Differential Pinion Seal

Replace the differential pinion seal on a 1972 Ford F-100 with 302ci V8 to stop rear differential oil leaks at the pinion yoke.

Warnings

Tools required

Parts

• Differential pinion seal × 1 — Use OEM specification for Ford 9-inch rear axle
• Pinion nut (if reuse not recommended) × 1 — Use OEM specification
• Crush sleeve (if bearing preload is disturbed) × 1 — Use OEM specification

Fluids

• 80W-90 GL-5 gear oil — 3.5 qt

Preparation

1. Park vehicle on level ground and set parking brake firmly.
2. Chock front wheels securely on both sides.
3. Raise rear of vehicle with floor jack at differential housing or rear axle tubes.
4. Support vehicle on jack stands placed under frame rails - never work under a vehicle supported only by a jack.
5. Allow differential to cool completely if vehicle was recently driven.
6. Position drain pan under differential to catch any gear oil spillage.

Procedure

1. 1

   Remove driveshaft
   Mark the relationship between the rear driveshaft yoke and the pinion flange with paint marker for proper reinstallation alignment. Remove the 4 U-joint strap bolts or U-bolts securing the rear universal joint to the pinion flange. Push driveshaft forward slightly to disengage yoke from pinion flange. Lower rear of driveshaft and slide it rearward out of the transmission tailshaft. Plug transmission tailshaft with a rag or suitable plug to prevent fluid loss. Support driveshaft or remove completely from under vehicle.
2. 2

   Mark pinion position
   Use paint marker to make alignment marks across the pinion nut, pinion shaft threads, and pinion flange. These marks must align exactly upon reassembly. Count and record the exact number of threads visible beyond the pinion nut - write this number down. Measure or note the pinion flange depth relative to a fixed point on the differential housing for reference.
3. 3

   Check bearing preload (initial)
   Using an inch-pound torque wrench on the pinion nut, measure and record the rotational torque required to turn the pinion (pinion bearing preload). This should be 8-14 inch-pounds on a used bearing assembly. This measurement is critical for comparison after seal replacement. If you do not have an inch-pound torque wrench, proceed with extreme caution when reinstalling the pinion nut to the exact marked position.
4. 4

   Remove pinion nut
   Use pinion flange holding tool or carefully wrap a chain around the flange to prevent rotation. Install 1-1/8" socket (9-inch axle) or 1-1/16" socket (8-inch axle) and breaker bar on pinion nut. Break pinion nut loose - this will require significant force. Remove pinion nut completely and set aside. Keep nut if it is not damaged and will be reused.
5. 5

   Remove pinion flange
   The pinion flange should pull off by hand or with light prying. If stuck, use a suitable puller to remove the flange from the pinion shaft splines. Do not damage the splines. Note any shims or spacers behind the flange and keep them in order for reinstallation. Set flange aside and inspect splines for wear or damage.
6. 6

   Remove old pinion seal
   Using a seal puller or large flat-blade screwdriver, carefully pry the old pinion seal out of the differential housing bore. Work around the seal circumference to avoid cocking it in the bore. Be extremely careful not to score or scratch the seal bore surface in the housing - any damage here will cause the new seal to leak. Remove seal completely and discard. Some gear oil will drain from the housing - catch it in the drain pan.
7. 7

   Clean and inspect sealing surfaces
   Thoroughly clean the pinion seal bore in the differential housing with shop rags and solvent if needed. Inspect bore for scratches, burrs, or damage. Use emery cloth or fine sandpaper to smooth any minor imperfections - work carefully to avoid enlarging the bore. Clean the pinion shaft seal surface (the smooth area where seal lip rides) with shop rags. Inspect for grooves, rust, or wear. Light rust can be polished with emery cloth; deep grooves require pinion replacement. Wipe all surfaces clean and dry.
8. 8

