Short, practical guide to diagnosing and repairing transmission (gearbox/final drive) oil leaks on Fiat tractors 55‑60 / 60‑90 / 79‑90 / 80‑90 / 90‑90 / 100‑90. Written for a beginner mechanic: what every part is, why leaks matter, common causes, tools and parts, step‑by‑step fixes for the usual leak points, tests, and pitfalls. No fluff.

Quick theory (why this matters)
- The transmission contains gear oil that lubricates gears, shafts and bearings. Think of the oil as the gearbox’s blood: without enough, metal parts overheat and wear quickly.
- Seals and gaskets keep that oil where it belongs. They’re cheap compared with bearings, gears or housings. A simple leak that’s left can lead to bearing damage, shaft wear, gear failure, or final‑drive destruction.
- Many tractors of these Fiat families use a gearbox sump (oil bath) and final drives that either share oil with the transmission or have separate final‑drive sumps — check which your model uses. Overfilling, blocked breathers, worn bearings, or physical damage causes leaks.

Main components (what you will see and work on)
- Gearbox housing / casing: cast housing that encloses gears and oil sump. Mating faces for covers and flanges.
- Input shaft (from clutch/flywheel): brings engine power into gearbox. Has seal(s) where it passes the housing.
- Mainshaft / layshaft / gear clusters: internal parts—normally not removed for simple external seal repairs.
- Output shaft / axle stub / flange: transfers power to final drives / rear axles; common leak point where a radial oil seal sits.
- Final drives / planetary hubs / hub gears: usually have their own oil cavity and seals at the wheel/hub.
- PTO shaft: rotates and passes through a housing with a seal.
- Drain plug / filler plug: plugs with crush washers or O‑rings—common leak sources.
- Breather/vent: small cap/pipe to allow pressure equalization. If blocked, internal pressure can force oil past seals.
- Seals: radial lip seals (rubber with metal outer shell) around rotating shafts. Must face the oil side with the lip against oil.
- Gaskets / sealant joints: flat gaskets or gasket maker between housings and covers.
- Bearings / bushes: support shafts. Worn bearings allow shaft runout that destroys seals.
- Hub nuts, brake drums, hub flanges: need removal to get to some seals.

Tools, supplies, and PPE
- Basic metric socket/ratchet set, wrenches, screwdrivers.
- Torque wrench. If you can’t get exact specs, tighten progressively and evenly; avoid overtorque.
- Seal puller, pick set, soft‑faced mallet, brass drift or appropriate seal‑driver.
- Gasket scraper, cleaning solvent (degreaser), lint‑free rags.
- Bearing puller and press (if replacing bearings).
- Drain pan, funnel, new gear oil (see “oil”), spare rags, gloves, eye protection.
- New seals (correct sizes), gaskets, crush washers for drain/fill plugs, O‑rings. Possibly new bearings if worn.
- Optional: UV dye for tracing leaks, heat gun (warm seal to fit), penetrating oil, anti‑seize.

Common oils (typical; verify for your tractor)
- Many Fiat tractors use EP gear oil: SAE 80, 85W‑90, or SAE 90 gear oil. Older transmissions with synchromesh sometimes prefer API GL‑4; hypoid final drives may use GL‑5. When in doubt, follow your model’s workshop manual or use GL‑4 if mixed transmission/clutch. Use correct capacity; typical gearbox capacities vary widely — check manual.

Safety first
- Work on level ground. Chock wheels. Engage parking brake.
- If lifting tractor, use rated lift points and heavy‑duty jack stands or a shop lift. Never rely on a hydraulic jack alone.
- Support heavy parts (hubs, brake drums, PTO shafts) with a jack or hoist when removing.
- Oil is slippery—clean spills immediately. Dispose of used oil at a recycling center.
- Wear gloves and eye protection; used oil can irritate skin.

Diagnosis: find exactly where oil is coming from
1. Clean the suspected area thoroughly with degreaser and let dry. A dirty area hides the leak.
2. Run the engine (or move tractor slowly) briefly and watch for fresh oil. Use a mirror/flashlight. If safe, rotate wheels by hand or engage PTO briefly in neutral to pressurize system; beware moving parts.
3. Use UV dye and a black light to find slow leaks.
4. Note whether oil appears at: filler/drain plug, seam/gasket, around output flange, between brake drum/hub, at PTO shaft, or from breather. That identifies which repair route to take.

General repair flow (simple leaks)
- If leak = filler/drain plug: replace crush washer/O‑ring and torque. Clean threads and check for stripped threads.
- If leak = breather: clean or replace breather.
- If leak = cover seam: remove cover, replace gasket, clean mating faces, use correct gasket maker if required.
- If leak = shaft seal (output, PTO, input): remove the rotating component (hub, PTO shaft) to access old seal, remove and install new seal correctly, inspect shaft and bearing, reassemble.
- If leak = final drive/hub: remove hub/drum, replace hub seal, inspect bearings and splines.
- If seal failure due to worn bearing or shaft damage: replace bearing/repair shaft or the whole final drive as required.

Detailed repair procedures (step‑by‑step common cases)

A) Drain/filler plug leak
1. Place drain pan under plug. Remove plug and drain a small amount (or fully) if you need to relieve pressure.
2. Inspect plug threads and seat. Replace crush washer or O‑ring with new. Clean mating surface.
3. Reinstall plug and torque to spec. If threads stripped, repair with HeliCoil or replace the plug/insert as needed.
4. Refill to correct level.

