What is the case screw length and thread pitch for the 2010 MacBook 13" Model 7,1? I have a 2010 MacBook 13" Model 7,1. It has recently gotten some upgrades and one of the screws wandered off and was eaten by a grue. 

What length and thread pitch are the eight screws holding the bottom panel on to the laptop body on this model? 

These screws and panel are shown in this iFxit tutorial. I see that these are 4mm screws. I would like to know the screw size so that I can just order them as generic screws in my next McMaster-Carr delivery. 

Metric screws are measured by the outside diameter of the threads, the spacing between threads and the length as in Ma-b X c. iFixit tells me the length is 4mm. So I have Ma-b X 4mm. I need to know a and b. I suspect that this may be a non-standard or atypical screw size.
You didn't mention which screw you lost.  It's important to know because there are a number of different screws specific to that model:


3 mm Phillips #0. (Head: 2.75 mm. dia.) 
4 mm Phillips #0.  (Head: 2.75mm dia.)
6.25 mm Phillips #000. (Head: 4 mm. dia. x .5mm thick)
3.25 mm Phillips #000. (Head: 4 mm. dia. x 4mm thick) 
11 mm Phillips #00, with Shank (2.2mm dia. x 2 mm len.) (Head: 3.2 mm. dia. x .5mm thick) 
.25 mm Phillips #00, with Shank (2mm dia. x 3.75 mm len.) (Head: 3.2 mm. dia. x .5mm thick)
5.2 mm Phillips #00, with Shank (2.3mm dia. x 3.5 mm len.) (Head: 3.2 mm. dia. x .5mm thick) 
11 mm Phillips#00 in the middle of the case. (Head: 5mm dia. x .75mm thick)
14.5 mm Phillips #00 (Head: 5mm dia. x .75mm thick)


Source:  ifixit.com

Your best bet is to browse through the repair guides on iFixit, identify the screw that you are missing then search for it online.  However, if it's just one screw, you will probably be just fine.