Please help identify a screw

First topic message reminder :

Got a new to me pistol today with frame mounted red dot. Mount is held by 5 screws each side. One screw missing and I would like to make it whole again. Do not have a caliper at this time, but here are photos of screw in the gun and 6-40 5/16 for reference. 6-40 on left, what I am looking for on the right. Thread pattern is tighter. Can someone please take a stab at identifying?  I know this is totally unscientific unless caliper is used, but maybe some clues?
Thanks!

Although tools are the ultimate judge, sometimes pictures can replace tools.  With that pictures provided by OP clearly showed three identifiers:
1. different thread angel in comparison with known #6-40 (metric threads have 60 degrees);
2. major diameter of the thread is just slightly over 3mm;
3. 9 full threads at the slightly more than 5mm length, - 9x0.6 = 5.4mm
Conclusion - it is bastardized from the longer screw 3.5-0.6 x 5.5

The only other option might be 3-0.5 x 5.5 if pictures (or my eyes) have distortions.
That is IMHO.
AP

Inch and metric are 60° on standard screws. Indications point to 3.5mm X .6mm but in this application #6-40 will also work except for larger head diameter. #6-40 and 3.5mm X .6mm are so close that in short thread engagement (.050") both will screw-in.
 Tap drill for 3mm X .5mm is 2.5mm (.098") Tap drill for 3.5mm X .6mm is 2.9mm (.114") it is very possible due to short female thread length (.050") the 3mm screw will screw into 3.5mm. If I were the OP and had access to a definite #6-40 screw I would use it to determine thread size.

Frank, the 6-40 is the one on the left in pic. Need finer thread pitch, likely 6-48 or 4-48. I got 3-0.5, but that's too small and cap too big. 3.5-0.5 may be good, but I cannot find online with a socket cap.

Tripscape wrote:I got 3-0.5, but that's too small and cap too big. 3.5-0.5 may be good, but I cannot find online with a socket cap.

3.5mm will have a .5mm larger head diameter than the 3.0mm screw.

https://www.mcmaster.com/socket-head-cap-screws/socket-head-screws/system-of-measurement~metric/thread-size~m3-5/

AP

Thanks AP. Looking at those measurements my screw is 4mm vs 6 head diagonal and 2mm height, not 3.5, so it may be some special variant.

That don't make sense. If it were 4mm X .7mm the thread pitch will be courser than 40TPI on the #6. The screw in the photo next to the scale (ruler) is 3mm, maybe 3.5mm. Plus the hex wrench will not fit3,3.5 and 4mm heads. Simply put the #6 screw will go into the short female threads if it's 3.5mm X .6mm. Are you positive the #6 in 40TPI? Having the screw next to the scale it looks as if the threads are .6mm which is very close to 40TPI. What you need to do is determine thread size first. Having the correct thread the head of the cap screw can be altered to fit!
4mm is also larger than #6 and again due to short female thread length the #6-40 should screw into 4mm x .7mm hole!

Thread pitch must be the same I understand that but if the thread pitch is close to another and the length of female threads is short one will fit the other. 40TPI thread distance as measured in metric screws is .0250", .6mm is .0236" that means that in one turn the 40TPI will advance .0250" and threads .0236 or .0014" less so in 2 turns it will be .0028" less. In addition the material thickness being .050 and the threaded hole having a slight countersink to remove burrs and sharp edge then most likely there is less than .050" of thread length. Tap drill size is set up for a 75% thread so that there is clearance. It is very possible that 6-40 screw will fit better in a 3.5mm X .6mm threaded hole .050" long better than the 3.5MM screw!
 I would make sure the #6 screw is 40 not 32.
 Also if your saying that the 3mm X .5mm screw is too small but 5.5mm head diameter is too big then the difference between it and the 4mm screw diameter is only 1.5mm or .75mm(.0295")/side that means the space between the threads and the head must be less than .030"/side much too small to do any good especially in aluminum! If the 5.5mm head is too big and we assume it's 5mm than that results in a 1mm increase above 4mm then the head will be .5mm(.020)/side larger than the screw. That will looks as if there is very little difference between screw size and head diameter.
 There is a 4mm x .75mm but that is courser yet. Remember the higher the TPI in inch system the finer the thread while the higher the number in metric system threads the courser the thread. Metric thread number is distance between the threads

Don't know where you are but NC has Fastenal.  Store which is 90% screws.  Lengths are usually a little off, but nothing a dremel can't fix.  Never not found what I need there for shooting related stuff, and I come from smallbore!  So many screws...

