F-Body DIY Ford 9" Rear End Buildup
We all are painfully aware of the Achilles heel all 82-02 F-bodies share, the 7.5” and 7.625” Corporate 10-bolt rear end. What makes these rears so weak? Essentially, it’s the size of the ring gear. At 7.5” it is simply too small to support high torque loads or severe shocks. With most F-bodies weighing in around 3600 lbs, the situation is even worse as the loads on the gears are increased as the engine tries to overcome the inertia of the car sitting still. In a 2900 lb Fox-bodied Mustang, the 10-bolt would probably be at least somewhat more durable.
Generally, if you are racing an automatic, you’re going to be safe on the stock 10 bolt well into the low 12 second ET range. Once you hit 11s you may want to start saving up for a rear because a catastrophic failure is now only a matter of time.
There are measures you can take to upgrade and strengthen the 10-bolt. None of these measures will enable it to survive behind an M6 car with Drag radials or slicks, but for Autos into the 11s and stick cars on street tires in the high 12s you can get by with an upgraded 10-bolt with the following modifications:
- Weld the axle tubes to the differential housing. OEM uses only two small spot welds
- Moser or other aftermarket axles, these are as much as 30% stronger than the stock axles and will reduce the chance of sheering off the splines or snapping an axle in two
- T/A or
other brand performance rear end girdle cover. The preload caps on the cover help hold the differential bearing caps in place to prevent heavy torque loads from throwing the casing and gears out of alignment.
- Main bearing stud kit…replacing the stock bolts further increases bearing cap stability.
- Solid pinion spacer aka Crush Sleeve eliminator. Another method of death for the 10 bolt is when, due to high load, the crush sleeve, which is used to set pinion depth, collapses and allows the pinion gear to walk up the ring gear. This misaligns the teeth and then the gears pretty much eat themselves. The Solid spacer takes the slack out of the setup and will not allow the pinion to move backwards into the ring gear.
These upgrades should allow you to reliable hold 350rwhp in an M6 car on street tires and up
to 400rwhp in an automatic with any tires and a higher stall converter. The high stall speeds actually soften the shock to the gears.
However, even with these upgrades, the ring gear can only be so strong due to its size. It is recommended you use the highest quality gear set you can find, which means Strange/US Gear or Motive. Also, stay away from 4.10 or higher ratios. The higher ratios require more ring gear teeth, meaning they have to be narrower and therefore more brittle. For best 10-bolt life I’d recommend going no higher than 3.73s.
But alas, ultimately even the 10-bolt housing is a weakness and can hold only so much power. It is not uncommon to hear of cracked housings as the method of death for the 7.5” 10-Bolt.
So what’s an F-body owner to do about rear end
durability? There are several options. Most common are the GM 12-bolt assemblies offered by Moser and Strange. Each is a modern version of the GM 12-bolt that originated in the early 1960s and is the strongest live axle rear end ever offered by GM. For $2300-$2500 depending on options, you will get a complete rear end, with posi, high strength axles, choice of ABS configuration if needed, your choice of gear ratio, etc. These are ready to bolt-in setups, with the exception that you may want to paint them before installing as they come bare. The Strange setup is a bit better than the Moser in a couple of respects. Most notably the Moser uses larger than stock axle tubes, requiring you to get an additional sway bar mount kit. Strange uses stock sized tubes, which truly makes installing the Strange simpler and more convenient. ‘98+ owners will have to either send in
their disk brake backing plates to have them welded onto the axle tubes or pay an additional $230 to have Strange or Moser put a set of theirs on for you. Pre-98 assemblies have a basic plate installed that accepts the typical LT1 disk brake package at no additional cost.
With a ring gear size of about 8.8”, the 12-bolt is probably the strongest rear any automatic car will ever need. However, they are not unbreakable. Any M6 car pushing over 500 hp could possibly break one of these under the right conditions (like 5K rpm clutch dumps). The downside to the 12-bolt is really cost. After shipping and if you include the optional aluminum rear cover girdle, you could be looking at $2600+ for a 12-bolt setup. That’s more than most of us would spend on a heads/cam package.
There are other alternatives, however. Extreme Chassis
(www.Extremechassis.com) takes Ford 8.8” rears (the stronger rear offered in the Mustang) and adapts them for F-bodies by installing the control arm brackets and spring perches, fabbing up a torque arm mount, etc. to create a bolt-in setup. Starting around $1880 before options, the units are completely rebuilt used rears with your choice ratio new gearset and I believe a new posi unit. As the ring gear is over 1” larger than our 10-bolt, this is a much stronger rear end. The downside is they are still not unbreakable, the fabricated torque arm mount can be of questionable strength, and most of all the lead time and people who have purchased have reported customer service has been poor. Just do a search on LS1tech.com for Extreme Chassis and read up. So, buy at your own risk, but this is a way to save upwards of $500.
You can, of course, fab up
your own rear if you are handy with a welder. You can source a GM 12-bolt from a salvage yard and cut the axle tubes to the correct width, weld on your backing plates, cut off the brackets from your 10 bolt and install on the new rear, and go through the gears. However, the sticking point again is the torque arm mount. It takes quite a bit of ingenuity to construct and attach a mount that will stand up to the torque it will see. Availability of the 12-bolt in junk yards is getting scarce as well. Another option is the late model 8.5 inch 10-bolt that comes in the GM full size trucks. You will have to do the same fabrication but the rears should be much easier to find.
