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Discussion Starter · #1 ·
I have a cheap Harbor Freight tap/die set that I used on another project. I've got all the "clip nuts" off for things like the fender. I'm trying to find the correct size/pitch and don't understand the numbers.

I use the gauge and it says "18"

A tap that seems to work is marked "5/16" and the die that seems to fit is marked "3/8 NC 16"

The tap and die seem to fit together. I run both of these on a sample nut/bolt pair and if work, I can operate with fingers only.

I don't understand, I have 3 sets of numbers:
18
5/16
3/8 NC 16

Yet they all seem to fit the same nut/bolt pair.

I'm tapping all the nuts I can before the last assembly and just in case any paint gets in there.

What's the magic key to reading these tap/die sets and am I doing something wrong? The nuts fight a bit, but nothing close to being like a full re-thread.
 

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You are kidding, right..........? In case you really don't know see below.

18 is the thread pitch, in this case 5/16-18 is a coarse thread bolt. 5/16 and 3/8 refer to the bolt diameter, they are not interchangeable.

By the way for cleaning up threads or restoring damaged threads you are better served with a thread restorer set.
 

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Hi,

Perhaps an old machinist can help. Take 5/16-18 UNC for example. 5/16 is the nominal diameter of the threaded body of a given bolt, whereas 18 represents the number of threads per inch (or pitch) of that bolt. UNC stands for "Unified National Coarse" and its counterpart UNF for "Unified National Fine". The "UN" thread form superseded "N" decades ago, although they are nearly identical dimensionally. UNC and UNF are industry standards for commonly used threaded fasteners. Such standards dictate what dimensional values for each size of thread are acceptable when manufacturing bolts and nuts. They insure that a nut from one manufacturer threads onto the same size of bolt machined by a different manufacturer.

A 5/16-18 tap can sometimes fit into a 3/8-16 clip nut because: (1) 5/16 is smaller than 3/8, allowing for diametral clearance, or "slop"; (2) 18 pitch (threads per inch) is not all that different from 16 pitch; and (3) old, worn...or poorly made bolts and nuts will sometimes allow for fasteners of different sizes to crudely fit together.

If a 5/16-18 tap threads into a 3/8-16 die throw one or both away, as they are either worn out, have broken cutting edges, or are junk. You don't, however, need to spend big bucks to buy an American made, industrial grade tap and die set. Most of the offshore stuff is more than adequate for occasional use.

You don't need to be an engineer or machinist to decipher thread forms, sizes and dimensions. A "Machinery Handbook", often referred to as the "machinist's bible" is an excellent resource. It is an excellent source for most any mechanical or industrial subject matter.

I hope I've helped and not confused you further. Good luck!
 

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Discussion Starter · #4 ·
Ok it looks like the tap is not fully marked. It is an 18 pitch as is the bolt and nut.

The die marked 3/8 NC 16 is pretty close and the bolt actually does thread down just using fingers.

So I ran the tap and tried the 5/16 18 die and it worked. It was dam hard to get it all the way down and it really cleaned things up. However, it really didn't make that much difference.

I can use fingers on the untouched or the 5/16 18 and there really isn't much difference. In looking at the other test bolt, after running it on 50 nuts, it's clear the bottom of the threads aren't being touched.

I'm going to run the wire brush bench grinder and see how everything works.
 

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Discussion Starter · #6 ·
You are kidding, right..........? In case you really don't know see below.

18 is the thread pitch, in this case 5/16-18 is a coarse thread bolt. 5/16 and 3/8 refer to the bolt diameter, they are not interchangeable.

By the way for cleaning up threads or restoring damaged threads you are better served with a thread restorer set.
I actually wasn't kidding, it ends up that getting deep down into the threads was a real pain. The 3/8 and 5/16 actually do thread on the same bolt, the 5/16 18 works but a fight all the way down.

Probably should soak and wire brush all the thread and try again.
 

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Discussion Starter · #7 ·
sounds to me like you just screwed up a set of bolts and clip nuts
Thankfully I didn't run the die, only the tap. I ran a test on a few parts before doing them all. The correct pitch is 18 and 5/16 is the correct size. The problem was that deep in the thread of the bolts is full of crud and made it dam hard to clean.

I found some cherry bolts I just pulled today and was able to confirm the size and pitch.

My mistake on not cleaning deeper before trying to check all the threads.

The good news is they flow like glass now.
 

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I've been wire-wheeling bolts for 50 years or more. Growing up, my dad always had a belt-driven grinder with a wire wheel of some kind on one end.

I now have the same old set up on one of my benches.

Sure miss my dad....
 
