Wamsutta
Well-known member
Is the H1 thread limit better for chasing threads?
Less chance of enlarging the threaded hole?
Less chance of enlarging the threaded hole?
Taps with H1 Thread Limit
- Thread starter Wamsutta
- Start date
OccupantRJ
Well-known member
That is what I tend to use for general refreshing unless I see a larger limit is needed, but I have the available taps to do so. Be aware that there are also undersize taps available. I predict this thread will open the proverbial can of worms relating to thread chasers. I don't own any, and see no need to own any. If a hole was tapped with an H1 at the factory, what is the tolerance of the thread chaser one would be using? How about if the hole was tapped with an H7?
I had heard years ago that the auto makers sometimes used H-1 or H-2 for tapped holes in blocks.
They got specially designed (?) taps for cast iron.
Keep in mind that each number amounts to half a thou, (.0005), in diameter.
Not near enough to matter in the real world.
They got specially designed (?) taps for cast iron.
Keep in mind that each number amounts to half a thou, (.0005), in diameter.
Not near enough to matter in the real world.
Technically might be but H-1 are quite a bit less common than H-3 in the real world and the .001 diameter difference won't matter.
Heeeeeeere we go...
If you have a bolt for the thread you're trying to clean, you can make a thread chasing tool for less than $1 in about 30 seconds.
Using taps is a method that works most of the time, but can cause problems if you're not careful. Using a bolt which a groove cut in it is much more idiot proof.
With regard to .0005" 'not mattering'.... Ha. The pitch diameter tolerance window for a 1/4 bolt is only 28 tenths wide. In the 'real world' quality fastener manufacturers hold tolerances very close to the center of that window. This is the beauty of threads which are rolled and not cut- single-tenth process control is not only possible, it's common. Which means that a change of half a thou is significant, and requires a machine setup change to make parts of a different class. If you think it doesn't matter, go build some machined parts with 1A/B fasteners and assemble them, and then repeat with 3A/3B fasteners.
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In the real world, you are using thread chasers because a hole is cross-threaded.
If you stick a sharp tap into a bolt-hole that is already damaged, you run the risk of cross-threading the tap. While you will get new threads cut, if you thing those threads are h1 or whatever limit your tap is, you're being hopelessly naive. Those threads are often (if not usually) junk, and what you are relying on is the previous threads still to exist and remain near spec. But the tap being off just damages the existing threads the more you insert it, once its off...longitudinally...by a small amout.
Tap limits are diametric not longitudinal for the most part.
This is why you never use a sharp tap to clean dirt and grime. For that you want a dull tool that won't risk cutting into metal (essentially cross threading). You can uses a loose-fit tap, but a higher precision tap is not helpful inho because its more likely to cross-thread once it gets off slightly for whatever reason.
This is why chasing is usually done with dull/sanded cutting taps or those crappy looking things that look like grade 2 bolts with grooves in them.
Just my $0.02, but I think its worth considering.
Also....aluminum is the material most likelt to be a problem. Its way to grabby and way to soft to chase (repeatedly) using close-fitting sharps....so maybe with steel things are slightly different....but again experience says its the softer stuff that is needing most of the work.
If you stick a sharp tap into a bolt-hole that is already damaged, you run the risk of cross-threading the tap. While you will get new threads cut, if you thing those threads are h1 or whatever limit your tap is, you're being hopelessly naive. Those threads are often (if not usually) junk, and what you are relying on is the previous threads still to exist and remain near spec. But the tap being off just damages the existing threads the more you insert it, once its off...longitudinally...by a small amout.
Tap limits are diametric not longitudinal for the most part.
This is why you never use a sharp tap to clean dirt and grime. For that you want a dull tool that won't risk cutting into metal (essentially cross threading). You can uses a loose-fit tap, but a higher precision tap is not helpful inho because its more likely to cross-thread once it gets off slightly for whatever reason.
This is why chasing is usually done with dull/sanded cutting taps or those crappy looking things that look like grade 2 bolts with grooves in them.
Just my $0.02, but I think its worth considering.
Also....aluminum is the material most likelt to be a problem. Its way to grabby and way to soft to chase (repeatedly) using close-fitting sharps....so maybe with steel things are slightly different....but again experience says its the softer stuff that is needing most of the work.
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Leg17 is right.
A maximum H1 tap, and a minimum H3 tap is only 0.0005" apart in pitch diameter. With a maximum of 0.001" between the two sizes.
But the class of bolt/screw you use will play a bigger roll. For example in 1/4-20.
So you are looking at over 5 times the difference between two acceptable 1A threaded bolts, compared to the difference between H1 and H3 at their biggest difference. Class 2A which is the most common, is still nearly 4 times greater in difference then the H1 to H3 jump.
Dimitri
A maximum H1 tap, and a minimum H3 tap is only 0.0005" apart in pitch diameter. With a maximum of 0.001" between the two sizes.