The ultimate solution for the F-body is the same solution hotrodders have been using for decades now, the Ford 9” rear end. With a 9 inch ring gear, these rear ends are nearly indestructible.
NASCAR teams use Ford 9” rears under all their cars. Moser and Strange sell bolt-in packages ready to go, and include all new components. These units will save you only a few bucks verses their 12-bolt units however. The secret to the budget build is to source your own components!
The single most important piece to the 9 inch build is the center case. Unlike most rear ends where the housing is also a structural member, the Ford 9 inch utilizes a removable center “chunk” which houses the ring and pinion and the bearings, etc. This chuck bolts in to a housing that is more like a giant rear cover, with the axle tubes and mounting points installed on it. One reason for the 9 inch’s popularity is that this arrangement allows for quick gear changes, and many professional teams have several centers already set up with various gear ratios ready to
The 9 inch cases come in several configurations, and the only one you need to know about at this point is “nodular iron”. Many OEM applications used a cheaper iron center case, and these cases are good for barely more than 400hp in most applications. That’s not going to gain you much over your stock 10-bolt. The nodular iron cases, however, are far stronger, but much more rare. Look for a bit “N” on the front of the case to confirm its nodular content. Moser and strange both make new nodular cases for around $300, and Strange has a new, slightly lighter duty nodular case that runs around $200. This new Strange case is made out of the same material, but it has less reinforcement ribbing, which also makes it slightly lighter and should still hold up to 650hp easily.
If you are overly concerned with weight, be warry of the
9 inch. A complete unit is going to weight over 40 lbs more than your 10-bolt setup. Its also less efficient. The larger ring gear absorbs more horsepower, but the real killer is how low the pinion gear mates with the ring gear. This is a less efficient spot from which to drive the ring gear and as a result the 9 inch is about 3% less efficient than the 12-bolt, which itself is about 2% less efficient than the 10-bolt. So expect to give up about 15hp on a typical modified F-body just to drive the rear end. Not a happy thought, but look at it this way, the extra 15hp isn’t doing you any good if you snap the 10-bolt every time you race the car.
Luckily for us, Moser sells a complete 9 inch housing package, with the spring perches, control arm brackets, and torque arm mount for $985, which includes Moser axles with your choice of spline count. You are going
to want at least 31-spine axles if you plan on any serious ET efforts. This deal gets the housing out of the way. Now, for you 97 and earlier guys, your existing brake backing plates will have to come off the 10 bolt and be put on the axles prior to pressing on the bearings. On 98+ F-bodies you’ll have to send in your caliper mounting plates to have them welded on to the Moser housing, or you’ll have to pay them extra to put a set of theirs on (Verify this is accurate before ordering).
Next you will need a center section. You can buy a 100% brand new centers off of ebay cheap. The Strange Pro-Series nodular Iron case, the strongest of the different types, will run you about $300 and will take basically as much power as you can throw at it. It includes a reinforced pinion support bearing area, which is a crack-prone spot on the stock centers.
Next up is the Strange S-Series case. It is also nodular, but contains a bit less material and is not quite as strong. It is, however, only $200 and can still support up to 650 horsepower. For cars where weight is an issue, Strange makes a $400 aluminum case which is very light, but don’t plan on putting a lot more than 450hp to it. There are several other brands of 9” centers out there, so just shop around. If you stick with an OEM piece just make absolutely sure it is nodular and check for cracks at the rear pinion support. Look for the big “N” on the front of the case.
Once you have a case, you will need a pinion support. On the 9”, the pinion is mounted to the support, which then bolts into the case with shims. This eases installation because you don’t have the trial/error of installing shims between the pinion and pinion bearing.
The most popular style is the “Daytona” pinion support. It’s the strongest and comes in aluminum and nodular. The support is quite strong for what it needs to hold, so don’t fret over which material to get…try for the aluminum if possible to save a few pounds, this rear end is probably going to be 60+ lbs heavier than your 10-bolt as it is.. Expect to pay around $100 for the pinion support.
The remainder of items you’ll need include a setup kit with bearings, races, pinion support shims, the pinion nut, bolts, washers, etc. Jeg’s or Summit has these kits cheap. You’ll also need a set of gears obviously, I’d recommend Motive or Strange for strength, though there is a new company out there called Yukon that offers quality gears at a reasonable price. Finally, don’t forget a pinion yoke.
For maximum strength, get the 1350 series yolk, it will run you around $100.
A quick word on driveshafts: With a Ford 9” behind it, the driveshaft will now be a weak link. Consider a custom made steel driveshaft with upgraded U joints and trans yoke. The stock driveshaft will fit if you use the short-style pinion yoke on the rear end, otherwise you’ll have to have a custom length one built. NAPA sells a Precision brand “447” conversion U-joint for $30 that adapts your 1310 series driveshaft to the 1350 series yoke if you end up sticking with your stock driveshaft.
Finally, you’re ready to assemble. I’m not going to write that up here, partly because I failed miserably at it, and also because it’s a whole other article in and of itself. A good tutorial can be found at
Moser 9 inch housing w/tq arm mount and 31 spline axle package $950
Motive 3.70 gears
Timken install kit
Strange S Series nodular iron case
Daytona aluminum pinion support
Auburn Pro-series posi, used, plus Moser sway bar mounting kit, used $230
Cost to set up gears $100
Cost to have axle bearings pressed on $60
Total cost: $1917