Summary theory (brief)
- Function: the output-shaft (tailshaft) seal is a lip-type elastomer seal pressed into the transmission/axle housing where the rotating output shaft exits. Its lip rides on the shaft surface and retains gear oil while excluding dirt.
- Failure modes: hardening, lip wear, torn/chewed rubber, distortion, installation damage, shaft groove or scoring, housing bore damage, or axial/runout misalignment. Result = oil leak, loss of lubricant, contamination ingress, overheating and bearing/gear failure.
- Repair principle: remove the damaged seal, restore/replace any damaged shaft or bore surface that prevents proper lip contact, and install a new correctly sized seal seated to correct depth and orientation so the lip re-establishes a tight, even interference with the shaft to retain lubricant.

Ordered procedure (explicit steps with theory and what each step fixes)
1) Safety and preparation
- Park on level ground, set parking brake, chock wheels, lift and support vehicle securely if needed.
- Drain transmission/axle oil to below the output seal level. Collect fluid. Theory: removing pressure of fluid prevents mess and avoids oil being forced out when the shaft is disturbed.
- Gather tools: appropriate sockets/wrenches, puller or pry tools for coupling/yoke, seal puller, seal driver or appropriate-diameter socket, snap-ring pliers, torque wrench, screwdriver, cleaning solvent, lint-free rags, new OEM output shaft seal (correct part number), new mating gasket/packing, possibly new retaining nuts/bolts, lightweight oil for seal pre-lube, anti-seize, and a dial indicator if checking runout. Theory: using correct parts and tools avoids installation damage and ensures correct interference fit.

2) Access the output seal
- Remove any obstructing components: driveshaft/propeller shaft flange, universal joint yoke, center bearing (if applicable), exhaust bracket or crossmember as required for access. Mark driveshaft orientation to maintain balance. Theory: access and correct reassembly maintain dynamic balance and enable seal extraction without additional component damage.
- If the output coupling uses a flanged yoke bolted to the output shaft, remove bolts and slide off the yoke. If there's a retaining nut and washer, remove them. Use a pry method or puller to separate the flange from the shaft. Theory: the seal sits behind the flange; removing the flange exposes the tailshaft housing and seal.

3) Remove the old seal
- If a dust cover or snap ring is present, remove it first.
- Using a seal puller or small hooked tool, carefully extract the old seal from the housing. Avoid gouging the housing bore. Theory: removing the old seal clears the bore and reveals the shaft condition to diagnose failure cause.
- Clean the bore and shaft with solvent and a lint-free rag to inspect surfaces. Remove any gasket material and oil residue.

4) Inspect shaft and housing
- Visually and tactilely inspect the shaft running surface: look for grooves, scores, corrosion, pitting, burrs or wear. Measure runout with dial indicator if shaft wobble suspected. Check the housing bore for nicks or deformation.
- Theory: the seal lip needs a smooth, continuous surface and correct radial interference. Grooves or high spots prevent even lip contact and will leak even with a new seal. Excessive runout (>0.05–0.2 mm depending on application) will accelerate lip wear.

5) Repair or replace shaft if necessary
- Minor surface corrosion or burrs: remove light ridges with very fine emery or polishing film, de-burr with a crocus cloth, then clean thoroughly. Use a thin coat of oil to check finish.
- Significant grooves or scoring: do not rely on a new seal alone. Options: fit a hardened sleeve (shaft repair sleeve) pressed over the damaged area or replace the output shaft/shaft carrier per workshop manual. Theory: the sleeve restores a true, smooth sealing surface and corrects diameter for proper interference. A new seal on a grooved shaft will extrude into the groove and leak.
- If bearings or seals were failing and contaminated bearings, consider bearing replacement while the area is open. Theory: oil loss often accelerates bearing failure; replace worn bearings to prevent recurrence.

6) Prepare the new seal and housing
- Confirm seal orientation (lip faces the fluid). The spring-loaded lip is installed toward the fluid to keep pressure against the shaft. Theory: reversed installation allows fluid to push past the lip.
- Lightly lubricate the seal lip and shaft with the same lubricant used in the transmission/axle or light assembly oil to reduce dry-start wear and ease installation.
- Clean housing bore; check for any foreign matter. Apply a light smear of suitable assembly grease to the outer diameter of the seal if recommended by manufacturer. Theory: lubrication and clean surfaces prevent immediate wear and facilitate correct seating.

7) Install the new seal
- Use a seal driver or a socket that matches the outer diameter of the seal to press it evenly into the housing. Tap evenly around circumference until seal is fully seated to the specified depth or flush with housing. Do not cock the seal. Theory: even seating ensures the seal lip engages the shaft concentrically at the designed radial interference; cocked or partially seated seals leak.
- Replace any snap ring or dust cover removed earlier.

