"The project so far has been fun but relatively simple and unchallenging, however, that is all about to change as we are now venturing into the heart of the IRS project, narrowing of the half shafts, narrowing the LCAs, and changing the bolt pattern on the wheel flanges.

Where we have been and where we are going:
In “Mustang IRS” page I, we looked at the original Ford IRS design for the Mustang, the reasons for upgrading to IRS in a classic Mustang, as well as the reasons to use a Jaguar unit rather than some of the other options available in the IRS market.  In Mustang IRS page II, we looked at the logistics of finding a Jaguar IRS unit, the task of tearing it down into its basic components and the labor-intensive job of cleaning it up.  The project so far has been fun but relatively simple and unchallenging, however, that is all about to change as we are now venturing into the heart of the IRS project, narrowing of the half shafts, narrowing the LCAs, and changing the bolt pattern on the wheel flanges
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Getting the correct track width:
As with most aspects of this project, there are several schools of thought on making an IRS unit fit in your application.  One option is to use the Jaguar unit as it was designed and then run wheels with extreme back spacing. The idea is that the width of the rear end plus the minimal offset of the wheels will be similar in width to a standard rear end assembly with normal backspacing and off set.  In theory, this is a good idea, however, by requiring wheels with extreme back spacing, your wheel choices are greatly reduced.  Also, the hub on an IRS unit takes up quite a bit of space and some wheels with extreme backspacing may end up hitting the hub.  Another thing to consider in the case of a Mustang install, is that the nearly 4” difference in track width, between an early Mustang rear end and XJ Jaguar assembly, may not be completely compensated for by large backspacing wheels, and modifications to fenders may still be required to make the assembly fit.  For these reasons, I chose to narrow my Jaguar unit for a correct fit.

When narrowing any rear end, whether it is a live axle or IRS, you need to consider what you have planned for the car with special attention paid to wheel and tire widths.  Some wheel and tire combinations, such as modern Mustang rims, may require a slightly wider rear end. The stock 65-66 Mustang rear end measures 58” hub to hub, so a target width of that, plus or minus an inch, should be your goal depending on your plans and application.  NOTE normally you measure a Ford live axle rear end housing minus the brakes from housing flange to housing flange, and such a measurement on a 65-66 Mustang would be 52.25”.  For this project, I measured from the outside of the hub instead because there is no other place to get a measurement that translates on the IRS unit.  The width of an XJ6 or XJ12 IRS unit is 61.75”, wheel flange to wheel flange, so to maintain the 58” width of a 64.5-66 Mustang, 1.875” needs to be removed from each side.  In the case of my application, I chose to remove 2.125” from each side for a final width of 57.5” hub to hub.  I chose this for two reasons, first, I wanted a little extra clearance for my tires, and two, the half shafts start at 16.125” u-joint center to u-joint center and having them shortened to an even 14” just seemed to make sense somehow.

Narrowing the half shafts:

On the Jaguar IRS system, the half shafts serve triple duty.  They provide the connection between the differential and the wheels to drive said wheels, they act as upper control arms keeping the hubs vertical during suspension travel, and their connection to the differential provides a means of adjusting camber.  With so many functions, these parts are very important and the utmost care needs to be taken in narrowing them.  I am a competent fabricator, however, the greater part of what makes me “competent” is knowing when the task that lies before me is beyond my skill set.  Narrowing half shafts is one of those tasks that, for all but a select few with the correct skill set and access to the correct equipment, should be farmed out to a driveline shop. NOTE if a half shaft were to fail while in use, the results could be catastrophic, so weigh the potential consequences and think very carefully before attempting to narrow the half shafts yourself.

If the Jaguar half shafts were like a typical driveline made from a piece of mechanical tubing with a yoke pressed in and welded to each end, then there would only be one option for narrowing them.  However, when Spicer originally designed and manufactured the half shafts for the Jaguar, they decided on solid forged steel pieces, much like the axles on a live axle system. This creates challenges in narrowing them, however, it also allows for more than one option on how to do it.  Even though, in most cases, the work will be done by someone else, you, as the consumer, have three choices as to how you end up with the correct half shaft length: “Bevel and weld”, “Turn down, cut, press & weld”, “Fabricate from scratch”, and with the aid of your machinist a decision can be made as to which is the best option for your application and financial resources.

