Metric thread question

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Wino1442

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Hi all....a small part I am making is tapped on each end and I need to pick up the taps to complete the job. I have checked the threaded ends of the mating parts and am a little confused. One of the threaded ends measures 7.75mm in diameter and a 1.0mm thread gauge fits perfectly, so I'm assuming that would be a 8.00mm x 1.00mm tap for that end. When I measure the other part end it measures 3.00mm across the threads, but the diameter just past the thread measures 4.00mm. The pitch seems to be .6mm. So I'm not sure which tap I need for this. I'm thinking I need a 3.5 x .6mm, but I'd like to get your opinions before I order them.
Thanks
 
I would buy some metric nuts in the sizes you think you need and try those for fit.
 
It is usually my philosophy that if one end is imperial, the other will be also. Not a "hard and fast" rule, but fits everything I have ever come across, except Ford timing covers. Further, I have never seen a 3.5mm fastener. They may exist, but I've never seen one.

That said, the small end looks to me to be Nr6-32. I have chased that size on several occasions when my mind was down the road somewhere. Finished size unthreaded OD=0.138", so close to 3.5mm that it can't be measured with a micrometer. As it happens, 3.5mm is a special number in one of my hobbies, so I'm usually watchful around that number. 32 TPI is also very close to the measured 0.6mm.

Therefore: if one end is imperial, the other end is quite likely to be also. So, to figure 7.75mm by 0.03937= 0.3051. That's so close to 5/16 inch at 0.3125" that taking the "truncated" cone of a "V-sharp" thread makes it a hard call. 1mm pitch is 25.4TPI. (5/16-24?)

As a quick and dirty test, the cover plate for an electrical light switch is held on by Nr6-32 screws. Since there are two(2) screws holding the cover plate, borrowing one will not cause a hazard. As an aside, Nr6-32 is a very common size for residential electrical work. Others are Nr8-32 and Nr10-32. While not relevant in this case, good numbers to remember.

Now, for my rant. . . If we had "metrified" a couple hundred years back when Tommy Jefferson tried to, we wouldn't be dealing with this at all.

.
 
.6m would be equivalent to 42.33 tpi. 1mm would be equivalent to 25.4 tpi. 8mm and 5/16" are only .002" apart so take your pick. 3.5 mm is .1377" and a #6 - 40 is .138" so it is a likely candidate. As Bill stated, it is unlikely that Imperial and metric threads would be mixed so I would go with 5/16" and #6-40.
 
But he actually measured closer to 3.0mm rather than the guessed 3.5mm. This is why I said 4-40, which is a standard.

For those that haven't seen metric and imperial mixed, you need to experience British cars during their switchover in the 70s and 80s. Any part that was redesigned had to be changed to metric. Thus you have, say, a timing cover held on with 5/16-18 bolts. When they added A/C for the US market, two bolts were made longer. Those two bolts became M8. The cars were riddled with these changes. And not just the British did this. GM did with some of their automatics designed in the 70s also.
 
RandyWilson said:
But he actually measured closer to 3.0mm rather than the guessed 3.5mm. This is why I said 4-40, which is a standard.

For those that haven't seen metric and imperial mixed, you need to experience British cars during their switchover in the 70s and 80s. Any part that was redesigned had to be changed to metric. Thus you have, say, a timing cover held on with 5/16-18 bolts. When they added A/C for the US market, two bolts were made longer. Those two bolts became M8. The cars were riddled with these changes. And not just the British did this. GM did with some of their automatics designed in the 70s also.
Click to expand...
Some vehicles such as Land Rovers had Whitworth/SAE/Metric. Had to have a very good selection of spanners and sockets.
 
Wino1442 said:
Hi all....a small part I am making is tapped on each end and I need to pick up the taps to complete the job. I have checked the threaded ends of the mating parts and am a little confused. One of the threaded ends measures 7.75mm in diameter and a 1.0mm thread gauge fits perfectly, so I'm assuming that would be a 8.00mm x 1.00mm tap for that end. When I measure the other part end it measures 3.00mm across the threads, but the diameter just past the thread measures 4.00mm. The pitch seems to be .6mm. So I'm not sure which tap I need for this. I'm thinking I need a 3.5 x .6mm, but I'd like to get your opinions before I order them.
Thanks
Click to expand...
I think you may be right. The male thread is usually under the nominal size and the female thread slightly larger than nominated. What the part is for is often a clue as to whether it is Imperial or Metric. I have a small 40X magnifying glass that is helpful when looking at threads and using a thread gauge particularly on the small stuff.
Although I have some vivid memories of the change over to Metric and the mixes as mentioned by Randy Wilson these days things are generally one or the other.
Converting from Metric to Imperial is a good way of checking /identifying,(which is where the magnifying glass comes in handy).
Having some known reference samples to compare with as well is a big help.
I can't find a really good Metric chart I have but here is a tap and drill chart that may come in handy.
 

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Thanks for the responses all...the part I'm making is a coupling to allow the "mop" to clean a 12ga. shotgun to attach to the cleaning rod from another gun. The thread pitch on the mop head looks to be 8.00x1.00mm and the thread pitch on the rod appears, after measuring and re-measuring many times, to be 44. The diameter of the rod, across the threads, measures .119, which makes it close to a #5..no? The pieces come from 2 different cleaning kits so that's why I think I may have both imperial and metric.
 
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Bi11Hudson said:
It is usually my philosophy that if one end is imperial, the other will be also. Not a "hard and fast" rule, but fits everything I have ever come across, except Ford timing covers. Further, I have never seen a 3.5mm fastener. They may exist, but I've never seen one.

That said, the small end looks to me to be Nr6-32. I have chased that size on several occasions when my mind was down the road somewhere. Finished size unthreaded OD=0.138", so close to 3.5mm that it can't be measured with a micrometer. As it happens, 3.5mm is a special number in one of my hobbies, so I'm usually watchful around that number. 32 TPI is also very close to the measured 0.6mm.
Click to expand...

On Honda D-series motors, the distributor rotor is retained by an M3.5x.8 screw (according to the parts list). Trying to find some socket head screws to replace the phillips heads supplied with aftermarket rotors, I was struck by the extremely odd nature of this screw (very coarse pitch for the diameter). Standing in the fastener aisle at OSH doing some mental calculations, it struck me. It was a damn 6-32 with the dimensions translated to metric! Why the hell they would have picked that particularly weak thread is still a mystery.
 
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