M3 & M4 screws?

[callisto]

Hi everyone,

Just for my sanity is the number after the letter refering to the screw width in millimetres?

Reason I ask is that I ordered a pack of 4 nylon M4 screw's from fleabay to replace some on my guidescope rings... the ones I received were thinner than my original screws (mine 4mm theirs 3mm).

After sending them a message and images they said  "theirs are M4 and mine M5"?

Am I going mad?

(My 'M4' screw is on the right)

 

[DaveL59]

Yep the trailing number is the mm diameter of the threaded part (to include the thread). Does look like the one on the right is M5 from the pic. I have a mix of M4 and M5 depending on which scope and if using OEM or other holders so bought M3 M4 and m5 nylon screws to cover all eventualities.

Edited October 21, 2023 by DaveL59

[vlaiv]

Best to measure with a set of calipers.

M4 screw will have thread diameter (not body but threads) a bit less than 4mm, depending on tolerance class.

From your image, right screw seems to be M5 because shaft of the screw is more like 4mm than whole thing with threads.

If you look at this diagram:

Major radius differs from minor radius by factor of 5 * H/8 where H is related by pitch by pitch * sqrt(3)/2

M5 has pitch of 0.8mm so H in this case is ~0.693 mm, and hence 5 * H / 4 which is difference of diameters  (twice difference of radii) is ~0.866mm - or close to whole mm, so if you measuring shaft to be ~4mm - then actual thread diameter is ~5mm

[Carbon Brush]

Assuming you have a handful of small drill bits...
Drill holes in a piece of scrap metal or plastic. 
Drill a 4mm hole and a 4mm screw should just go through. Works for all sizes of metric screws.

If you start asking about imperial screws things are different.

Looking at your photo it looks like you have M4 & M5 screws.

HTH, David.

[rjsc2001]

Buy some nuts, 3/4/5 mm and try them on the bolts 

[Mick J]

A nice phone app called Ruler Pro has handy thread pitch and screw size screens.  Once app is calibrated real size is shown.

[callisto]

Cheers for everyone's replies 👍 

I've ordered a mixed bag to cover all bases 😀

 

Mark.