Bevel and weld:
Hot rod builders have used this first technique to narrow the half shafts of Jaguar IRS units for their street rods since Jaguar came out with IRS in the 60s. The concept is simple, place a half shaft in a lathe and cut out the section needed to achieve the desired length.  Then on each end piece bevel from the cut edge back toward the yoke so that the shaft of each yoke ends up with a point on it.  Line the two shafts up, point-to-point in a piece of angle iron, making sure that the yokes are perfectly in phase with each other, and begin filling the area between the two pieces with weld. A competent welder should be able to fill in the shaft and return it to a solid piece. Using this technique results in maintaining a solid shaft just as the unit was originally designed, and the welded section will probably be stronger than any other part of the shaft.   However, there are two difficulties in this technique, one, getting the yokes lined up, and two, ending up with a straight half shaft that is not slightly bent or warped.  If the half shaft yokes are out of phase even 1 or 2 degrees or the shaft is bent, the slight variation will result in horrible vibrations and/or broken u-joints.  Assuming everything is correctly welded, together this technique has proven to be strong and reliable.

Red lines show beveled cuts and the green area is the weld

Turn down, cut, press & weld:
Another option is to use the yokes from the solid half shafts to make a mechanical tubing type half shaft.  This is done by turning down a section of the half shaft near each yoke end to a size that will be a .002 to .01 press fit in the mechanical tubing that will be used to connect the two ends.   Then cut the yoke ends off of the half shafts, leaving about 1”- 2” of turned down shaft attached to each yoke.  As mentioned, thick wall mechanical tubing cut to the correct length will join the two yoke ends together by means of a press fit as well as a bead of weld around the base of the yoke and mechanical tubing.  One major advantage to using this technique is by converting to a tubing type shaft, weight is shed and, if repairs are needed in the future, they are simple to do just as you would with any other tubing type drive shaft.  You may wonder how tubing could be strong enough, especially when you consider the original shafts were solid, but the reality is that the modern materials used to make thick wall mechanical tubing are stronger ounce for ounce than the materials used to forge the original shafts and the strength is probably fairly close, if not better, on the modified shafts.  This option is usually less expensive than the one mentioned above because it is easier for a driveline shop to index the yokes and avoid having the half shafts warp.  After discussing the options with several machine shops, as well as others who had narrowed a Jaguar rear end, I decided to have my shafts narrowed via this technique for both cost and reliability.  NOTE the larger the diameter of the mechanical tubing used, the stronger the half shaft, just as an example, a tube 1.75” O.D. is almost 4 times stronger than tubing of the same wall thickness with only a 1.25” O.D.  Also, for ideal penetration while still maintaining strength, 1⁄4” wall tubing will be best for this application.  Several aftermarket companies that narrow up Jaguar rear end assemblies use 1.75” O.D. and 1.25” I.D. DOM mechanical tubing.

Red lines show modified yoke ends, blue lines represent mechanical tubing, and green lines represent the welds.

Fabricate from scratch:
Another option is to have custom half shafts made from new yokes or used drivelines.  The yoke on the Jaguar half shafts is sized for a standard Chevy u-joint so new yokes can be used or used Chevy drive lines can be cut down to fit between the differential and the hub.  The advantage to this is a solid quality piece in many cases made from all new components.  The down side is using drive line yokes will make the shafts wider than the original half shafts, which my require spacing out the coil over shocks so that there is clearance between the springs and half shafts.  Also, finding used Chevy drive lines that are around 2” or less may be difficult and when I looked into ordering new yoke ends, it was going to cost me over $100.00 a piece just for the yokes, not including the machine work.  That is the main reason why I chose to have the original half shafts used to make the yokes.

This is what my shafts looked like after they were narrowed at a local driveline shop.  
After this picture was taken, they were sand-blasted and powder coated.

Narrowing the LCAs:
Narrowing the LCAs is easier than the half shafts in that it can be done at home but don’t let the DIY aspect of the process overshadow the importance of narrowing them correctly.  Half of the rear end weight will be on each of these half shafts and making sure they are narrowed properly is critical.  Paying attention to the details and knowing the limitations of your skill set will ensure this process is completed correctly.

Taking measurements and making indexing marks:
The first thing that needs to happen in the process is to take a whole bunch of measurements so that you know exactly what the LCAs looked like prior to modifying them. YOU MUST do this step because, if you do not, you will not have any frame of reference as to how long things need to be. Once you think you have taken enough measurements, take a few more.  Not only take measurements and notate them but also notate where the measurements were taken from so you can measure for the exact same points when measuring the mocked up pieces after material has been removed.  Another wonderful point of reference is to make indexing marks on the side of the tubing so that when you put it all back together you can check to see that the lines are lined up and know that everything is being put back together correctly.  To make lines, place a piece of angle iron on the LCA tubing and score a line along the side of the angle iron on the tube.  I only did 1 indexing line per LCA.  Make sure the line is longer than the section of material to be removed so that it will be on both halves after the material is removed.