8) Reassemble driveshaft/flange and torque
- Reinstall the output flange/yoke onto the output shaft. Replace any paper gaskets or apply specified sealant per workshop instructions.
- Reinstall bolts/nut and torque to the manufacturer's specified value in the correct sequence. If the flange uses a crush washer or locking device, replace as required. Theory: correct torque and hardware ensure the flange runs true; axial movement or misalignment will damage the seal lip or cause leaks.
- Refit any removed brackets, crossmembers, and the driveshaft in the marked orientation.

9) Refill fluid and check
- Refill the transmission/axle to the correct level with the specified lubricant. Start with the amount recommended and top to level via fill plug.
- Run the engine and operate through gears (or cycle the driveline) to build pressure and spread lubricant to the seal area. Inspect for leaks while rotating the shaft slowly and under light load.
- Check after a short road test or loaded operation; re-torque flange fasteners if required by manual after heat cycle. Theory: confirming under operating conditions ensures the new seal maintains contact and there is no leak path due to dynamic movement.

10) Final inspection and follow-up
- After initial run, recheck fluid level and inspect for seepage. Re-inspect for oil on adjacent bearings or housing that would indicate other leaks.
- Advise monitoring oil level in the following days to catch a slow persistent leak.

How this repair fixes the fault (concise)
- The seal replacement restores the elastomer lip-to-shaft interference that physically retains the lubricant. By removing the old, hardened or damaged lip and installing a new correctly oriented seal, the lip re-establishes a continuous, elastomeric sealing surface.
- Inspecting and correcting shaft or bore damage prevents fluid from escaping through channels or gaps that a new seal alone cannot bridge. Proper seating and correct flange alignment ensure the lip runs true, preventing early wear and re-leakage.
- Restoring proper lubricant level and cleanliness returns bearing and gear lubrication to designed conditions, preventing secondary damage caused by oil loss.

Important cautions (short)
- Do not reuse a torn or deformed seal. Do not hammer a seal directly on its inner lip; only drive on the outer case.
- If the shaft has deep grooves, don’t just fit a new seal—use a sleeve or replace the shaft. A new seal on a grooved shaft will fail quickly.
- Use correct seal orientation: spring/lip toward the fluid.
- Always use correct torque values and OEM-specified lubricants.

Common diagnostic signs confirming a successful repair
- No visible oil leak at output housing after road/test cycle.
- Stable oil level over subsequent days.
- No abnormal noise or overheating from bearings/gears previously suffering from oil loss.

End.
rteeqp73

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1991 HINO FE For Sale https://www.truckpaper.com/listings/trucks/for-sale/211084531 1991 Hino FE17 COE Refrigerated Van Truck, S/N ...

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Tools & materials
- Metric socket set and ratchet, extensions and universal joint
- Torque wrench (range to 0–100 Nm)
- Combination wrenches
- Flat and Phillips screwdrivers
- Pliers (hose clamp pliers or channel locks)
- Drain pan (large)
- Funnel or vacuum coolant filler (preferred for bleeding)
- Gasket scraper / wire brush / clean rags
- Small pick or O‑ring tool
- Infrared thermometer or long‑reach digital probe thermometer (optional)
- Boiling pot (for bench‑testing thermostat) or kettle
- Gloves, safety glasses
- New thermostat (correct part for your Hino model and engine)
- New thermostat gasket or O‑ring (do not reuse)
- New hose clamps if existing are corroded
- Fresh coolant (Hino spec — typically ethylene‑glycol based, premix or concentrate)
- Thread chaser or bolt cleaner (optional)
- Waste container and absorbent for spills; container for used coolant (proper disposal)

Safety precautions
- Work on a cold engine. Cooling systems are pressurized when hot — wait until fully cool.
- Wear eye protection and gloves. Avoid skin contact with coolant.
- Catch coolant in a pan and dispose per local regulations.
- If truck must be raised, use jack stands on firm ground and chock wheels.
- Relieve any pressure slowly: remove radiator cap only when cold and use a rag to protect hands.

Overview
Thermostat replacement is straightforward: drain coolant below the thermostat housing, remove hoses and housing bolts, swap thermostat and gasket (spring side toward engine block), clean mating surfaces, reassemble, refill and properly bleed the cooling system, then confirm thermostat opens at operating temperature.

Step‑by‑step procedure
1. Preparation
- Park on level ground, chock wheels, set parking brake.
- Ensure engine cold. Disconnect battery negative if you will be working near electrical components (optional but recommended).
- Gather parts and tools.

2. Drain coolant below thermostat level
- Place drain pan under radiator/drain cock or under engine block petcock.
- Open drain cock and drain enough coolant so level falls below thermostat housing (you don’t need to fully drain the system in most cases; just below housing).
- Close drain cock once drained.

3. Access and exposure
- Remove any air intake ducts, engine covers or brackets blocking access to thermostat housing.
- Locate thermostat housing — typically at engine end of upper radiator hose or coolant outlet on cylinder head.

4. Isolate coolant flow
- Use hose pinch pliers or pinch the upper radiator hose if needed to minimize spillage while disconnecting.
- Loosen hose clamp and remove upper radiator hose from housing. Be prepared for residual coolant to spill.