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Another handy tool (and its cheap) is a thread pitch gage set. It looks like a feeler gage set kind of but has visual examples of the various threads and they are marked. Just place the thread gage over the thread profile for an accurate comparison. Of course a cheapo vernier caliper can get you a very accurate diameter reading also. You will rarely ever run into a UNF (fine thread) on many car projects but you sure can run into metric bolts that are close and mimic an American standard bolt etc. Of course there are metric thread gages too but they are marked as the distance between threads ie: 12mm dia with a .8mm pitch. it will be marked .8. for example. Good Luck!

Wallydog
 

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Discussion Starter · #11 ·
Another handy tool (and its cheap) is a thread pitch gage set. It looks like a feeler gage set kind of but has visual examples of the various threads and they are marked. Just place the thread gage over the thread profile for an accurate comparison. Of course a cheapo vernier caliper can get you a very accurate diameter reading also. You will rarely ever run into a UNF (fine thread) on many car projects but you sure can run into metric bolts that are close and mimic an American standard bolt etc. Of course there are metric thread gages too but they are marked as the distance between threads ie: 12mm dia with a .8mm pitch. it will be marked .8. for example. Good Luck!

Wallydog
The kit does have the gauge tool, I was able to confirm 18 pitch. There was a few problems. The set wasn't clearly marked, the nuts/bolts weren't well cleaned before I started working with them.

I did get them to the point where they work with only fingers. I'll have to look thru all the bolts and see how they look.

I think a bit of acetone and wire brush should help. Another part of the problem was how these were stored over the years. This project has been going on since the 70's and some of the nuts weren't stored well.
 

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So far you have gotten some good advice. Using a tap to clean threads can sometimes remove metal that you might want to keep. So, there are tools called "Thread Chasers" which are intended for cleaning threads. Here is what appears at Summit when you Google Thread Chasers. Of course there are a number of other places that can sell you these.

https://www.summitracing.com/search/part-type/thread-chaser-sets
 

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I have the Craftsman set which is shown in the above link on top of the page. In my opinion they work much better then a tap or die IF you just need to clean up or restore existing threads. I use them all the time.
 

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I made the mistake of buying a HF tap and die set mainly to get the small numbered taps for a likely one time use. It's junk IMO. Sizes are off.
 

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I have the Craftsman set which is shown in the above link on top of the page. In my opinion they work much better then a tap or die IF you just need to clean up or restore existing threads. I use them all the time.
Yup. I agree that these are the way to go, when you want to just clean up existing threads. Taps are designed to cut new threads.
 

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As a shade tree mechanic the general rule of them I use is
9/16 bolt head usually is a 3/8
1/2 " bolt head = 5/16
7/16= 1/4"
5/8=7/16
 

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Discussion Starter · #17 ·
Well the HF handle broke. The kit comes with 2 holders for taps, 1 for dies. I thought it was odd that some taps had a pointed back tip and some didn't. I was able to use the other handle by spreading open the edges.

The good news is that it is really working well. I can finger tighten these bolts and that'll be nice when putting everything back together.

I don't remember what I paid for the HF set, but it was used quite a bit in another restore about 14 years ago, but overall, not a quality set.

Quite a bit of HF stuff is use till it breaks, then toss it.
 

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A drill index or open end wrench can help determine some common diameters.
I have a few of the plastic gauges from Fastenal, Wurth and KAr Products.
Thread files, nut size thread chasers and tap and die set all come in handy.
Some holes need a tap if they are too deep for a chaser.
These are also very handy Chase 'em Back Tools

The tools should be worked back and forth with some cutting oil , the same as if you were cutting threads.
There shouldn't be anywhere near enough force to break a handle.

There are starting tap, bottoming taps and a third I forget.
The starting are tapered ("pointed back tip" per KarlJay) to make it easier to start cutting new threads.
A bottoming tap has threads to the end to cut threads to the bottom of a blind hole.
 

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I still reuse and "restore" a lot hardware, but lately I have been buying AMK products. Not sure about 100% concours, but many of their bolts/nuts have the correct marking and finishes, come it kits.

It costs money, but saves time having to tap/die/clean/refinish nuts and bolts... And despite being in California, I still seem to ge ta few of those nuts that brake off the clip because the bolt/nut was corroded...

Thru NPD, I think a "Master" body fastener kit is about $200 (460+ pieces for 1966 Chevelle body)... Under $50 for the entire front end fastener kit...
 

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Thanks for sharing your experiences with Harbor Freight taps and dies, Von. Others have told me such products work OK. I guess i am fortunate that when I purchased a Bridgeport milling machine it came with two Kennedy toolboxes and rollaways stuffed with (American-made, industrial grade) taps, dies, endmills and other machinist tools. The tooling is actually worth much more than the mill...which is a very nice Easy Trac II model.
 
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