I am measuring from two fixed points on the jig while it is installed on the unnarrowed LCA

You can see the indexing marks line up perfectly, even after the LCA has had a section removed.

Building a LCA shortening jig:
Once you have measurements, it is time to build a jig. (Homemade Tool # 3)  I built mine by starting with the pivot mounting holes.  When building the jig, I set the LCA on the table with the flat machined surfaces of the LCA facing up so that I could use them as surface for the jig to fit up against.  I cut small pieces of steel that could be bolted to the outside of the pivot points on the LCAs.  On the hub end of the LCA, two .625” bolts were used, but on the differential end of the LCA, I had to come up with a spacer and went to the hardware store with my digital calipers in hand.  What I found that had the correct O.D. was a plastic 5/8” pipe coupler.  The O.D. was perfect but the I.D. was still a little big so I took a lug nut, drilled out the threads and rounded off the corners and then pressed it into the coupler.  This gave me a perfect .5” hole in the center to bolt the jig to the LCA.  Once each arm had a sidepiece bolted to it, I connected the two sidepieces together with a sold piece of steel that was clamped to the arms up against the machined surfaces and then welded the connecting piece to the sidepieces.  This creates a solid bolt-in mount for each end.  I then cut two short pieces of angle iron and rounded the outside corners so they fit perfectly inside a longer piece of angle iron.  After that, I placed the two pieces, one on each end, on the LCA and connected them with the longer piece of angle iron clamped with hose clamps to each one and the LCA.  The next step was to weld connectors between the two short pieces of angle iron and the bolt-in mounting ends.  I used scrap that I had on hand.  The concept of this jig is that once made it can be removed, and then the LCA cut, the two halves can be bolted to the two halves of the jig and by using the longer piece of angle iron and the hose clamps, the two ends can be held perfectly in place, indexed correctly, for tack welding.  Once the jig is made, bolt it to the LCA prior to cutting and find a place to measure on the jig.  This will give you one more reference point when measuring the position of the two halves to ensure that you end up with the correct length LCAs.

These are the two halves of the narrowing jig.

Half of the jig is installed on half of the LCA with the connecting angle iron clamped to it.

Cutting the LCAs:
In order to narrow up the LCAs, you will need to cut out a section of the tubing.  The professional way to do this is put the LCA in a lathe and use the lathe to cut it.  This takes a large lathe and probably the services of a machine shop.  In the pursuit of maintaining a DIY project, I devised a way to do it at home.  I started by setting both LCAs side by side and made a mark on the differential end of each LCA where the cut would be, so that the cuts would be the same on both LCAs. I then placed tape around the tube so that I had a nice straight line to follow, and then measured the distance I wanted to remove and applied a second piece of tape at that measured distance. Using the edge of the tape, and being careful not to damage it, I took a triangular file and made a small grove along the edge of the two pieces of tape.  This gave me two groves 2.25” apart.  I know you are thinking the half shafts were only narrowed 2.125” and we will discuss the discrepancy later in this article.  I was then able to use a large 3-blade pipe cutter to cut out the 2.25” section.  The reason I cut the groves with a file first is I have found that it is easier to use a large pipe cutter and get an accurate cut if there is already a groove for the blades to rest in.

Tape can provide a perfect edge to follow when
cutting a grove.

A large pipe cutter makes cutting a section out of
the LCA simple.

Getting the pieces indexed and measured for welding:
Prior to welding the halves together, the jig will need to be installed, measurements will need to be taken on both the arm and the jig to ensure the correct length and the indexing marks need to be checked.  It is also a good idea to lay the parts out on a flat surface and make sure that the distances between the eyes of the LCA and the flat surface are the same.  This will ensure that both ends of the LCA are correctly indexed.  I measured mine by making sure the hub end was shimmed so that the centers were the same distance to the surface and then measured the distance between the machined surface and the table on the differential end.  I shimmed the hub end and measured the differential end because the wider differential end will show greater differences than the hub end, due to the extra width.  When I measured mine, the jig only allowed a.02” difference, which probably would have been fine but I couldn’t let it go and adjusted it by loosening the clamp on the differential end, tapped the arm lightly with a hammer, measured again and, once the measurements were identical, tightened the clamp back up.  With everything held in place, the half shafts are ready to be welded.

Once the two halves are installed in the jig, measurements need to be taken to end up with the desired length.

Using a flat surface and a set of digital calipers allows the two halves of the arm to be indexed perfectly.

Welding the pieces together:
As with the half shafts, there are several schools of thought, and thus, different options on how to best accomplish the task of joining the two halves back together.  Prior to narrowing my LCAs, as with the half shafts,  I did much research and came up with three techniques/options for welding things back together :  “Bevel & butt weld”,  “Sleeve” and “Gusset”.

