Well I got one set back. Sorry, Mike. Turns out they don't fit every C8, but they fit my two C8's and and another gentleman who bought the other set loved them as much as I do. So I am offering this set to the first person with ten dollars who actually measures their screws. A black tube C8 is what has worked, but save us both some postage and check the size.
These are the size that fits my celestron C8. I have two different OTA's, one is fastar compatible and the other is the non-fastar black tube that is just about four years old. They should work in any tube that uses a 6-32 x 5/16" or 6-32 x 3/8" collimation screw. If you don't have a screw thread guide, just take one to a hardware store and check it. Never remove more than one at a time. If you are afraid to take it out then you probably never adjust your collimation anyway and these won't help you. 8^)
The screws are solid brass and have a knurled raised knod that makes collimation easy. I love mine. And yes they do still fit under the plastic lens cover. They are only about 3/8" tall. I will include a picture of them below.