Apple III case screws

I think that they are #4-40 x 3/8" long phillips or slot head screws.  I've seen both types.

The next SAE machine screw size up is a 6/32.   Maybe that would work. 

The shaft diameter of 4/40 is 0.112" (2.84 mm), 6/32 is 0.138" (3.51 mm), 8/32 is 0.164" (4.17 mm).  The second number (40 or 32) is the number of threads per inch or 25.4 mm.  You could count the number of threads and divide by the number of mm and multiply by 25.4 to get the threads per inch.

The most standard SAE sizes are 4/40, 6/32, and 8/32.   Similar metric sizes are M2.5, M3, and M4, but generally American hardware in the 1980s is going to use SAE.

Agreed.  You could try M3 and that might match the thread pitch. 

The diameters look similar, though.  But the thread pitch on the original screw is higher than "40"

profdc9  posted the measurement of 6/32 already : 6/32 is 0.138" (3.51 mm).

https://www.applefritter.com/comment/98824#comment-98824

So if M3 is to small and M4 is to tall... it must be something in between... which seems to be 6/32.

guibrush wrote:

I got the screws today: 6/32 isn't the right spacing. Now I'm puzzled...

 

https://imgur.com/a/1ss6ru7

 

 

left is the 6/32 screw I got today, right is the original one of the Apple III...

 

 

Those both look like 6/32...  BUT...  one looks like it is COARSE thread, the other FINE.

https://us.sfs.com/content/files/images/Comparison%20Chart.png

I had a look in the Apple III Service Reference Manual, looks like there is a parts list in there. It's a bit hard to work out which screws these would be, but it looks like on page 337, there are some screws mentioned with quantity 19. Screws to hold the motherboard pan, plus the power supply gets up around this number.

They are M3.5x 0.6 x 10, so looks like they might actually be metric.  Apple Part No in the ref manual: 420-1006

Link is here for the doc on internet archive:

https://archive.org/details/Apple_III_Service_Reference_Manual-1982

/Rob

Screws are a PITA to do like this, best bet is always to take what you got and head to a good hardware store with a variety of speciatlity screws. In the US we have Ace as a national brand which is pretty good. There are two important things... first screw diameter, second thread spacing, legth is easy. 

I agree these may be metric, but could also be a standard size too I found M3.5 is a perfect match to 6/40 both of these are course thread. There are also fine thread, and for my Apple IIs I found I could use M3.5 and both 6/32 or 6/40 when those where needed. From your first picture (assuming the top is the original fastener) ,  the bottom one being the 4/40 3/8" looks close, is it just too small a diameter? The threading looks slighly off, but that could just be the picture. Best to line the threads up like you tried to do but just overlap like 10 theads so the heads don't get in the way with the threads nested it's easy to judge spacing. 

If you have a pair of calipers head over to McMaster-Carr look at the pan heads found here: McMaster-Carr 

This is a good resource for cross-referencing sizings from your measurments too (this is my favorite) : Understanding Metric Fasteners - Fastener Mart

My guess you need a 6/32 3/8" screw, but I just saw your second image and honestly that one on the left looks nothing like 6/32 to me. It's got half as many threads as the one on the right! If you put that next to the 4/40 they should be close but I suspect you'll find they aren't. 6/40 is similar to your 4/40 but with a larger diameter and may closer match the original "thicker" diameter (if that's what you mean by the first try was "too small". That's not very specific.... cuz there are lots of dimensions here.