If you have the factory gears in place, and are in good condition (i.e. no noise, whining, clunking, etc.), you have a perfect target dimension to start from (after all, Ford should have set it up with one of their gauges, LOL!). If not, the "master housing dimension" of 4.4199" should be used. This is the theoretical depth from the axle centerline to the back side of the pinion head. If you do not have a "good" reference set installed, you'll need to do little careful measuring and math (based on set-up procedure) to establish the proper target dimension, or use the OTC tooling.
The good: The best method for establishing proper pinion depth for an FMS gearset is to use the pinion depth gauge, part number T79P-4020-A. OTC makes this tool for Ford, and I understand it to be quite costly; somewhere in the neighborhood of $600 (complete; I've heard the essentials are about $250.)! Unfortunately, it is as it's described: a gauge. You cannot read an actual pinion depth number; you install the gauge and slide one shim at a time (better have a stack of them!) until you feel drag. That shim will set the pinion depth properly. If you have access to a machine shop, and are inclined to build your own, I have details of the depth gauge (AutoCAD drawings).
So what if you have Richmond, or another set that dictates an optimum pinion depth measurement (scribed on pinion normally)? The OTC gauge is completely useless in that situation; you need a depth checking tool. Essentially, it's a jig that locates off the diff case bearing bores, is squared up somehow, and projects to allow you to read a number (see the Chambers site for a good picture). I have no idea of cost or availability of these units: if anybody finds some details, please forward them (of course you'll get credit!
The bad: Shortly after I did my first set of gears (on my car, BTW) using the NPDM (see "the ugly" below), I thought, "there has to be an easier way to check pinion depth". What I cooked up is an aluminum plate, 1" thick (thickness isn't a major concern, as long as you know/measure the "actual"), that will bolt to the housing using the differential cap fastener locations. Bolt the plate to the housing using four 1/ 2"-13 socket head fasteners, and torque to 20-30 lb-ft or so. See the drawing at right:
For FMS gears: Using a depth micrometer, insert the plunger through the hole at the bottom of the plate. Hold the mic firmly to the base, and touch the pinion face. Record the depth, rotate the pinion, and take another reading. Measure and record several locations on the pinion to get a good average (Dpavg). This is the target pinion depth for your new FMS pinion.
For non-FMS gears: Using a depth micrometer, you touch off the bearing bore (Dbb) and record a number. Then, touch off the pinion in several spots while rotating the pinion, record, and average (Dpavg). Measure the diameter of the bearing race (Dbr; Timken specs lists LM603012 cup diameter as 3.0625"), and record. The pinion depth will be the average pinion depth minus the bearing bore depth, plus one half of the bearing race diameter (PD = Dpavg - Dbb + (Dbr / 2)).
The ugly: The "Neighbors pinion depth master" (NPDM), or combination square for short. After fretting for several days about pinion depth (and little to no help tracking any information down what-so-ever), it dawned on me one night: the gauge doesn't really do anything, other than provide a reference point. Aha! I was on to something...Once I realized this, that reference point can be anywhere, provided it doesn't move relative to the pinion. Axle centerline, the bumper cover, your next door neighbor's fence...as long as it's a repeatable measurement. I took the rule out of my combination square, and attached it to the "short" square side of the body with two small C-clamps (this makes an "L", with the "long" square side and one edge of the scale on the same plane). I then clamped this contraption to the axle cover mount surface (nice, flat, machined surface), using two spring clamps, locating the "rule" portion approximately over the pinion centerline (make note of the initial location, as you will want to install the NPDM in the same position during pinion installation). Using calipers (I used digital, but dial works also), measure the depth to the pinion by holding the end of the calipers square on the edge of the scale, and slide the calipers out, making sure it contacts the face of the scale (this gets you on a parallel axis with the pinion centerline). Drop the depth part of the caliper down until it contacts the pinion (on a flat area, not in the letters!). Record several measurements while rotating the pinion, and average. Now you have a benchmark...
Once you decide the pinion depth obtainment method, take all pertinent measurements (I like to get pinion bearing pre-load also), then remove the pinion assembly. Attach a pinion retaining tool with two driveshaft fasteners (if the car is jacked up to the right height, the end of the angle will rest on the concrete, allowing "hands free" operation), as illustrated right, or use two fasteners and a long pry bar. Unless you need the exercise, use a 1-1/16" impact socket and wrench to remove the pinion nut. The factory minimum torque is 140 foot-pounds, so it will take some effort to break loose. Remove the pinion flange with a two or three-jaw puller (I tried my harmonic balancer puller initially, but it didn't work for two reasons: 1. The driveshaft fasteners are bigger than the "slots" in the puller, and 2. The proximity of the flange to the housing precludes the use of long, smaller bolts with a nut...). The pinion should fall loose from the front bearing, but if it doesn't, a little "persuasion" with a hammer will do the trick. The front pinion bearing and slinger will remain in the housing, captured by the pinion seal. Remove the seal by driving a regular screwdriver under the lip (between the lip and the housing), and pry out; grab the bearing and slinger as you do.
Clean all the loose parts with carb, brake, or parts cleaner, keeping in order. Set aside and let dry, or blow off with compressed air. Pull out as much oil as you can by hand from the bottom of the housing, scooping it out and into the drain pan. Get the oil out of the axle tubes by jacking up one side (at a time) of the housing slightly (to get the oil to run downhill...) and spraying with cleaner (I also stuffed a couple of paper towels in and pushed through with a broom handle). Blow the housing and tubes out with compressed air.