Short version first: flywheel resurfacing is removing the damaged top layer of the flywheel friction face so the clutch can bite correctly again. It requires removing the transmission and clutch, taking the flywheel to a lathe or resurfacer, machining it flat and concentric within factory limits, then reassembling with new or inspected clutch parts and correct torque/sequence and seals. Below is a full beginner-friendly guide with component descriptions, why it’s needed, how the system works, step‑by‑step instructions, inspection limits and common failure causes.

Theory / “Why this repair is needed”
- Purpose of the flywheel: the flywheel is a heavy, round steel plate bolted to the crankshaft flange. It provides:
- a flat friction surface for the clutch disc,
- rotational inertia to smooth engine pulses,
- a place for the ring gear (starter engagement),
- a pilot seat for the transmission input shaft/pilot bearing.
- Why resurfacing: over time the flywheel friction face can develop hot spots, glazing, scoring, ridges, cracks, or become warped because of heat and pressure from clutch slip, poor clutch adjustment, or contamination (oil/grease). These defects cause clutch slip, chatter/pulse when engaging, rapid clutch wear, and vibration.
- Analogy: think of the clutch surface like a cutting board. If it gets deep grooves or burn marks it won’t hold your knife straight — resurfacing is like planing the top flat again so it cuts cleanly.
- When you resurface: you remove a thin layer to restore flatness, remove hot-spoted material and restore the correct surface finish so the new or existing clutch disc can mate properly.

Basic system operation (how it works)
- At idle the pressure plate clamp springs (diaphragm or coil springs) press the clutch cover against the flywheel; the clutch disc is squeezed between flywheel and pressure plate so engine torque goes to the transmission input shaft.
- When you press the clutch pedal, the release/throw-out bearing pushes on the pressure plate fingers, releasing clamping force; the disc is freed and the transmission input shaft can turn independently.
- The flywheel must be flat and concentric to allow even pressure across the clutch disc and to keep engagement smooth.

Major components you will encounter (what each is and why it matters)
- Engine crankshaft flange: where flywheel bolts attach. Inspect for damage and runout.
- Flywheel: cast/forged disc with friction face, bolt holes, and ring gear. Contains stamped minimum thickness sometimes.
- Ring gear: press-fit gear on flywheel outer diameter; engages starter. Inspect teeth for wear.
- Flywheel bolts/studs + washers: high-strength fasteners that secure flywheel to crank. Often single-use (stretch type) — replace as required.
- Dowel pins: locate the flywheel concentrically on the crank flange.
- Clutch disc (friction plate): contains friction material that contacts the flywheel. Always inspect/replace when resurfacing.
- Pressure plate (clutch cover): clamps disc to flywheel. Inspect for cracks, heat spots, spring height and flatness.
- Pilot bearing/bushing: inside crank or flywheel that supports transmission input shaft; often replaced when clutch removed.
- Throw-out bearing (release bearing): presses on pressure plate; inspect/replace.
- Bellhousing: gearbox cover you remove to access clutch.
- Transmission input shaft: splines that engage clutch disc; inspect splines for wear.
- Rear main seal: crankshaft rear seal that is often easier to replace with the flywheel out — check/replace.
- Clutch alignment tool: used to center the disc when reinstalling.
- Flywheel lathe or resurfacer: machine used to turn the flywheel face flat and restore finish.

Tools & consumables (basic list)
- Vehicle lift or heavy jack + stands and transmission jack
- Engine support bar or engine/transmission support (if needed)
- Socket set, breaker bar, impact wrench (careful with flywheel bolts), extensions
- Torque wrench (capable of specified torque)
- Clutch alignment tool
- Pry bars, screwdrivers
- Dial indicator (to check runout)
- Straight edge & feeler gauges (to check flatness)
- Micrometer or vernier caliper (to measure flywheel thickness)
- Puller or pry tools for bellhousing/clutch parts
- Marker/paint to mark orientation of bellhousing/transmission
- Thread locker (if specified by factory)
- New flywheel bolts (recommended), new pilot bearing, new release bearing, new rear main seal (if required)
- Protective gloves, eye protection

Safety notes (must follow)
- Disconnect battery before starting.
- Support vehicle securely on stands or lift. Use a transmission jack for the gearbox; do not rely on floor jacks alone.
- Flywheel is heavy — handle with care; it can fall and cause injury.
- Use correct tools; do not use makeshift supports.
- Hot or oil-contaminated friction surfaces must be cleaned with solvent and then re-machined if required — oil on the surface will cause slippage.