Bevel & butt weld:
The first technique is not only the method used when the LCAs were original made, but also part of the other two options.  Once you have cut into the LCAs, if you look inside the tube on the differential end you will see weld slag where the forged steel end was welded to the tubing.  This is nothing more than a butt weld and a butt weld is a sufficient way to join the two halves.  For best results, however, bevel the edges to improve penetration. With the halves in the jig, you should notice a .125” gap between the two pieces from cutting out more than the 2.125” the arms will be shortened, and this is there to be filled with weld and create a better penetration and a solid joint.

To the right we have the inside of the LCA.  Notice the ring of weld slag that is the result of the forged steel end being butt welded to the tubing.

An improved option over the butt weld option is to also sleeve it and then butt weld.  By cleaning up the inside of the tubes, you can machine a tube to fit inside and help line things up.  This will keep the tubing straight and add quite a bit more strength.  This also allows for plug weld holes to be drilled through the LCA tubing on the arms so that more than just the butt weld can hold the sleeve to the arm and gives you another place to make tack welds.

Sleeves sized to fit inside the LCA tubes.

Sleeve installed in the LCA.

Plug weld holes in the LCA tubing.

Yet another improved option over the butt weld technique is to also install gussets after butt welding the two halves together.  The gussets will overlap both the original butt weld joint and the new butt weld joint on each side and provide another way to connect the two pieces together.  Also, this improves rigidity of the LCA and completely eliminates any LCA flex. 

The way I see it is if I am going to have gussets, I might as well make them look good!!

Not being one to do things in a less then over built way, I chose to use both sleeves and gussets. Adding the gussets not only added strength but also afforded me the opportunity to have some really cool lightening holes cut out.  When I put these together, I tack welded the two halves of the LCA together and then took them into a professional welder to be welded up, including the installation of the gussets.   NOTE as I said before I am a competent fabricator, however, the greater part of what makes me “competent” is knowing when the task that lies before me is beyond my skill set.  Could I weld up the LCAs, yes, would they be strong enough, probably, but why run the risk of not having professional quality welds when it would not be that expensive or time consuming for me to take the tack welded arms to a certified welder and have them finished up.

Notice the deep channel between the two halves of the LCA with the sleeve in the middle, this channel will fill with weld and allow for excellent penetration. Also notice the tack welds in the channel and plug weld holes.

 This is what the LCA looked like after I tack welded it.  Since the gussets will go over the plug weld holes, they will not be installed until the plug weld holes have been filled and ground smooth.
Picture yet to come
Here we have the welded up LCAs complete with running horse gussets. These are the correct length, should be stronger than the originals, and a little powder coating will make them look incredible.

Changing the bolt pattern:
When it comes to wheel bolt pattern, if you are trying to put a Jaguar IRS in a classic Chevy, you have it made because the Jaguar bolt pattern is the same as a classic Chevy, 5 on 4.75”, but if you want to put one of these units in a classic Ford, the bolt pattern is different, Ford uses a 5 on 4.5” pattern.  As with other aspects of this project, there are several options in dealing with this issue.  The first is to run different pattern wheels on the back than on the front.  Problem with this is that you need to carry two spare tires.  The second option is to use hub adaptors that change the bolt pattern.  This option is not ideal because bolt pattern adaptors are .75” to 1” thick and, if you are planning on this option, the unit will need to be narrowed even further.  Also, adaptors like this sell for around $50.00 a pair.  Another option is to buy the correct bolt pattern wheel flanges from places like CWI.  They sell hubs that have been modified and have the correct bolt pattern machined into them.  This option gives you a quality flange with the correct pattern but has the down side of being fairly expensive. 

I didn’t really like any of these options and, there again, was striving for this to be as much of a DIY project as possible, so I decided to drill out the pattern myself.  When I first suggested to some other car enthusiasts that I was planning to drill the pattern myself, I was told not to drill the pattern because it would be almost impossible to have all 5 holes be in the correct spot.   I pondered the issue and, at first, thought about making a 10-hole jig. (5 holes each pattern)   Problem is, I still would have a hard time getting all 10 holes in the correct position. One day while contemplating this issue, I had an epiphany, instead of making a 10-hole jig, make a 3-hole jig and rotate it from position to position. I made the jig (Homemade Tool # 4) by drawing 2 circles, a 4.75” circle with a 4.5” circle inside it.  I then measured the distance between two studs on the Jaguar wheel flange and plotted those 2 points on the outer circle.  Using these two plotted points I measured between them and plotted a center point on the inner circle.  The drawing was then transferred to a piece of steel and the three holes were drilled out, .5” inches on the outer ones and .125” for the center hole.  The jig can now be bolted to the flange and a pilot hole drilled.  The beauty of this jig is that even if the pilot hole is not perfectly centered, as long it is on the 4.5” circle, it will be in the same location between the two studs on all 5 holes, giving a perfect star pattern.  After creating the jig, I purchased a cheap Chevy rotor and drilled out the Ford pattern to ensure that the jig was accurate prior to drilling the hub.

