Where do I get really long limit screws?
 

I want to make a custom 3-speed Cassette. I want to run the gears in the middle of the freehub for optimal chainline, in order to do this I need really long limit screws for both the high and the low. What is the specific type of screw that I need and where do I get it? What length do you think I need?

Thanks

I'd pull one out to get the exact size and head to the hardware store. Doubt you'd need much length as the limit screws are near the pivot. Probably 5 - 10 mm longer would do the trick.

Most hardware stores do have a selection of metric screws in stock, but be careful, because most of what they have is not suitable for outdoor use. You don't want to mess up your RD by having something rust in place. Look for stainless or plated brass screws, or be sure to coat the regular ones with oil, grease or something like LPS-3, and inspect them from time to time for signs of rust.

Ace Hardware usually has M4 and M5 bolts in stainless steel.

Turn them around....

Most limit bolts are M3 (3.0mm x 0.5mm).

Take a look in the derailer at the surfaces that stop against the limit screws. These rotate as the derailer moves and are angled so that they are close to perpendicular to the screws when they contact them under the normal range of adjustment. If you extend the screws a long way, the stop surface will be sloped compared to the screw, tending to push sideways on the screws, which now are now unsupported over a longer length. I don't know if this will be an issue or not, but it's something to consider.