no difficulty identifying grey cast iron because the drill will make dust.

Now the only exception to this is a casting made of Ductile iron also marketed under the term semi steel. Drilling this could produce a swaff as seen when drilling steel. While it is very unlikely that you engine block is made from this, unless it has cylinder liners, should you come across it then Yes you should use rapid tap or some other cutting fluid.

As an engineers Pattern maker I apprenticed in a general engineering works in Britain 1966 to 1971 it was run by tradesmen long past normal retirement age. Some born in the 1880's it was a requirement to work in all departments, Core making, sand molding, casting both ferrous and Non ferrous metals, machining, fitting, and forging. including fire welding, which I did before learning to use oxyacetylene & arch welding, We also did plug making for fiberglass moulds. I have also worked in about five different boat building companies during times when the demand for foundry work was slow here on the west coast. So I have taken a life time interest in trades and manufacturing processes. Do what you will with the advice given.

[Wotname]

Originally Posted by coastalexplorer

...........
in the machine shop all grey cast iron is machined dry. During the process of drilling the tapping sized hole you will have no difficulty identifying grey cast iron because the drill will make dust.

Now the only exception to this is a casting made of Ductile iron also marketed under the term semi steel. Drilling this could produce a swaff as seen when drilling steel. While it is very unlikely that you engine block is made from this, unless it has cylinder liners, should you come across it then Yes you should use rapid tap or some other cutting fluid..........

I love detail. Thanks for the explanation of the different types of "cast iron" and yes, the engine does have a wet liner so I will be observing the drill debris with a sharp eye.

[sailah]

I tap a lot. Plenty of NPT in my tap drawer up to 2.5"

My thoughts:

Use proper drill bit esp with NPT threads, creates a fair amount of force to tap tapered.

Tap cast iron dry, use tapping fluid on everything else. Anyone telling you to use lube on cast iron has not worked in a machine shop. Always tap, drill and machine cast iron dry. Old timers would use bacon grease if they felt the need. Cast contains graphite which acts as lubricant. You want to back the tap out 1/2 turn often and that's to clear the chips/dust which can cause tap to bind.

Use a proper tap handle if you can find one. A crescent wrench is a sure way to tap crooked

Don't tap full depth with NPT. I usually stop 3-4 threads before bottoming out tap and then check my fitting.

I use Loctite 592 & 567 thread sealants. If there is a better pipe sealant I haven't found it yet. Both rated to 400 degrees.

The dust/chips created by tapping is not something you want in your engine. Remove component if possible. If not possible, use heavy grease on threads (see tap lube lol) to catch chips

[Wotname]

Originally Posted by sailah

I tap a lot. Plenty of NPT in my tap drawer up to 2.5"

My thoughts:

Use proper drill bit esp with NPT threads, creates a fair amount of force to tap tapered.

Tap cast iron dry, use tapping fluid on everything else. Anyone telling you to use lube on cast iron has not worked in a machine shop. Always tap, drill and machine cast iron dry. Old timers would use bacon grease if they felt the need. Cast contains graphite which acts as lubricant. You want to back the tap out 1/2 turn often and that's to clear the chips/dust which can cause tap to bind.

Use a proper tap handle if you can find one. A crescent wrench is a sure way to tap crooked

Don't tap full depth with NPT. I usually stop 3-4 threads before bottoming out tap and then check my fitting.

I use Loctite 592 & 567 thread sealants. If there is a better pipe sealant I haven't found it yet. Both rated to 400 degrees.

The dust/chips created by tapping is not something you want in your engine. Remove component if possible. If not possible, use heavy grease on threads (see tap lube lol) to catch chips

Thanks.

It looks like we are getting a broad consensus to tap cast iron dry (except for cast ductile iron as mentioned by coastalexplorer)

I note gotta agree about the removal of the debris, right now the block is naked, absolutely bare, not even the liner is in and it will remain that way until the job is done!

[Wotname]

Job is finally completed!

A BIG thank you to those who advised to drill / tap cast iron DRY. I was amazed how easy it was. I have drilled and taped hundreds of holes in Al, mild steel, SS (304 & 316) but today was the first time in cast iron. Both drilling and tapping was a piece of cake!
FWIW, the cast iron head was much easier than the sump plug. The sensors fitted are temperature sensors (oil & water) for a custom set of temperature gauges.

I have worked a way around having the sensors penetrating the sump or the water jacket so the swarf issue was a non event.

Again for the advice offered in this thread.