I did mine on a bridgeport and drilled a 7/16 hole and then used a 14mm end mill. I went as deep with the 14mm as the retaining clip on the injector is. You definitely need a step in there to keep the injector down.

In hindsight, i think 14mm is a little loose, but all my research showed that it is a 14mm hole that is used.

 

I never even thought about using the retaining clip area distance as the depth for the rail. :-damnit

Are you running any retaining clips or just using the steps in the intake/rails to keep the injectors in place? Most people do not seem to run any retaining clips in the aftermarket stuff.

These are about the best I have seen but are from Australia shipping seems to only be 15 bucks though

https://www.efihardware.com/products/1508/injector-retaining-clip

 

The thing that is nice about the ross machine tool is less tool changes. I had to do 4 changes for each hole. Small pilot, 7/16, 14mm, chamfer bit. Ross machine tool should be a 1 shot deal after locating hole #1 on each rail.

 

I've never put a step in them. I machine them 13.5mm clear through. The distance from the bung to rail should be 2" for the common length injectors. That's what keeps it from moving too much.

 

I use a 17/32" drill bit (13.5mm is _really_ close), that seems to give enough crush for both o-ring sizes (there's a thick and thin o-ring) that I've never seen a leak. 14mm is the size of the smaller o-ring so a 14mm hole will not give any crush on that o-ring, and not enough on the fatter ones.

If you have enough material for it to seat on the shoulder or you use clips like the one linked a few posts up you can just drill then clear through and then just chamfer the holes. If you want it to seat/bottom in the rail, first drill it a smaller size (anything smaller will work, 1/4" is larger than the filter inlet on most injectors, I think I've done something in the 1/4-3/8" range typically, it's not really critical). That makes each hole 3 steps, drill the smaller through hole, open it up part depth to 17/32" and then chamfer, but if you're doing it on a mill it can be done very fast. If you're doing it with a drill press it's 3 swaps/hole (maybe 4 if you're anal retentive and want to use some sort of point to locate the hole), but lets face it, you're typically only doing 8 so it still won't take that long.