#4's and 5's both use a 3/32 key. The slightly larger 2.5mm might fit better in them. Most socket screws are now 'headed' and the sockets suck ;-)

Based on the scale, it looks to be 3mm or very slightly larger. A #5 is .125, and M3 .118. And there is a fine thread #5 (not that I've ever seen one), a 5-44 vs the 'standard' 5-40.

A decent caliper would go a long way with this. And even somewhat useful for other ordinary everyday activities, like ... reloading ...

Going back and forth with metric I assumed 4mm not #4
 #4 is ,112" in screw diameter, 3mm is .118" so if 3mm is too small, #4 is smaller.
 A #5 (.125") is larger than the #4 but smaller than the #6 yet the screw looks larger than the #6 (.138) in the photo.
 Calipers are good, Micrometer is better. Toolmaker Microscope is best!

Guys, I have caliper coming in Monday. Eyeballing screw in question is slightly larger than #6. Don't know what to make of it. Super frustrating.
Next step would be to send to Frank or Dave for exact measurement. If all else fails then re-drill and tap for larger screw.

Re-drilling then tapping for larger screw is not a good option unless you use a larger size with very close thread pitch like .6mm and 40 TPI. Next size bigger in wire gauge tap will be a #8 or a 4mm.  However thread pitch changes so if 4mm is used you will get a lousy female thread, increasing to #8 will probably be OK but it's quite a bit larger.
 You have a screw I assume from another mounting hole, I assume it fits. Therefore screws must be available since the low number of scope mounts that would be made would not warrant special screws made.
 As I mentioned earlier a 4mm cap screw that will have a head diameter smaller than a 3mm since you said the 3mm is too small but the head is too big will have only .5mm/side increase in size over the 4mm screw diameter. If the 3mm screw you got has the same hex key size as all the other screws in the mount that should indicate it's not 4mm so 3.5mm or #6 is the only option. (key for #6 and 3.5mm are close in size)  I have found that usually inch screws are slightly smaller in actual diameter than listed size and metric are usually near size so that might be why #6 is slightly smaller than 3.5mm.
 I think your best option is to get it measured and buy a box of screws from McMaster or ebay so that you have others to fit the mount.
 Full strength of a screw requires 3 threads so if thickness of material is threaded the pitch of the threads that will have 3 full threads in the material being threaded. It might be that threads finer than .6mm are used on the 3.5mm screw or finer than 40TPI on a #6. I have a chart that has at least 2 thread sizes for every metric size so it's possible finer threaded screws are available

brownels

Ok, so caliper came in. Screw dimensions are in inches (across 9 screws):
Diameter at the thread =.1345 - .1350
Diameter at the cap = .1930 - .1950
Cap height = .1215 -.1240

Got the m3.5 philips head and my 6-40.

6-40 diameter = 0.1365
Screw starts threading and then stops. Never goes all the way. Centers properly.

M3.5 diameter = 0.1360
Goes all the way on about all the holes. Seems to be more correct than 6-40.

Now, given that the 1) cap is odd size and will require fitting unless you guys can point me to a place with right screws and 2) given that the mount is painted like cr@p and I would love to strip and redo it, but don't have media blaster nor anodizer -- does it make sense to spend $100 on a new Weigand mount? Hole spacing is same .5 and .65 in the back. Screws will be 6-40 which should be fine. If yes, ten shoud I tap the hole or just muscle the screw in all the way as it will tap new thread anyway? As long as it's centered.

