This RACK and PINION is the most useful one you will find!
It uses a standard servo (20mm x 40mm) that i turned into a gear box.
This has a 200mm rack, I have added a hole to each end of the rack for mounting.
The printer build plate determines the size I can print. if you need a longer one, you can do the modification with one of the many on-line programs.

You will need to buy one part, it is the servo spline post that will need to be glued in.
http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXWWG3&P=7

6 - 2.5 x 8 screws hold the cover on and 4 - 3 x 8 washer head screws hold the housing to the servo.

WANHAO Duplicator i3

No

Yes

.25 works fine

25 to 50 depending on application

support is needed for the housing only.

I have never been able to find a good working rack and pinion that i could use for projects, so....... I designed one my self!