Step-by-step procedure (removal, inspection, machining, reassembly)

A. Preparation and removal
1. Park, chock wheels and disconnect battery negative.
2. Drain and/or remove any components that prevent access (airboxes, crossmembers, drive shafts as applicable).
3. Support the engine if you will be separating the transmission. Use engine support if engine mounts or torque rods are loosened.
4. Remove the transmission:
- Mark orientation of bellhousing/transmission relative to engine so you can re-align later.
- Disconnect linkage, hydraulic lines (clutch slave cylinder if external), speedometer cable or sensor, electrical connectors.
- Support transmission with a transmission jack, remove bellhousing-to-engine bolts, and carefully lower transmission straight back. Be careful of the input shaft and clutch disc falling.
5. With transmission removed, inspect and remove clutch assembly:
- Loosen pressure plate bolts in a crisscross pattern gradually to avoid warping.
- Remove clutch disc and pressure plate. Keep orientation marks so you know which side faces the engine if reusing component (but typically you replace the disc and inspect/possibly replace pressure plate).
6. Remove pilot bearing/bushing if needed (often driven out from the back with an appropriate tool).
7. Mark flywheel orientation relative to crank if you plan to reuse it (original balance marks) — ideally keep orientation if reusing. Remove flywheel bolts in a star/crisscross pattern to ease it off without distortion (or as manual recommends). Watch for dowel pins and note their location.
8. Inspect rear main seal area for leaks; this is a good time to replace the rear main seal.

B. Inspection before machining
1. Visual: look for heat discoloration (blue/black spots), visible cracks, heavy scoring, ring gear damage, ridges at the edge where the clutch lip sits.
2. Flatness/runout: mount flywheel on a surface plate or the crank flange and use dial indicator on the friction face to check axial runout and radial runout. Typical allowable multi-point runout is small — factory spec is best. For many flywheels allowable runout <0.05–0.10 mm (0.002–0.004 in); consult Hino manual.
3. Thickness: measure at several radial positions using a micrometer from the friction face to a defined reference (or check stamped minimum thickness on flywheel). If you must remove more metal than allowed or the thickness is below minimum, replace the flywheel.
4. Cracks: use a bright light and magnifier. For cast flywheels, crack repair is not recommended — replace.
5. Ring gear: damaged teeth require replacement or ring gear replacement (press-on style) or flywheel replacement.

Decision tree:
- If cracks, severe heat checking, or below minimum thickness → replace.
- If minor scoring, glazing, or slight warpage within allowable limits → resurface.
- If ring gear severely damaged or bolt holes elongated → replace.

C. Machining / resurfacing (what happens at the machine)
If you don’t have machining equipment: take the flywheel to a reputable machine shop that machines truck flywheels.

What the machinist will do:
1. Clean the flywheel and check balance and runout on a dedicated flywheel lathe.
2. Mount flywheel on lathe using the bolt holes or arbor to ensure concentricity.
3. Cut a thin layer from the friction face with a carbide tool to remove hot spots and scoring. The goal is a uniform, concentric surface. They typically remove only the minimum necessary.
4. Check for flatness and concentricity after the cut.
5. Re-profile any lip at the edge where friction meets the step (a slight radius may be added depending on spec).
6. If necessary, balance the flywheel (if significant mass has been removed or if the flywheel is balance-critical).
7. Re-check ring gear fit and retap bolt holes if needed.
8. Clean and degrease. Avoid contaminating the friction surface with oil.

Desired surface finish:
- The face should show fine concentric machining lines. Too smooth (glazed/polished) is bad; too rough will accelerate clutch wear. Machine shops aim for a finish that allows the clutch to bed in properly (consult spec; typical roughness ranges used by shops are moderate — fine visible lines).