If you would like to purchase a CNC cut 3-hole bolt pattern Jig rather than make your own I now offer them for sale.  To purchase one click here  

Finding wheel studs:
Before you can drill your wheel flanges, you need to purchase your wheel studs so you know what size to drill the holes.  Finding the correct press fit wheel studs can be a challenge, due to the fact that moving the holes in 1⁄4” moves them very close to the machined surface on the backside of the wheel flange.  With that in mind, the new press stud holes need to be no bigger than 17/32”.  Most 1⁄2” wheel studs have 9/16” – 5/8” Knurl Diameters and so would be too big for the application.  To come up with the correct stud, I went to my local tire and wheel supplier with my digital calipers in hand and asked to look through their wheel stud selection. I found a stud that has a Knurl Diameter 0.558, which makes it a perfect press fit in a 17/32” hole so I purchased 10 of these for my project.  At the time, I was not able to cross-reference the part numbers I had for the studs to find out what they were normally used for.  Some time later, I went back to get 10 more for another Jaguar assembly I have that is going in my 62 Galaxie and could not find the same studs so I found another stud that would work.  The new one has a Knurl Diameter of 0.536 making it an ideal press fit for a .5” hole.  Not being able to find the original stud concerned me, in case I ever needed to replace one, so I once again began a search for what application either of these studs were designed for.   This time I was successful and the first set of studs I purchased are Dorman part #98141 used on the 1980-1983 Ford F-100.  The second set of studs I purchased are Dorman part #610258 used on 1998 Jaguar XJ8 and the 1967-1978 Cadillac.

3-hole jig, two in the Jaguar pattern and one in the Ford pattern. To buy a CNC cut jig click here

The jig bolted to a cheap Chevy rotor to test it and ensure that it was accurate

Once I had my jig built and I had my wheel studs so that I knew what size to drill my holes, I was ready to modify the Jaguar wheel flanges.  To do this, I removed the original Jaguar wheel studs by grinding of the back crimped edge of the wheel studs and unscrewed them.  I had to install old lug nuts onto the studs and weld the nuts to the studs in order to use a breaker bar to bust them loose.  The threaded holes left by the wheel studs were ideal to bolt the jig into place. With the jig bolted in, I drilled a pilot hole and then positioned it on the next two studs, making sure the same side of the jig always stayed up.  I drilled the pilot holes one bit size at a time up to .5” all the way through and then 17/32” halfway through from the backside to accommodate the .558 Knurl Diameter of the new studs.  Prior to using them I modified the press studs by trimming of one edge so that there was proper clearance between the studs head and the hub housing.  I then pressed the studs into place.  This gave me wheel flanges with the correct Ford bolt pattern.

New studs installed in the Ford bolt pattern, with the original pattern holes left open.

Stud with one edge ground off for clearance.

Notice the two sized 1/2" at the bottom and 17/32" at the top to fit the press studs.

The hub is a perfect fit in this 5 on 4.5 bolt pattern rim

With everything modified for the purpose of narrowing the unit and the correct bolt pattern drilled, this project can now go into the assembly phase.  In the next article, page IV, we will look at the differential rebuild, drive shaft rebuild, and the rebuilding of the hubs.

Want more information about putting a Jaguar IRS in a classic Mustang??  Check out Page I, Page II Page IV, Page V Page VI and Page VII of this project.

BUT if you can't wait to see what a Jaguar IRS looks like under a classic Mustang check out Mustang IRS Success Stories

Disclaimer on Daze Tech Tips
      I am not an expert in this field. I have performed these modifications myself with very good results. I am passing along restoration and performance tips for the purpose of education.  If you are concerned about reliability or safety issues, I do not recommend that you or any other individual perform these changes or attempt to modify your cars from stock configuration except under your own volition.  I do not assume nor accept any liability for the use of this information or how it is applied.

2013 DazeCars
The words / logos for Ford, Jaguar, Mustang, Galaxie, etc are used for descriptive and reference purposes only. DazeCars is neither affiliated with Jaguar Land Rover North America LLC, Ford Motor Company nor the manufacturers/distributors of Ford or Jaguar automobiles.