Safety first
- Work on level ground, parking brake on, wheels chocked. Block wheels both front and rear.
- Stop engine, remove ignition key and disconnect battery negative.
- Wear gloves, eye protection, and steel-toe boots. Have a fire extinguisher nearby.
- Lift only with rated jacks/stands if you need access under tractor. Never work under unsupported machine.
- Keep gearbox oil catch pan and absorbent pads ready for spills.

Tools and consumables
- Metric socket set (8–24 mm), ratchet and extensions
- Combination wrenches
- Flat and Phillips screwdrivers, pry bars
- Snap-ring pliers and small drift punches (2–6 mm)
- Small magnet, picks and needle-nose pliers
- Soft-faced hammer
- Torque wrench (0–200 Nm range)
- Pick-up tool / telescoping magnet
- Brake/parts cleaner, lint-free rags
- Gasket scraper, paper gasket or gasket sealant
- Replacement gasket for shift housing OR suitable RTV (suitable for gearbox oil)
- New detent parts: detent balls, springs, plunger/pawl (OEM kits recommended)
- Gearbox oil (type and quantity per manual)
- Threadlocker (medium strength), light grease (lithium), and a small dab of anti-seize where appropriate
- Service manual for torque specs and exploded diagrams (recommended)

Symptoms that indicate detent repair
- Gear pops out of gear into neutral under load
- Selector not locking into gear detents or feels “sloppy”
- Hard or imprecise shifting between gears

Step-by-step detent repair (general procedure used on Fiat 55-60 → 100-90 series)

1) Preparation
- Park tractor on level ground, chock wheels, disconnect battery negative.
- Move shift lever to neutral to ease removal of linkages.
- Clean around shift tower/gearbox to avoid contamination when opening.

2) Remove shift lever and external linkages
- Remove knob and gaiter/cover boot.
- Disconnect shift linkages and any interlock rods. Note or mark positions for reassembly (a quick photo helps).
- Unbolt and remove shift lever assembly from the top of the shift housing. Use appropriate drift or puller if lever is tight on splined shaft.

3) Access the detent assembly
- Remove shift housing/top cover bolts. Expect gearbox oil residue — place pan under area. On many Fiat tractors the top cover houses the selector forks/detent plungers.
- Carefully lift top cover; if necessary break the gasket seal with a gasket scraper—do not gouge mating surfaces.
- Remove the top cover and set it on a clean bench on paper/cloth.

4) Identify detent components
- Locate selector rods/shafts, detent plungers (spring-loaded plungers or ball + springing seats), and the indexing notches on rails.
- Typical detent parts: spring, ball (or plunger), detent sleeve or plunger guide, circlip/retaining pin.

5) Remove worn detent parts
- Use small magnet or pick to lift detent balls out of their pockets. Carefully remove springs and plungers — keep parts in order.
- If plunger is held with snap ring, use snap-ring pliers. If pinned, drift out pin with small punch.
- Inspect shaft grooves, selector rods and plunger bores for wear, pitting or elongated holes.

6) Clean and inspect
- Clean all passages and bores with parts cleaner and lint-free rag. Blow out with compressed air if available.
- Inspect springs for loss of height or corrosion. Measure length against new part if available. Replace any suspect springs.
- Inspect detent balls for flat spots or pitting; replace if not perfectly round.
- Inspect plunger tips and selector grooves. If grooves are badly worn, selector rails/forks may need replacement or regrind. Minor wear: new detent components can restore engagement.

7) Replace parts
- Install new springs and balls or plungers into their pockets. Apply a light coat of grease to prevent corrosion, but do not fill bore with grease — just a film.
- If there are retaining clips/pins, reinstall and secure with proper orientation. Use threadlocker where specified on studs/bolts.
- Replace the top cover gasket with a new paper gasket or apply RTV (per manual). Ensure mating surfaces are clean and dry.

8) Reassemble shift housing
- Carefully lower top cover straight down to prevent displacing detent parts. Tighten cover bolts finger-tight first, then torque to spec in a criss-cross pattern. If you don’t have the spec, tighten evenly but do not over-torque—consult the workshop manual for exact numbers.
- Refit shift lever onto splined shaft, securing nut/bolt to spec. Reconnect linkages and gaiter.

9) Refill/check gearbox oil
- If oil was lost, top up gearbox to correct level with the correct oil type. Check for leaks around the housing and cover gasket.

10) Adjust and test
- With tractor stationary and engine off, shift through all gears. You should feel distinct detent engagement for each gear and lever shouldn’t “float” out of gear.
- Start engine and with clutch, road-test under light load. Try to hold gears under normal loads and check for popping out.
- Recheck fluid level after test and inspect for leaks.

How the tools are used (practical notes)
- Magnet/pick: retrieve small balls/springs from cavities without dropping into gearbox.
- Small drift & hammer: remove pins or stubborn retaining rings carefully to avoid enlarging bores.
- Snap-ring pliers: remove/install circlips in plunger bores.
- Torque wrench: ensure cover bolts and lever nut are tightened correctly — prevents leaks and deformation.
- Parts cleaner & rags: remove old oil and grit from detent bores before installing new parts.
- Feeler gauges are rarely needed for detent removal — but a caliper can measure spring free length and ball diameter for correct replacements.

Common pitfalls and how to avoid them
- Dropping balls/springs into geartrain: work over a clean tray and use a magnet/pick immediately. If anything drops into lower gearbox, remove cover and recover before running engine.
- Reusing worn springs/balls: they’re cheap; replace rather than gamble on partial repair.
- Not replacing gasket: leads to leaks. Use correct gasket or RTV rated for gear oil.
- Failing to inspect selector rails/forks: worn selector grooves will continue to let the box pop out of gear even with new detent parts — replace worn forks/rails as needed.
- Over-greasing detent bores: excess grease attracts grit and interferes with detent action. Use only a light film.
- Over-tightening bolts: strips threads or distorts the housing—use torque wrench and manual specs where available.
- Skipping test drive: you must verify under load to ensure the repair solved the problem.

Replacement parts typically required
- Detent ball(s), detent spring(s), plunger/guide(s) — available individually or as a detent repair kit from Fiat/parts suppliers.
- Shift housing/top cover gasket (or RTV)
- Possible additional parts if inspection shows wear: selector forks, selector rods/shafts, springs, retaining pins, shift lever spline nut, O-rings/seals.

Final notes
- Whenever in doubt about torque specs or part numbers, consult the Fiat workshop manual for your specific model (55-60 → 100-90 family).
- If selector rails or fork shoulders are appreciably worn or fork teeth are chewed, gearbox strip down and fork replacement is required — detent parts alone won’t fix severe wear.

Follow these steps and replace the small detent components; if after that the transmission still pops out or shifts poorly, expect deeper internal wear requiring full gearbox servicing.
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