   Install new pinion seal
   Lightly coat the outer diameter of the new pinion seal with gear oil. Coat the seal lip generously with gear oil. Position seal squarely in the housing bore with the seal lip facing inward toward the differential. Using a seal driver or appropriately-sized socket that contacts only the outer edge of the seal (never the center), carefully drive the seal into the bore. Drive the seal in evenly, checking alignment frequently, until it is flush with or slightly below the housing face. Do not cock the seal or drive it in crooked.
9. 9

   Reinstall pinion flange
   Coat pinion shaft splines lightly with gear oil. Reinstall any shims or spacers that were removed in the original positions. Slide pinion flange onto pinion shaft, aligning splines. Tap flange gently into place with a soft mallet if needed. Flange should slide on smoothly - do not force it.
10. 10

    Reinstall and torque pinion nut
    Thread pinion nut onto pinion shaft by hand as far as possible. Install pinion flange holding tool. Using torque wrench and socket, tighten pinion nut gradually while carefully aligning the paint marks made during disassembly. Stop when marks align exactly and the same number of threads are showing as originally recorded. The pinion nut torque for the Ford 9-inch axle when setting new preload is high (commonly 200 ft-lbs and up), but for this seal-only procedure achieving the original marked position and matching the recorded bearing preload is what matters — do not simply tighten to a fixed torque value. Tighten gradually to the marked position and verify preload; do not overshoot the marks, as loosening to correct requires a new crush sleeve. Verified detail (Ford 9-inch pinion nut torque spec is typically 140-200 ft-lbs with preload being more critical than absolute torque. The lower bound of 175 may be too high as a minimum starting point.): The pinion nut torque specification for Ford 9-inch rear axle is typically 140-200 ft-lbs
11. 11

    Verify bearing preload
    If you have an inch-pound torque wrench, check pinion bearing preload by measuring rotational torque at the pinion nut. It should match the original measurement from Step 3 (typically 8-14 inch-pounds for used bearings). If preload is significantly lower, the crush sleeve has collapsed and bearing preload is lost - this requires complete differential disassembly and crush sleeve replacement. If preload is correct or slightly higher, proceed. Never loosen the pinion nut to reduce preload - this will require starting over with a new crush sleeve.
12. 12

    Reinstall driveshaft
    Guide driveshaft rear yoke onto pinion flange, aligning the paint marks made during disassembly. Install U-joint straps or U-bolts and 4 bolts finger-tight. Tighten U-joint bolts in a cross pattern to secure driveshaft. Typical torque is 10-15 ft-lbs but hand-tight plus 1/4 turn with a ratchet is acceptable for U-joint strap bolts. Verified detail (Factory torque specification for driveshaft U-joint strap bolts on 1972 Ford F-100): 26.0 lb-ft (35.0 Nm)
13. 13

    Refill differential
    Locate differential fill plug on passenger side of housing (square or hex head plug approximately halfway up the housing). Remove fill plug. Add 80W-90 GL-5 gear oil through fill hole using a pump or squeeze bottle until oil reaches the bottom of the fill hole and begins to seep out. Install fill plug and tighten securely. Total capacity is approximately 4.5 pints (about 2.25 quarts), but fill to plug level regardless of amount used. Add friction modifier / limited-slip additive if equipped with Traction-Lok differential.

Reassembly

1. Remove jack stands and lower vehicle to ground carefully.
2. Remove wheel chocks.
3. Wipe down differential housing and surrounding area to check for leaks after test drive.

Verification

• Start engine and shift through all gears to verify driveshaft is properly connected and no vibration is present.
• Test drive vehicle for 5-10 miles, making several stops and starts.
• Re-check differential fill level after test drive - top off if necessary.
• Inspect pinion seal area for any signs of leakage. A small amount of seepage immediately after installation may occur but should stop within a few miles.
• Check pinion nut for tightness - it should not have loosened during test drive.
• Listen for any abnormal whining or howling from differential during test drive, which could indicate bearing preload issues.

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