D. Reassembly & installation
1. If the pilot bearing, release bearing, or rear main seal are to be replaced, do those now.
2. Clean the crank flange and locate dowels. If you marked orientation when removing, align marks. If not, ensure dowels are seated correctly.
3. Position flywheel on crank. Use new flywheel bolts if specified. Tighten hand-tight in a crisscross pattern.
4. Torque the flywheel bolts to factory specification in stages using the correct torque sequence. Many heavy-duty engines require a final angle-tightening. Check Hino FG17/FG19 manual for exact torque and sequence. Do not reuse stretch bolts unless specified as reusable.
5. If specified, apply threadlocker to bolts as directed.
6. Install new clutch disc and pressure plate. Use the clutch alignment tool through the disc into the pilot to center the disc.
7. Tighten pressure plate bolts gradually and evenly in a crisscross pattern to the specified torque.
8. Reinstall transmission carefully, aligning the input shaft splines with the disc and pilot bearing. Use the transmission jack to support and guide; ensure it seats to the bellhousing face before inserting bolts.
9. Reconnect all lines, linkage, electrical connections and mounts. Refill fluids if removed.
10. Reconnect battery.

E. Adjustment, break-in and testing
1. Adjust clutch free play or pedal as per manufacturer specification.
2. Start engine and check for abnormal noises or vibration.
3. Test drive with light engagement to bed the clutch — avoid heavy torque or towing for the first few hundred kilometers/miles to allow proper bedding.
4. Re-check torque on flywheel and pressure plate bolts after initial heat cycles if factory recommends (some recommend re-torquing after a period).

What can go wrong (common mistakes & failure modes)
- Not replacing flywheel bolts: many are torque-to-yield and must be replaced. Reusing can cause failure.
- Over-machining: removing too much material reduces flywheel mass/inertia, upsetting balance and clutch engagement; may exceed minimum thickness → replace needed.
- Warping after machining: caused by poor mounting or using a grinder rather than a lathe. Only machine on proper flywheel lathe.
- Unseen cracks: if heat cracks are missed, flywheel can fracture under load; always inspect thoroughly.
- Contaminating the surface: oil/grease on the friction face causes immediate or early slip. Never touch the machined face with oily hands; clean with brake cleaner and handle with gloves.
- Wrong surface finish: too smooth (glazed) causes slip; too rough accelerates wear — use a qualified resurfacer with experience on clutch flywheels.
- Missing pilot bearing damage: a worn pilot bearing damages the clutch disc splines and input shaft—replace pilot bearing when clutch is out.
- Improper torque/sequence on flywheel: can cause crank flange distortion, bolt failure, and imbalance leading to vibration.
- Not replacing the clutch disc or pressure plate when needed: resurfacing an old flywheel but fitting a worn clutch disc still results in early failure.
- Reusing damaged ring gear: starter engagement failure; check teeth and replace or resurface ring gear if necessary.
- Improper reassembly of transmission: misalignment can damage input shaft splines or pilot bearing.
- Not supporting the engine/transmission properly: risk of injury or damaging mounts.

Specific Hino FG17/FG19 considerations
- Always consult Hino factory service manual for exact torque values, bolt replacement rules, runout and thickness specs, and any model-specific procedures. Truck flywheels often require high torque and sometimes angle torquing; factory procedures must be followed.
- Heavy-duty truck practice often calls for replacing certain fasteners and inspecting balance marks and ring gear condition more strictly than light vehicles.

Useful checks and measurements (practical checks a beginner can do)
- Visual: heat spots, cracks, deep scoring.
- Runout: mount flywheel on the crank and use a dial indicator at multiple radii; anything obvious (more than a few thousandths of an inch) means machining or replacement.
- Thickness: micrometer across the friction face at several points and compare to minimum (stamped or manual).
- Flywheel-to-crank bolt torque: verify spec before reassembly.
- Clutch alignment: ensure disc is centered with alignment tool before tightening pressure plate.

Final tips
- When in doubt, replace rather than resurface a flywheel with suspicious cracks or very heavy damage — the cost of a new flywheel is less than the cost of a failed flywheel in service.
- Use a qualified machine shop with flywheel experience.
- Replace the clutch disc, pilot bearing, and release bearing as a set when doing flywheel work.
- Keep the friction faces clean and do not handle them with greasy hands.
- Record the work done and bench-test runout/balance readings if possible.

This covers the full workflow and components you’ll meet as a beginner mechanic doing flywheel resurfacing on a Hino FG series truck. Follow the Hino service manual for exact torque values and tolerances, and use a proper flywheel lathe for machining.
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From the Archives: Hino FG Series Old footage of the Hino FG Series truck.

From the Archives: Hino FG Series Old footage of the Hino FG Series truck.

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