5. Remove thermostat housing
- Remove the bolts holding the thermostat housing to the engine. Use proper socket and extension.
- Note bolt lengths/positions — some housings use bolts of different lengths.
- Gently pry housing off. If stuck, tap with plastic mallet or wedge a screwdriver carefully — avoid gouging mating surfaces.

6. Remove old thermostat and gasket
- Pull out the old thermostat and discard.
- Remove old gasket or O‑ring; use a pick/scraper but avoid scratching the mating faces.
- Inspect housing and engine mating face for corrosion, pitting or warping. Clean thoroughly with gasket scraper and solvent.

7. Bench‑test old (optional)
- If you want to test the old thermostat: suspend it in boiling water and observe. It should begin to open near its rated temp and be fully open near boiling. Replace if uncertain — best practice is to install a new one.

8. Install new thermostat and gasket
- Confirm orientation: spring (coil) faces the engine (toward water pump/block) and the thermostat valve flange faces the housing/radiator outlet. (On most Hino engines the spring goes toward the engine.)
- Fit new O‑ring/gasket. If O‑ring type, lightly coat with clean coolant to seat.
- Place thermostat squarely in seat.

9. Refit housing and bolts
- Clean bolt threads; apply small amount of anti‑seize or light oil if recommended. Do not use threadlocker unless specified.
- Fit housing and tighten bolts by hand, then torque to manufacturer specification. If you do not have the exact torque spec on hand, torque evenly in a crisscross pattern to a conservative value (typically 8–20 Nm for small housing bolts) — but obtain exact spec from the workshop manual as soon as possible.
- Reattach radiator hose and secure with hose clamp (replace clamp if corroded).

10. Refill coolant and bleed air
- Refill cooling system with the correct coolant/water mix using a funnel or vacuum fill tool to reduce air entrapment.
- If the engine has a bleed screw near the cylinder head/thermostat area, open it while filling until a steady stream of coolant (no air) appears, then close it.
- If no bleed screw, start engine with heater on high and cap off or use a funnel; watch for air bubbles and top up until level stabilizes. Use infrared thermometer or touch top radiator hose — thermostat opens when hose feels hot (careful — avoid burns).
- Run engine to normal operating temperature and observe that the upper radiator hose heats up (thermostat opening). Keep adding coolant as necessary.

11. Check for leaks and verify operation
- With engine at operating temp, inspect thermostat housing, hose connections and drains for leaks.
- Confirm engine reaches normal operating temperature and does not overheat. Check temperature gauge and feel that heater output is normal.

12. Finalize
- Tighten any clamps or bolts as required to final torque.
- Dispose of old coolant and parts responsibly.
- Recheck coolant level after a short run and again after a thermal cycle (cold engine) and top up.

How each tool is used (quick)
- Ratchet/socket: remove housing bolts; use extension and universal for awkward angles.
- Torque wrench: final torque on housing bolts to spec to prevent leaks/warping.
- Pliers/hose clamp pliers: remove and install hose clamps safely; prevent hose damage.
- Gasket scraper: remove old gasket material; clean mating surfaces for proper seal.
- Funnel/vacuum filler: fills and helps de‑air system — vacuum filler is best to prevent airlocks.
- Infrared thermometer: confirms thermostat opening by measuring hose temp.
- Boiling pot: bench‑test thermostat opening if desired.

Replacement parts required (typical)
- Correct thermostat assembly for your Hino engine
- New thermostat gasket or O‑ring
- Possibly new hose clamps or upper radiator hose if cracked
- Coolant (size depends on truck)
- Optionally thermostat housing bolts if corroded or stretched

Common pitfalls & how to avoid them
- Not draining low enough: causes large spills and difficult removal. Drain below housing level first.
- Wrong orientation: installing thermostat upside down prevents it opening properly — install spring toward engine block.
- Reusing old gasket: causes leaks — always use new gasket/O‑ring.
- Scratched/contaminated mating surfaces: leads to leaks — clean surfaces thoroughly.
- Over‑torquing bolts: can crack housing — use torque wrench and manufacturer specs.
- Air pockets (airlock): will cause overheating and false diagnosis of thermostat failure — bleed system properly (use vacuum filler or bleed screw).
- Using wrong thermostat temp rating: can affect warm‑up and emissions — use OEM part or correct spec.
- Not checking heater operation: heater performance confirms coolant flow through heater core.
- Ignoring corroded hoses/clamps: can fail shortly after repair — replace if questionable.

Notes specific to Hino trucks
- Hino medium‑duty trucks often have thermostat housings accessible but sometimes partially covered by intake or piping; remove obstructing components carefully and note sensor locations (CTS, coolant temp sensor) to avoid damage.
- Always confirm the exact thermostat part number and torque values from the Hino workshop manual for the FD/FE/FF/SG/FA/FB series and the specific engine fitted to your vehicle.

Done.
rteeqp73