Trying to source some special metric screws, and have a sample. I looked on google and says to measure a metric screw simply measure across the thread. I put my micrometer on it and appears to be 4mm, but going by the specs of the thing I am putting them in they should be either 3mm or 5mm? Am I doing something wrong or if they measure 4mm across the thread they are 4mm

Your 3,4 and 5mm are the outside diameter of the thread. They fit in 3.1, 4.1 and 5.1mm, or slightly bigger holes. Depending on who drilled them. 

 

Where in BKK> There is a good nut and bolt shop om Rama 4 approximately opposite the BMW dealer, just down from the Honda dealer.

 

EDIT: Shop above does not sell wood screws.

Along with the nominal size you will need to measure the pitch.

 

Using a decent scale, measure the distance from thread to thread. This may be easier to do by counting how many threads in a cm and dividing. 

 

 

 

buy a thread gauge . could be coarse or fine thread 

16 minutes ago, VocalNeal said:

Your 3,4 and 5mm are the outside diameter of the thread. They fit in 3.1, 4.1 and 5.1mm, or slightly bigger holes. Depending on who drilled them. 

 

Where in BKK> There is a good nut and bolt shop om Rama 4 approximately opposite the BMW dealer, just down from the Honda dealer.

Here is a link to that shop. It shows a picture but you can change it to the map view to see exactly where it is.

https://www.google.com/maps/place/บริษัท+ตงเลี้ยง+(1995)+จำกัด/@13.719173,100.5640485,3a,75y,90t/data=!3m8!1e2!3m6!1sGVbB9UxNM9IAAAQJOPKFTw!2e0!3e3!6s%2F%2Flh4.googleusercontent.com%2Fproxy%2FDrFp_HpXLr0ogLxmuCusDl45UIwKRV1XTZLC5RrIoj9yfSXmvwhrbOi5m905YxYYQ7hraGkCU3-LD_YWRbhhcQVnisirddS5DQwYyCbK9_16PWSJkdnYLTV0cWbzVwxy-38hTI49l45ayScZvxdtp12eH68ShngxT8eRb22Bc73r%3Dw114-h86-k-no!7i2133!8i1600!4m8!1m2!2m1!1shardware+store!3m4!1s0x30e29f09999c94e7:0x35164f4bbc97a0c7!8m2!3d13.7191094!4d100.5640736

 

 Don't really understand your question, are you screwing into a threaded item or is it a thru' hole?

 

If the OD you are measuring is 4mm it's a 4mm thread of either 0.5 or 0.7 pitch, although I would expect the diameter to be slightly less than 4mm ie. 3.97 - 3.85 OD. Many thread charts on the net for accurate detail. 

https://www.lazada.co.th/products/winton-thread-gauge-mminch-i561610992-s1030340230

 

https://www.lazada.co.th/products/apex-whitworth-metric-52-i1569900465-s4232886439

 

They're also available on Lazada from China for less than 30 baht...

 

https://journeymans-workshop.uk/isometric.php

 

I have since got advice that they are a standard 4mm screw, actually for a guitar. They have a special head on them, knurled knob for turning by hand. I managed to source some off Ali express. Thanks for all the replies