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Spartan Locker Install Ford 8.8 IRS

Paul Fithian

Elite Explorer
October 12, 2016
Reaction score
City, State
Long Beach, IN
Year, Model & Trim Level
2007 Job 1 RWD V8 Limited
I completed installation of a Spartan Locker SLF8831 in the 8.8 IRS differential on my 2007 Sport Trac. Drives great with better traction!

Although the Spartan catalog lists this unit for 2001-2010 Sport Tracs, it does not fit the 2007-2010 IRS version without modification. The axles cannot be pushed in far enough, they contact the carrier pin before the axle circlip can expand into the area inside of the side coupler.

In order for the axles to clip into the side couplers, a 0.100" relief must be made for the axle circlip to expand into. See attached PDF on how to do this to an existing locker. I had correspondence with Gus at Randy's Worldwide, he advised that their engineering department will implement this change on future products. Note this change enables use of a Spartan Locker in post 2001 Explorers and Mountaineers with 8.8 IRS differentials.

Part of this job was to replace the axle and pinion seals. The pinion seal was leaking due to driveshaft movement caused by a failed center bearing (7A2Z-4A499-A Bracket), which was replaced as part of this work.

I had to fab up some tools to install the pinion and axle seals as well as the Front Insulators 7L2Z-4B431-A (bushing). Seals installed were:

Pinion: SKF 18136
Driver Axle: SKF18005 (new style, made in Taiwan by KOK)
Passenger Axle: NAPA 27SS2850 (new style, made in Taiwan by NAK)
Spartan Locker Ford 8-8 IRS Pin Interference1.jpg
Spartan Locker Ford 8-8 IRS Pin Interference2.jpg
Axle Seal Compare1.jpg

Ford 8-8 IRS Insulator Install.jpg
Ford 8-8 IRS Seal and Insulator Install Tools.jpg


  • Spartan Side Coupler Mod Ford 8-8 IRS.pdf
    159.4 KB · Views: 164

Update, I've put about 4,000 miles on this since installation, most of this was towing a 5,000 lb trailer.

Zero issues, works perfectly.