Who is it for? 2015-up Mustangs.
Why do you want it? Because your stock stuff is weak. Literally.
If you are going in to do the differential bushings, specifically the BMR BK051 or BK054, we highly suggest upgrading the actual hardware while you are already in there. The bolts in our Extreme Bolt Upgrade Kit are stronger than the OEM pieces, but more importantly the kit plays a real role in limiting differential movement, which the stock setup does not.
Billet sleeves eliminate bushing void
Converts front mount to through-bolt
Low cost, high effect
Who Needs This
- Anyone installing BMR BK051 or BK054 differential bushings. You are already there with the tools out. Doing the hardware separately later means doing the job twice.
- Anyone who launches the car hard. Diff movement under load is what changes pinion angle, and pinion angle changes are what kill axles and driveshafts.
- Anyone chasing a driveline vibration they cannot find. If the diff can move, the angle is not consistent, and neither is the vibration.
The Rear Bushing Problem: Too Much Play
In the rear differential bushing, the stock bolt is a smaller diameter than the hole it lives in. That gap lets the differential move. On our own car the bolt sat visibly off centre, and there were thread impressions left in the bushing from the force of the bolt being pushed sideways into it.
That movement changes pinion angle. Pinion angle changes cause vibration, and past a point they cause driveline carnage. It is not a subtle failure mode.
The Front Bushing Problem: Loading the Threads
The front bushing is a different failure, and a worse one. From the factory it is a fully threaded design, which means the mounting point is relying heavily on the threaded portion of the bolt.
Threads are the weak part of a bolt. A thread is a stress riser cut into the shank, so a bolt loaded in shear across its threads fails at a fraction of the load it would handle across the solid shank. We have seen these bolts shear clean off for exactly this reason.
Our kit changes the geometry. We drill out the bushing and replace the threaded factory setup with a solid through-bolt design. The cradle then bears on the solid shank of the bolt rather than the threads, which is a fundamentally stronger arrangement.
What the Kit Changes
| Location | Factory setup | Extreme kit |
|---|---|---|
| Rear bushing | Undersized bolt in an oversized hole, diff free to move | Billet sleeve fills the void, grade 12.9 bolt |
| Front bushing | Fully threaded design, load carried on the threads | Bushing drilled, solid through-bolt, load on the shank |
Doing the bushings at the same time, which is the whole point of doing this now:
Locks out diff movement
Pair with the Extreme bolt kit
Locks out diff movement
Pair with the Extreme bolt kit
And if you are addressing the rear end properly, the differential is one piece of it. Subframe deflection is the other, and that is what the Steeda Stop the Hop kit handles.
Frequently Asked Questions
What does the Extreme Differential Bolt Upgrade Kit do?
Two things. It fills the void around the rear differential bushing bolt with a billet sleeve so the diff cannot move, and it converts the front bushing from the factory threaded design to a solid through-bolt so load is carried on the bolt shank rather than the threads.
Why do factory differential bolts fail?
The front bolt carries shear load across its threads, which is the weakest part of a fastener. The rear bolt is undersized for its hole, so it allows movement and gets pushed sideways into the bushing.
What happens if the differential can move?
Pinion angle changes. That causes vibration and, under load, driveline failures. We broke a driver's side axle on Project Goldmember from exactly this.
Do I need this if I am installing BMR BK051 or BK054 bushings?
We strongly recommend it. You are already in there with everything apart, and the bushings address movement in the rubber while the bolts address movement in the hardware. Doing one without the other leaves half the problem.
What grade are the bolts?
Grade 12.9, which is stronger than the factory hardware.
Does the install require drilling?
Yes, the front bushing gets drilled out as part of converting it to a solid through-bolt design. That part is permanent.
Will this fix a driveline vibration?
If the vibration is caused by the differential shifting and changing pinion angle, then yes, this addresses the cause. If it is a driveshaft, u-joint or tire balance issue, it will not. Worth diagnosing before you buy.
Keep Reading
Questions? Call 877-2-LETHAL or email us.
Cheapest Axle You'll Ever Save
Grade 12.9 bolts, billet sleeves, and a through-bolt front mount. Do it while the bushings are out.
