Please help identify a screw

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!

#6 are 32, 40 and 48 tpi.
Weigand mount uses 5-40 I believe. Measure diameter of thread to confirm #5 or #6
Jon

Bag it, hardware store,

james r chapman wrote:Bag it, hardware store,

Good luck with that. Hardware stores rarely carry fine thread gunsmithing screws.
Jon

No way in hardware stores.  They don't carry that at all. To my eyes looks 4-48?

A good gunsmith just might have a few in his odds and ends box. At the least they should be able to firmly ID the screw particulars.

And never the #5's, which will really fool you. Yours looks like a 6-48.

Sometimes you can find sight screw kits in stores like Cabela's. Another strategy is to paw through the sight rails on the rack, looking for one with the magic fastener.

Nah, I will Bolt Depot it. Just wanted to ballpark the size.

Careful about the socket cap dimensions. Yours looks pretty low, height-wise. Sight screws are always "special".

Jon Eulette wrote:

james r chapman wrote:Bag it, hardware store,

Good luck with that. Hardware stores rarely carry fine thread gunsmithing screws.
Jon

Hackney Hardware, Dexter, Mich has a whole section dedicated to gunsmith screws.

M3.5

AP

Thanks AP, I reviewed and you are most likely right! Plus the metric socket from Holosun fit the screw perfect.

Assuming US manufactured frame mount, I find it hard to believe that it is a metric thread. #6 and 3.5mm are very close. Obviously the thread pitch of the two would need to be similar. 3.5mm are typically 0.6mm (0.023”) thread pitch which is very close to a 6-40 (0.025”).
I’m leaning towards 6-40.
Jon

Jon, it is not 6-40. I pictured 6-40 on left with the screw in question on the right. Screw in question is smaller. Smaller cap and smaller thread. I was initially leaning 4-40 or 4-48, but metric socket (that was used for M4) fit it.

So a bastardized metric thread pistol lol.
Thanks 
Jon

Well done Alex - you RCB!

What model pistol is this, anyway?

1911 custom build.

Metric 4mm cap screw head diameter is 7mm or .276" #6 cap screw head diameter is .226". Head diameters stay the same regardless of thread pitch and are different for each screw size. Key for hex cap screw 3mm is 2.5mm (0984"), 4mm is 3mm (.118") and 5mm is 4mm (.157") #6 hex cap screw key is 7/64 (.109") So Key size will determine screw diameter.
 Screw diameter for 4mm is .157" and #6 is .138"

Thanks Frank! Thisis even more indicative that the screw may be 3-0.5. I was wrong on Holosun, the screws are M3 and the key that fits that M3 was perfect fit for the screw in question. I ordered some 3-0.5 already. Now to get new caliper )))

Metric socket cap screws always have larger diameter heads, IME, than UN of a similar size. Which makes them better for use in aluminum, but ...
An M3 would be more of a visual match with a #4, .118 vs .112. So if it is metric, its likely a 3.5, and has a non standard head.
My guess would be a gun specific 6-48.
And I've run across both metric and inch screws that fit better with the opposite hex key. And I'm getting more and more inclined to use torx vs hex on the smaller ones.

Greetings,

You appear to have nine (9) screws remaining.

Remove one and have it measured.

I can measure it for you. Postage fees only. I have many gauges to check the part with.

Or, find out who made the scope mount and give them a telephone call.

Cheers,

Dave

Tripscape wrote: I was wrong on Holosun, the screws are M3 and the key that fits that M3 was perfect fit for the screw in question. I ordered some 3-0.5 already. Now to get new caliper )))

Where that might all go bad is that M3 and M3.5 screws both use the same size key...

Remove one and have it measured. wrote:
I can measure it for you. Postage fees only. I have many gauges to check the part with.

Thanks Dave! Now that would be great. I have some screws coming, if none fit I will very much appreciate your offer.
Yevgeny

3mm (.118") head diameter 5.5mm (.217")  3.5mm (.138") Head diameter 6mm and 6.2mm (.236" and .244") and yes DA/SA is correct both use 2.5mm hex wrench.
3mm X .5mm is about 51TPI and 3.5mm X .6mm is about 42TPI
2.5mm key (.099") Key to fit #5 is 3/32 (.094") screw diameter .125".
Not sure where you are Tripscape but if your near PA if WA is too far. I too can measure the screw, have tool makers Microscope, Optical Comparator, and more micrometers than I care to admit having. Yes I am a tool junkie! I can just about measure anything. Period on a piece of paper nooo problem!