B) Top cover or side cover gasket leak
1. Clean area. Mark bolt locations for reassembly order if helpful.
2. Remove bolts and lift cover carefully—parts may be heavy. Support cover.
3. Clean old gasket/sealant completely from both mating faces using scraper and solvent. Do not score soft aluminum surfaces.
4. Inspect for cracks or warped surfaces. Lightly flatten surface if very minor, but replace cover if badly damaged.
5. Fit new gasket (paper, cork or rubber as OEM) or apply correct type of RTV where OEM calls for it. Follow curing directions if using RTV.
6. Refit cover and tighten bolts in a crisscross pattern progressively to the workshop torque spec. Do not overtighten.
7. Refill and test.

C) Output shaft / axle seal leak (most common)
1. Secure tractor, chock front wheels. Jack rear and put stands under axle housing or frame—safe support is essential. Remove wheel if needed.
2. Remove brake drum/hub assembly to access the axle flange. Keep track of shims and bearing preload washer if present. Use a hub puller if stuck. Support brake components.
3. You’ll see the seal in the housing where the axle passes. Clean area thoroughly. Note seal orientation before removal.
4. Use seal puller to remove old seal. Do not gouge bore. Inspect shaft surface (journal) for scoring or pitting. A scored shaft will cut new seals — it must be smoothed or the sleeve/shaft replaced.
5. If significant bearing play exists, press out and inspect bearings; replace as needed. Shaft end float or runout can ruin new seals quickly.
6. Lubricate new seal’s lip with clean gear oil. Install new seal squarely using a seal driver or appropriate socket that only contacts metal outer shell. Drive to full depth but do not deform lip. Ensure correct orientation: seal lip faces the oil (lip toward inside where oil is).
7. Reassemble hub/drum, reinstall bearings and shims in original order, torque hub nut to spec and set bearing preload correctly. Reinstall wheel.
8. Refill to level and test.

D) PTO shaft seal
1. Remove PTO shaft cover or shield and any retaining nuts/bolts. Support shaft.
2. Pull PTO shaft out a bit to access the seal. In some cases the seal is external and can be replaced without full removal; in others the shaft must come out.
3. Replace the seal as above, inspect shaft splines for wear and corrosion. If splines are damaged, replace shaft.
4. Reassemble and test.

E) Final drive / hub oil leak (inner hub seal)
1. Remove wheel and brake drum. Use care: some drums are heavy.
2. Remove hub nut and pull hub; this will expose the wheel bearing and inner hub seal.
3. Remove old inner seal and replace with new. Inspect bearings; repack with grease or replace if worn.
4. Reassemble with original shim stack/clearances. Correct preload essential. If unsure, measure end float and match original assembly.
5. Refill final drive cavity if separate.

Inspection checklist while you’re in there
- Shaft journal finish: smooth, no deep grooves.
- Bearing radial/play: side‑to‑side or axial play indicates worn bearings. Replace bearings before seals when worn.
- Housing face for nicks or corrosion where seals seat.
- Threads: drain/filler plug threads should be sound. Helicoil if badly damaged.
- Breather condition: clear and fitted above oil level.
- Seal orientation and seating: lip faces oil side; outer shell flush with bore.
- Correct gasket/sealant type: some surfaces need gaseous sealant or anaerobic; others need paper gasket.

Refill, run and check
1. Refill gearbox/final drives to correct level using recommended oil. Use a funnel and clean fluid.
2. Run tractor at low engine speed and cycle through gears and PTO gently. Watch the repaired area for leaks.
3. Shut down and recheck oil level after warm‑up and after short run; top up as needed. Maintain level within specified marks.
4. Recheck after a day of use: some leaks show only after thermal cycling.

What can go wrong (and how to avoid it)
- New seal leaks immediately: causes—seal installed upside down, not fully seated, shaft scored, or housing bore damaged. Avoid by careful orientation, correct driver, and inspecting shaft/bore.
- Bearing failure after replacing seal: if old bearings were marginal, replacing only the seal hides the root cause. Always inspect and replace bearings with significant play.
- Overfilling oil: causes foaming and leaks. Fill to correct level only.
- Stripped threads: caused by cross‑threading or overtightening; repair with inserts or replace plug/insert.
- Using wrong oil (GL‑5 vs GL‑4): can damage yellow metals (bronze) in some gearboxes — use the recommended spec.
- Broken housing when using excessive force: use the correct pullers, heat, penetrating oil rather than prying on castings.

Troubleshooting quick guide
- Leak only when moving: rotating shaft seal or hub seal.
- Leak while warm but not cold: seal hardening or shrinkage; old seals.
- Sudden large loss: physical damage (puncture/crack) or removed/faulty plug.
- Oil on brake shoes or drums: hub seal or final drive leak—inspect hub area and brake seals.

Parts you will commonly replace
- Output shaft oil seals (sizes vary per axle; buy specified part).
- PTO shaft seal.
- Drain/filler plug washers / crush washers.
- Cover gaskets / O‑rings.
- Bearings and seals for hubs if play is present.
- Breather cap.

Final notes and cautions
- Always replace seals with correct OEM‑sized parts. A slightly wrong diameter or thickness will fail quickly.
- If you find excessive bearing damage, major shaft wear, or a cracked housing, this becomes a major repair — don’t try to jury‑rig seals only.
- If you cannot safely support heavy components (hubs, drum, gearbox), get help or lift equipment.
- Keep a log: note oil used and how much you added and the date—helps track leaks and future diagnostics.

If you follow diagnosis, clean the area, replace leaking seal/gasket with correct parts, check bearings/shafts, reinstall correctly and refill with the recommended oil, you’ll stop most transmission oil leaks on these Fiat models and prevent more costly gearbox damage.
rteeqp73

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