Does the outer diameter of 4.78 indicate an M4 or an M5 bolt?

Question

I measured the maximal diameter of this counter-sunk bolt.

Note: The bolt is not parallel with the jaws, but slightly rotated so that the bolt head is behind them. Therefore measurement is not of the bolt head, just the threads.

And tried to match it with the metric bolt dimension chart:

But I did not find any match. I find the difference between dimensions M,N and R,S to be mysterious and suspect that the answer to my quandary is hidden therein.

My guess is that this is an M4, since the diameter without the thread could be 4mm and perhaps in some countries the standard specifies the inner diameter instead.

I am almost sure this is a metric bolt, since it is from a EU-made machine.

Answer 1

If it's metric, it's M5, of a poor fit class (or with measurement error contributing) meaning the thread fit is somewhat loose compared to a better fit class - common for non-critical hardware where "it fits in the nut and grabs" is adequate. It could not possibly be M4, it simply won't fit.

Put an M5 nut on it and see if it will thread all the way, or if it gets stuck. If it gets stuck it's not metric.

S is not mysterious - it's the amount of the screw that's solid inside the threads. Though actually it's being given as the tap drill size (how big to drill the hole before putting a tap in to cut the threads.) That's usually a hair bigger than the actual minor thread diameter, as cutting "100% threads" is difficult and prone to breaking taps .vs. "95%" threads, say, with very little change in performance. I'd use a real drill chart to verify that the size is correct for that application...

Neither M nor R appear to be given in the chart.

Standards-wise, one of the most world-wide standard things around - mostly ISO standards, though the Germans do throw in a couple of DIN standards that are slightly different. Quality-of-execution-wise, all over the map, and the fastener may well be imported rather than made in the country the machine is from.

Answer 2

It is 10-32. Easy to check in GIMP. I cropped and scaled the image to the width of the caliper measurement. The length of eight threads is 1/4". (i.e. 8/32"). 10-32 screws have a pitch of 1/32" so it matches up perfectly. Note the measurement in the bottom corner of the image.

Answer 3

The dimensions chart you're looking at is only the base nominal dimensions, and does not take into account tolerance classes. Normally, nuts (internal thread) are 6H which ideally matches the nominal exactly, while bolts (external thread) use 6g or another class that may be undersized to leave clearance. I use this site for getting the real dimension ranges for a particular metric thread size in a particular tolerance class: https://amesweb.info/Screws/metric-thread-dimensions-calculator.aspx

For M5 in class 6g, the outer diameter range is 4.826 to 4.976, which your measured value is close to fitting in; in particular it's larger than the maximum pitch diameter of 4.605 for a nut, which means it will almost surely catch and hold at least half-decently. This is of course assuming it has matching pitch.

Answer 4

To know bolt size, you need a few things:

• Major diameter, e.g. on top of the threads.
• Minor diameter, e.g. bottom of threads.
• Thread pitch
• Thread form.

(By Jeremy Riley - Own work, CC BY-SA 3.0)

The two first one is relatively easy to measure - at least the first one. And it will give an indication, but it's not enough to know what threads you have.

To measure pitch, the easy way is to get a thread gauge, which easily let's you find the pitch.

For metric, the pitch is the distance in millimeters between threads. For imperial, it's specified as number of threads per inch. To measure without a thread gauge, measure a distance, and count the threads across that distance.

The thread form we can normally ignore, but there's a few differences worth knowing. Metric always has 60° thread angle. UN threads has 60°. British Whitworth has 55°. Bot UNC and Whitworth are imperial threads measured in inches, and the only difference is the thread angle. You can normally mix Whitworth and UNC, at some reduction in strength.

So - to figure out what bolt you have:

1. Figure out the outer diameter
2. Figure out the number of threads per inch, or the pitch. A thread gauge will tell you this with certainty in seconds. Without one, this may be difficult for small bolts.

Answer 5

It's too large for M4. An M4 bolt must fit inside an M4 hole. That's kinda the whole point.

Similarly, any size X bolt will fit inside a size X hole. Longer bolts aren't threaded all the way; they have an unthreaded shank for some distance. The shank, also, will fit a size X hole.

Answer 6

4.78mm is 0.188" 
4.7625mm is 0.1875" which is 3/16"

I think you have an older thread form that predated metric, called imperial or SAE or standard.

The next tool you need is a thread pitch gauge, to figure out how many TPI or threads per inch. Or one of those organised box-sets of fasteners and just try different nuts till one fits.

Answer 7

I can't believe that no one pointed out that the image is showing measuring the head diameter. That does not really tell you anything. Measuring the threads with the calipers would easily differentiate between M4 and M5 as M4 would have a diameter slightly under 4mm and M5 would have a diameter slightly under 5mm.

Revising my comment...

The M designation always applies to the nominal diameter of the hole/bolt. There is never a case where the measured diameter of an M4 bolt is larger than 4mm.

Answer 8

A machinist could identify that screw by sight… if not by feel. At the very least a machinist would have a thread gauge.
Since the measurement wasn’t very close to a whole mm…. I assumed it was imperial.
When I converted the metric measurement to imperial.
4.78mm = 0.188 inches Then converted to the fractional value 0.188 = 3/16”

A screw with 3/16” thread diameter is a #10 Judging by the fineness of the threads, looks like 10-32