But the class of bolt/screw you use will play a bigger roll. For example in 1/4-20.
- Class 1A - 0.2108 to 0.2164 or 0.0056" total tolerance
- Class 2A - 0.2127 to 0.2164 or 0.0037" total tolerance
- Class 3B - 0.2147 to 0.2175 or 0.0028" total tolerance
So you are looking at over 5 times the difference between two acceptable 1A threaded bolts, compared to the difference between H1 and H3 at their biggest difference. Class 2A which is the most common, is still nearly 4 times greater in difference then the H1 to H3 jump.
Dimitri
Clean the holes as much as possible with solvent and air first.
Then clean the hole with a 1/4 drill bit carefully. You don't want to make the hole any bigger, just get the junk out.
Get a conventional plug tap, H-1 if you can find it, or H-3.
Use a real tap wrench which will help you line up straight with the hole.
Apply some grease to hold the contamination.
Back off every half turn. You'll get the hang of that quickly.
Proceed slowly, be careful, so you don't break the tap. The contamination might try to tighten up the tap.
Remove it every couple of turns to clean the gunk and re-apply the grease or whatever you use. Look especially for new chips indicating a problem.
You should be able to 'feel' that you are in the existing thread.
Get an idea where the bottom of the hole is and stop a little short so you don't jam the tap tight and break it off.
It's not a race. Take your time. Its not rocket surgery.
The biggest enemies of using a tap are carelessness and impatience.
Let us know how you make out.
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tr0n
Well-known member
****, there's different sizes of sizes?
colour me a baffled mechanic.
colour me a baffled mechanic.
Bottoming taps are a last resort kind of thing. When you bottom out a plug, then start using a bottoming tap.
But a plug tap is easier to start (both when tapping a new hole, or giving threads a clean up). And is less likely to start cocked like a bottoming tap would.
Dimitri
But a plug tap is easier to start (both when tapping a new hole, or giving threads a clean up). And is less likely to start cocked like a bottoming tap would.
Dimitri
OccupantRJ
Well-known member
Let me baffle you further then. There are also thread forming taps which have no flutes and essentially do no cutting at all. They are driven into the proper size hole to displace the walls of the holes to create internal threads. For some holes there are internal thread milling cutters that travel around the perimeter of the hole while setup in a milling machine. Take a look at the links below.
http://www.formdrill-usa.com/formtap.htm
Most tapped holes are not bottom tapped.
Especially in production environments, like engine block transfer lines.
Unless some particular situation requires, the tap drill is deeper than the full length of thread allowing a plug tap to do all the work.
Take a look here, for example.
http://www.blinn.edu/brazos/matheng/jcoffelt/graphics/files/ege/fast/fast_page4.htm
No point in making more of this than necessary.
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leg17
What is the difference between H-limit and class of fit?
I see Union Butterfield has either a 2B/3B class of fit, or a 3B class of fit:
https://www.mscdirect.com/product/details/40746455
What is the difference between H-limit and class of fit?
I see Union Butterfield has either a 2B/3B class of fit, or a 3B class of fit:
https://www.mscdirect.com/product/details/40746455
The tap you linked to is H3.
That is the usual everyday common size of tap.
"H3" means that the nominal size of the tap is "high" by 3 half thousands over the basic pitch diameter. (If it was .0015 smaller, it would be "low" marked with an "L3")
This allows a screw thread that is close to 'zero' to have enough clearance to actually screw in.
(The more clearance between screw and tapped hole, the lower the class fit number)
Remember, EVERYTHING has a tolerance. NOTHING is "PERFECT".
Here is another reference for you. You can google for yourself and find all you could possibly want to know.
https://www.fastenal.com/content/feds/pdf/Article - Screw Threads Design.pdf
Have you ever used a tap wrench?
If not, you might heed others advice and try a thread restorer type tool instead of a tap, which is somewhat harder, thus somewhat more brittle.
You may be over-thinking this whole thing.
leg17
I've had extensive experience cutting threads and installing thread inserts. I've used Heli-Coils, Keenserts, and Time-Serts. What I don't have is much experience chasing threads. For me, overthinking is much safer than underthinking. I've made some very dumb mistakes by underthinking. I've got a pair of cylinder heads that took me two days to pull off a car in the salvage yard and I literally lost some blood over that job. I don't want to screw those heads up. Pun intended.
Thank you for your well thought out and carefully worded explanations. Much appreciated.
I've had extensive experience cutting threads and installing thread inserts. I've used Heli-Coils, Keenserts, and Time-Serts. What I don't have is much experience chasing threads. For me, overthinking is much safer than underthinking. I've made some very dumb mistakes by underthinking. I've got a pair of cylinder heads that took me two days to pull off a car in the salvage yard and I literally lost some blood over that job. I don't want to screw those heads up. Pun intended.
Thank you for your well thought out and carefully worded explanations. Much appreciated.