For this example, we will use a typical Ford 9" unit that is being modified to fit under a '32 Ford. This rear is out of a mid 70's Ford F-150 and the tag decodes this one as an open rear with 31 spline axles and 3:25 gears. The backing plate to backing plate width is 64 7/8". This is a large bearing rearend and the difference between the backing plate and the axle-housing flange is 2 3/8" on each side. So, the actual measurement of the stock housing itself (flange to flange) is 60 1/8" (64 7/8" - 2 3/8 x2 or 4 3/4" = 60 1/8"). Small bearing housings have a slightly different measurement between the backing plate and housing which is generally 2 1/2" but one should measure the unit they intend to use first because the large and small bearing units sometimes vary depending on the application. Remember: measure twice and cut once. The image above shows the rear after disassembly and cleaning. The spring perches and shock mounts were removed and the entire rear was ground clean of rust before we began measuring and marking the housing for narrowing.
OK, so we know the housing is 60 1/8" and the overall width backing plate to backing plate is 64 7/8". In this particular application, we are using wheels that have a full 4" backspacing, which is a bit more than what most people would use. We also want a minimum of 2" clearance between the body and the tires, so before doing any cutting we need to take this into consideration and calculate what the finished housing measurements (flange to flange) need to be to achieve this goal.
The tires we have decided to use on this project are 255/70 R 15 Goodyear Eagle II's. These tires have a section width of approximately 10 1/2" when mounted on 7.5" rims. We are using 8" rims, which will change the section width slightly, but not enough to make an appreciable difference. With a section width of 10 1/2", we know that the tire at its widest point will overhang the wheel by 1 1/4" on each side (10 1/2 / 2 = 5 1/4 - 4" = 1 1/4"). The backing plate to backing plate measurement must therefor be approximately 7 1/4" greater on each side than the width of the wheel housing at its widest point, (4" backspacing + 1 1/4"overhang + 2" clearance = 7 1/4").
On our body the wheel housing measurement is 45". We obtained this by dropping a plumb bob from the widest point of the wheel housing to the floor on each side, making a mark on the floor and then measuring the distance between the two marks. Now we can take the 7 1/4" measurement times 2 and add that to our 45 to get a finished (backing plate to backing plate) width of 59 1/2" (45" + 14 1/2" = 59 1/2"). Typically, rearends used for '32 Ford applications will have a backing plate to backing plate width of between 56" - 58". Given that we are using a wheel with 4" backspacing (about 1/2" more than most wheels), the extra 1 1/2" in width will more than offset the difference in backspacing and allow the tires to sit just about where we want them. It will also provide the 2" clearance we want from the inside wheel housing.
Now that we know how wide we need to make the housing, we need to make a decision. The Ford 9" has an offset pinion. If we cut the same amount off each side of the housing, then the pinion will still be offset the stock amount. This works fine in a lot of vehicles, but in a Model A or Deuce the driveshaft will be so short in most cases that any offset could result in excessive "U" joint angles. A little offset is fine and will help lubricate the needle bearings in the "U" joints but this can easily be handled by pinion angle adjustments. In our application, we are going to center the pinion in order to limit the angles we have to deal with when completing the driveline setup later. If you look closely at the picture below you will see that the rearend housing is not exactly centered in the frame. That is because it has been moved a little to the driver's side to center the pinion.
In order to center the pinion on a 9" it needs to be moved to the driver's side roughly 2". This can be easily accomplished by removing an additional 2" from that side of the housing. In our example we are going to cut a total of 5 5/8" out of the housing to achieve our target flange to flange width of 54 1/2" (60 1/8" - 5 5/8" = 54 1/2"). So, rather than cut 2 13/16" (5 5/8" / 2 = 2 13/16") from each side we will cut 4 13/16" from the driver's side and 13/16" from the passenger's side. This will give us a centered pinion and the proper housing width of 59 1/2". If the math overwhelms you, just draw everything out on a piece of paper so you can visualize what measurements are what. Just use this drawing and fill in the measurements.
