I would like to know the thread / size of mounting holes to mount a rear rack,

Think they are all standardized as 5mm. There should be plenty of them on Amazon for sale if needed.

To verify, call your LBS, they should be happy to provide info and even sell screws for you.

Do you mean these?

They are M5 x .8 thread pitch. They come in various lengths depending on what you're mounting.

Every bike store has them. Every Home Depot has them.

They are the standard size for just about anything you'd mount anywhere on a bike (handlebar stuff not withstanding). Bottle cages, rear racks, mini pumps, fenders, etc

You can get them at Home Depot and Lowe's to match the "color scheme" of your bike. If you like the look of all black, you can get those, or you can get chrome/stainless steel ones as well. They should be least expensive from HD/Lowe's (vs going to your bike shop).

Thank you all for the responses! Much appreciated...

I still find it amazing that I could not find the data on the Specialized site OR on the Zefal site...
I ordered a couple of sizes from the online version of the McMaster Carr catalog for various things..

Cool. I should also have said earlier -- I like pairing these with small fiber washers you can also pick up at your local home improvement store. It's like old school gasket material (that hard thin cardboard/wafer material). I like putting these washers between the bike frame and the accessory, to save the paint on the bike frame from being scratched/worn by the accessory. An OCD thing, for sure, but these things are about 60 cents for a little bag of four, so I back all of my metal accessories with them.

Reduces the chance of the screw rusting onto the frame too. That's less of an issue if you use a steel screw on an aluminum frame. That's more of an issue when both the screw and the frame are the same material (both aluminum or both steel)

But...still helps.

Also keeps water from seeping into the frame through the screw threading. Admittedly not a huge problem in the grand scheme of life. But for 60 cents per 4 pack...why not?

Actually it more of an issue if the are different metals because of galvanic corrosion. With an exception, stainless steel. Use stainless and grease.

I like stainless and blue Loctite. The blue formula holds the screw but stays soft enough to remove later.

If you have a fastener store like Fastenal, you can get screws in packs of 50 for what you would spend at Lowes or HD for 10. I like to fill all the holes, even where I don't have something mounted, just to keep water and dirt out.