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Metric tap is sooo close to UNF one

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  • Metric tap is sooo close to UNF one

    I was tapping a 1/2-20 hole and pulled an un-marked tap out of the extras box. I tapped the hole, but a 1/2-20 bolt was a poor fit. ?? On close examination, the tap was 13Mx1.25. You know how close 13Mx1.25 is to 1/2-20? 20tpi is 0.0500 pitch, 1.25 pitch is 0.0492 (1.6% smaller). 13mm is 0.5118, 2.4% large. Close, but no seegar.

    And, yes - it really is M13x1.25: it's clear with a pitch gauge.

    BTW - I have no idea how I came to have a M13x1.25 tap - when you collect stuff, it happens.

  • #2
    Try an M5 (x.8) and a 10-32.
    It's all mind over matter.
    If you don't mind, it don't matter.

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    • #3
      7/16 UNF and M11 x 1.25 are really close too.
      28, 32 and 36 TPI pitches are almost impossible to identify from metric counterparts unless the thread is long and very high quality.

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      • #4
        Would be nice have a chart of all the interchangeable thread sizes. I needed a nut for a free 10 in metric grinder left hand and was able to threaded on the lathe to work as if an exact relacement
        Ed
        Agua Dulce, So.California
        1950 F1 street rod
        1949 F1 stock V8 flathead
        1948 F6 350 chevy/rest stock, no dump bed
        1953 chevy 3100 AD for 85 S10 frame have a 4BT cummins motor, NV4500
        1968 Baha Bug with 2.2 ecotec motor, king coil-overs,P/S

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        • #5
          Originally posted by MrWhoopee View Post
          Try an M5 (x.8) and a 10-32.
          Just yesterday! M5-0.8 and 10-32 are both used for air fittings of the same type. Working on metric air parts and sure enough the ports were M5-0.8 not 10-32.

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          • #6
            Originally posted by MrWhoopee View Post
            Try an M5 (x.8) and a 10-32.
            Only real difference is their respective designations, as the M5x0.8 is part of a course thread series and the 10-32 is part of a fine thread series. In practical applications, interchangeable.

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            • #7
              Was looking at some threads the other day- trying to decide if it was 1/4-28 or a metric size. Turns out 6x1 is pretty close, though this is not as close a match as some of the others you've shown.

              Looks like I might have to start carrying a mini thread gauge on my person, along with the magnet and the knife. Along with the rest of the boy scout kit
              I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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              • #8
                When in doubt hold it against the bolt... easy.l
                it's not unusual for a pitch to be close, but having a 13mm tap , is sort of rare.. are the taps not marked ?
                don't you store metric separate from standard ?

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                • #9
                  I just ran into one of these. Took apart a used 4-jaw to check the backing plate because it wasn't running very true. Ended up remachining all the surfaces in contact. When I went to put the bolts back in I noticed they were only in by about 4 or 5 threads. They wouldn't screw in any farther. Checked the threads and the female threads on the backing plate are M20 x 2.5 and some clown forced 3/4"-10 SHCS into the holes. The pitch is off by about .0015" per rev, but since the O.D. is small they can go in a bit further. Close enough to thread in about 4-5 threads before they get tight. Dangerous.

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                  • #10
                    They may fit, but:

                    5mm = 0.1969" OD
                    and
                    #10 = 0.190" OD

                    So the M5 is clearly larger in diameter.

                    0.8mm pitch translates to 31.75 TPI
                    while
                    10-32 has 32 TPI

                    The M8 is clearly a coarser pitch.

                    The differences are small, but in a critical application the substitution, either way, could lead to a failure; if not right away, then perhaps after some time.

                    I once had to do a complete rebuild on a 16mm film projector where a single nut of the incorrect pitch was used the previous time it was rebuilt. This was a professional film projector that was used in a TV station to air 16mm films. It had a crankcase style oil chamber at the bottom where the end of a vertical shaft that drove everything ended with an oil impeller that was held on by a single nut. Like an automotive engine, that oil impeller was responsible for the lubrication of everything that moved in the projector. That nut and the thread on the shaft was a 1/4" OD, but it had a non standard pitch: not 1/4-20 and not 1/4-28. Nor was it a "standard" non standard pitch. Places like McMaster would not have the tap. The person who rebuilt it was not aware of this and he obviously just grabbed a 1/4" nut that was laying around somewhere. It may have seemed like it was tight, but it soon was not and the oil impeller was free wheeling on the shaft. Little or no oil was pumped around the mechanism. It worked for a few weeks but then bound up and would not turn. Since that nut was deep, deep inside the projector, a complete disassembly was needed to get to the problem. The maintenance manual's parts list clearly showed the actual size and pitch and I had to buy the proper nut from the OEM. The odd-ball pitch was a prime number and would have required an additional gear on any lathe. It was easier and a lot cheaper to just order it.

                    It was close, it did screw on and tighten, but it was no good. And it lead to that complete failure of the projector when the oil in the various bearings dried up. IIRC, I had to replace several bearings too. The nut was cheap, the bearings were not. And you wouldn't believe what the guys had to do when a 16mm movie was airing back-to-back with a commercial that was also on 16mm film and they had only one projector. Only a part of that procedure was a cardboard box on the floor instead of a take-up reel that they had no time to thread the film onto. There was an open and loud round of applause when I got that second projector back in service.



                    Originally posted by tom_d View Post

                    Only real difference is their respective designations, as the M5x0.8 is part of a course thread series and the 10-32 is part of a fine thread series. In practical applications, interchangeable.
                    Paul A.
                    SE Texas

                    Make it fit.
                    You can't win and there IS a penalty for trying!

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                    • #11
                      Doesnt even have to be confusion metric and imperial, sometimes both the taps can be imperial if youre stupid. Dont ask me how many times ive got 8-32, 6-32 and 10-32 mixed up in a design....

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                      • #12
                        Originally posted by Paul Alciatore View Post
                        .............The differences are small, but in a critical application the substitution, either way, could lead to a failure; if not right away, then perhaps after some time.............
                        While not a failure based on metric/SAE difference, your words reminded me of the British Airways Flight 5390- Blow Out.

                        Location: Saskatoon, Saskatchewan, Canada

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                        • #13
                          Originally posted by Arcane View Post

                          While not a failure based on metric/SAE difference, your words reminded me of the British Airways Flight 5390- Blow Out.
                          That has to be the one where a window blew out because the maintenance people used the wrong screw after some work. As I recall, they took out good screws and replaced them with some from stock that fit... sort of. Despite being blown half way out the window the pilot lived. It's an interesting story.

                          Dan
                          At the end of the project, there is a profound difference between spare parts and extra parts.

                          Location: SF East Bay.

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                          • #14
                            Mmm.
                            13mm is an unusual size for a metric fastener.
                            12 yes, various pitches, ditto 14, but 13?
                            That's a strange one.
                            Unlucky!

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                            • #15
                              Since so many of the metric and Imperial taps are so close there really isn't any good reason to go on using the metric taps at all.😂😂😂😂
                              The shortest distance between two points is a circle of infinite diameter.

                              Bluewater Model Engineering Society at https://sites.google.com/site/bluewatermes/

                              Southwestern Ontario. Canada

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