Ford 8.8 Locker 31 Spline with Heavy-Duty Cross Pin Shaft

A Ford 8.8 with an open differential will spin one tire and go nowhere the moment one side loses grip, whether that is a wet boat ramp, a muddy trail or a snowy driveway. This Spartan Locker, part number SL F8.8-31, turns that open carrier into a full locker without pulling the carrier or changing the gears. It is made for the 10-bolt-cover Ford 8.8 rear with 31-spline axle shafts and comes with a heavy-duty cross pin shaft, so F-150, Explorer, Ranger, Mustang and Expedition owners with the larger-spline axle get both drive wheels pulling under power.

What a lunchbox locker does in a Ford 8.8

In the stock open 8.8, the cross pin carries two pinion gears that mesh with two side gears splined to the axles. That gear set lets the wheels turn at different speeds but always sends torque to the easiest-turning side. The Spartan removes the gears and fits a pair of couplers and drivers, with springs and alignment pins, inside the same carrier case. Under power the driver teeth stay engaged and both axles turn together. In a turn, the outside wheel overruns, the teeth ramp over each other and the locker lets that wheel go faster, which is the clicking you hear on tight turns. Because nothing in the gear set changes, the ratio and the ring and pinion setup stay exactly as they were.

Symptoms that send people looking for a locker

• One rear tire spins while the other sits still on ice, mud or loose gravel
• Getting stuck pulling a trailer or boat up a slick ramp
• Worn open spider gears with play or chipped teeth, where the locker replaces the worn parts outright
• A cracked or grooved factory cross pin found on teardown

If your 8.8 is a Traction-Lok and it chatters in turns, that is a clutch and fluid issue; this locker will not fit a limited-slip carrier. Our Ford 8.8 rear axle problems guide covers both.

How to confirm this is the right locker

USA Standard Gear's install manual says the Spartan fits open carriers only and will not go into any limited-slip carrier, so check that first: jack both rear wheels off the ground, put the transmission in neutral and turn one wheel. If the other wheel turns the opposite way freely, it is open. Then count the splines on an axle end or check your build sheet. Our Ford 8.8 locker buyers guide for 28 vs 31 spline explains how to tell them apart, and the lunchbox vs full carrier locker comparison helps if you are still deciding.

Fitment

This unit fits the Ford 8.8 inch rear with the 10-bolt cover, 10 ring gear bolts and 31-spline axle shafts. It will not fit a 28-spline 8.8, and it will not fit the 2015-up Super 8.8, which uses a 12-bolt cover and 34-spline shafts. Many 8.8 axles were built in both spline counts across the same model names, so match the spline count on your own axle, or send us year and VIN. More 8.8 parts are in hard parts. If you only need the pin, the Spartan 8.8 replacement cross pin is sold on its own.

Install notes

• Drain the axle and pull the cover. Remove the cross pin bolt and pin, push the axles in, remove the C-clips and slide both shafts out partway.
• With deep ratios such as 4.88 or 5.13, the manual notes the ring gear may block the cross pin; then mark the caps, record backlash and pull the carrier. Revolution Gear lists 60 ft-lb for 8.8 bearing caps.
• Remove spider gears and thrust washers; the Spartan uses no thrust washers. Clean and inspect the case for cracks.
• Load springs into the alignment pins, hold them with wire, install couplers, spacers and drivers, then release the pins. Gap between the drivers should measure .145 to .170 in.
• Install the supplied cross pin and bolt, refit axles and C-clips, then test: one wheel should lock against the driveline and the other should click and rotate when turned the opposite way.
• Seal the cover with a gasket or RTV; a Yukon steel 8.8 cover is a solid upgrade. Refill with the gear oil grade on your axle tag or owner's manual.
• Keep both rear tires the same size and pressure; mismatched diameters wear a locker quickly.

Regearing at the same time? A Ford 8.8 3.73 ring and pinion with a carrier installation kit lets you do the whole axle in one teardown.

Will this fit my 28-spline Ford 8.8?

No. The SL F8.8-31 is made for 31-spline axle shafts only. A 28-spline 8.8 needs the 28-spline version or a swap to 31-spline axles. Count the splines on an axle end before ordering.

Can I install it in my Traction-Lok limited-slip carrier?

No. The install manual states the Spartan fits open carriers only and will not install in any type of limited-slip carrier. If you have a Traction-Lok, you would need an open carrier case first, or a full carrier locker.

Is the clicking noise in turns normal?

Yes. The drivers ratchet over each other when one wheel has to turn faster than the other, and that click is how this style of locker works. Steady banging, or a locker that never releases, means something needs checking.

Do I have to reset backlash or pinion depth?

Usually not. The locker goes into the existing carrier, so ring and pinion setup does not change. If the ring gear blocks the cross pin and you have to pull the carrier, keep the shims and bearing caps on their original sides and recheck backlash after reassembly.

Does it work in the Super 8.8 on a 2015 and newer F-150 or Mustang?

No. The Super 8.8 uses a 12-bolt cover and 34-spline shafts. This unit is for the traditional 10-bolt-cover 8.8 with 31-spline axles.

Do I need friction modifier with this locker?

A locker has no clutch plates, so it does not need limited-slip additive the way a Traction-Lok does. Use the gear oil grade Ford lists for your axle.