What you very likely have is a 6-48 Filister head scope base screw.   If 6-40 won’t screw in but M3.5 does (for the 3-5 threads needed), then this might just work.  6-48 should JUST ABOUT interchange.  A M3.5-.5 or .6mm pitch straddles the pitch of a 48 tpi thread and would very likely screw in the 3-5 threads of the hole.  It might be a false impression that “it fits” but the pitch diameter would not be right.   The screw illustrated is from Brownells and comes in several lengths.  The head diameter matches your reduced head dimensions.  

https://www.brownells.com/gunsmith-tools-supplies/small-hardware/metal-screws/torx-head-scope-ring-base-screw-kit-prod41830.aspx

On the other hand I just searched up 6-48 on brownells and ebay. Brownells part 080-924-683MB looks to be perfect head wise. Ebay auction 402741984242 looks dimensionally perfect. Being that the thread is fine do you guys think I should try 6-48?

Yes, I came to same conclusion!!! Omg, this was such a headache. Huge thank you to all that helped!!! I hope screws fit well with some trimming.

Tripscape wrote:Being that the thread is fine do you guys think I should try 6-48?

Again, yes. I think it's super unlikely anyone would drill and tap a 1911 as metric. And 6-48 is common among mounts.

Those existing screws in the photo do look as if they've been shortened by a grinder. Check carefully for not bottoming out, or they'll come loose.

Greetings,

Have a Bo-Mar rib or rear sight?

The elevation screw is #6-48.

Cheers,

Dave

It's possible it's 6-48, it's also available in hex cap screw that will match the others. Everything on the screw will be the same as the 6-40 except thread pitch. Distance between threads as used in metric screws is .0206", for 40TPI it's .025" and 3.5mm it's.0236". Head height of hex cap screws is same ad screw diameter which is the same for all 3. #6 is .138" and 3.5mm is .1378" but 3mm head heights is .118" and diameter is .2165"
 I have top of the line Starrett Digital Calipers, use them for rough measurements, Vernier Calipers also and it is more accurate. But if I need to make measurements in thousandths I use a micrometer. A digital caliper tends to indicate size smaller than actual. (inside measurement larger)
 Looking at the screw dimensions nothing seem to fit, head height is shorter so is head diameter. It seem the mount was intended to be used with 3mm and possibly later changed to probably 3.5mm or possibly #6 and head sizes altered.
 I assume you have a 2.5mm hex wrench, now try 7/32 the size if a #6 hex cap screw than should determine if it's #6.
I would check that #6 screw you have, possibly put it next to the scale (ruler) to confirm it is #6-40 because #6-40 should fit into 3.5mm X .6mm with female threads .050" But #6-32 will not.
 Seems no matter what screw you select the heads will have to be altered, being the 3.5mm has a slightly smaller hex wrench opening I would use the 3.5mm

Haven't really followed this "thread". 
If you are trying to confirm it as a 6-48, is that not what most rifle base screws are threaded?
You could try one  one of those or try your screw in a rifle

As I have mentioned the head dimensions for the #6 (same for all #6 regardless of thread) are larger, it seems odd to me that the mount was designed to need 10 screws with head dimensions altered. It's very possible that after use the screws were changed. I'm not going to get into any comments on the mount since the question was about screw dimensions. However when testing the screw I would remove a screw near the slide stop pin where the material is thicker so that the number of threads in the female threaded hole are greater. Since the mount is available there must be instructions for it's installation. If the counterbore was used to recess the head for screw size then tap size should be included. I would think a question to the vendor of the mount as to what size tap is needed to install the mount. You do not have to buy the mount or be honest with the vendor and tell them the situation and ask what screws are needed or tap size if screws are included with the mount. As I mentioned before dimensions posted suggest that mount was intended to be used with 3mm screws if the mount was not altered another cap screw with similar head dimensions is #5. It's possible that screws were changed possibly due to stripping so larger screws were use with the heads altered. Metric screw are not as rare as they onetime were. Metric Taps and Screws are easily obtained.