Now, a word about cutting. You can cut a rear with a reciprocating saw, cutoff wheel, or any of a number of different tools, but keep this in mind: the more accurate and square your cuts,I'd make a straight (scratch) on the top of each axle (longer than what I'm removing) where I want to make my cut the easier it will be to weld things back together ( aligning) when you're done. Here is a simple metal cutoff saw that was used on this project. It gives nice square clean cuts every time.
The picture below shows the rearend and the amount removed from each end. Magnets are holding the flanges on so we can confirm our overall flange to flange measurement is correct.
Now that the cuts have been made, we need to know the best way to weld everything back together without warping the housing. If the housing tubes warp, it will cause the axles to be out of alignment, which in turn will cause premature bearing failure (among other things). So, needless to say, welding on a rearend must be done carefully and with a bit of finesse.
If you are just narrowing the rearend, and not adding a whole bunch of brackets for a 4 bar suspension or ladder bars then the welding is not quite as critical. Simple alignment can be obtained by temporarily tacking four alignment bars to the axle tubes in order to hold the housing end in alignment when it is welded back on. This is the old "shadetree" method, which has been used successfully by thousands of self-taught mechanics for years. It is relatively simple and works pretty well as long as care is taken in welding. For our example, we will be adding spring shackle mounts, ladder bar brackets and shock mounts to the housing so the old standard method will just not work for us. In the picture below the rear has been mocked up in the chassis so the exact locations and angles of the ladderbar and shackle mounts can be marked. Whenever possible, use brackets that weld completely around the housing as these do. Using a bracket that only welds to the front or rear can often cause warpage since all the heat is applied to one side of the tube.
We will be using a narrowing jig, which is designed to hold the end of the housing in perfect alignment with the center section mounting flanges. The jig consists of a solid steel rod and several flanges. By using this tool, the axle tubes can warp slightly and it will not effect the finished alignment of the axle flange because the steel rod and flanges hold it in alignment with the center section. The jig has several flanges of different sizes that fit the different Ford 9" rearends and it's just a matter of using the correct size for your application. This one was purchased on eBay from one of the companies that manufactures them.
A tool like this is not inexpensive but is worth the investment if you will be narrowing several rearends. However, since we will be narrowing only this one, once the job is done the jig will be put back in the packaging and will go back up on eBay.
The key to using one of these jigs is a spare center section. Here you see a chunk with one of the jig's flanges in place. Another will go in the other side and then the unit will be installed in the housing. Below is the chunk installed in the housing. Next the metal rod will be installed and the "chunk" will hold it in correct alignment. The housing end can then be installed.
With the rod installed, the correct size jig flange is installed in the bearing race and the housing flange can be slid on the rod and secured. The metal rod assures correct alignement no matter how much welding was done on the housing. In this case there was no warping on the driver's side. The passenger's side was warped about a 32nd of an inch but the housing end was welded back on using the tool so it is perfectly aligned even though the tube is ever so slightly out of alignment.
Before installation and welding, both the flange and the housing tube were chamfered to create a better and stronger weld. You can see this in the picture above. Don't skip this step. You don't want your rearend to come apart while you're out on the road 40 miles from nowhere.
With all this done correct welding technique must be followed to insure everything stays in alignment. The entire flange is stitch welded at the 12, 6, 3, and 9 o'clock positions - IN THAT ORDER first. Let it cool down a few minutes before continuing. Once you can place your bare hand on the joint for a couple seconds then it's cool enough to proceed. On a side note -- this is the passenger's side and if you look closely you can see the flange and tube are slightly misaligned. The flange is in the correct position in relation to the center section here. It is the axle tube that has slightly warped because of all the welding that was necessary to mount the brackets. This is exactly why we are using this jig.
When it is cool enough go back and fill in another weld between each of the first four welds. Again STOP and wait for it to cool a bit. When you can put your hand on it for a couple seconds you can continue.
Eventually, after this process has been repeated 4 or 5 times you will have a complete weld as you see above. Now, if you're not too particular you're done. If you want to make it prety you can grind the weld smooth and no one will ever know you narrowed your own rearend.
Finally, the jig end flanges are removed along with the metal rod and then the center chunk is removed. Once that's done, it can be re-installed in the chassis and you're on to another part of the project.