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bottom end rebuild, engine rebuild, KTM 300 EXC, KTM maintenance, top end service, two-stroke engine

KTM 300 EXC Engine Rebuild Guide: Top & Bottom End 2024-2026

How to Perform a Full Engine Rebuild on KTM 300 EXC (EU) 2024-2026: Top and Bottom End with Seals, O-Rings, and Gaskets

A full engine rebuild on the KTM 300 EXC (EU) 2024, 2025, or 2026 restores the liquid-cooled 293.15 cm³ two-stroke engine, including top end (piston, cylinder, head), bottom end (crankshaft, bearings, conrod), transmission inspection, and all seals, O-rings, and gaskets. This major service requires engine removal and is recommended periodically per the owner manuals See Owner Manual (p. 1, 2024), See Owner Manual (p. 1, 2025), See Owner Manual (p. 1, 2026). Expect 20-40 hours for DIY; professional help advised if inexperienced.

Cylinder, Cylinder Head, and Crankcase Assembly - Exploded View

Diagram positions referenced (Cylinder, Cylinder Head diagram):

Safety Warnings: Work in ventilated area; exhaust gases toxic See Owner Manual (p. 1). Hot oils scald; wear gloves See Owner Manual (p. 1). Dispose oils environmentally See Owner Manual (p. 1). Use lift stand; secure bike.

Tools Required

  • Torque wrench (5-100 Nm)
  • Metric sockets, Torx T25-T50
  • Hose clamp pliers (60029057000)
  • Crankshaft puller/installer (KTM-specific)
  • Bearing press/puller kit
  • Piston ring compressor, feeler gauges
  • Loctite 2701/243
  • Engine stand, cleaner spray

Parts You'll Need

Compatible across models; source from diagrams like Crankshaft (60313), Crankcase (60193), Cylinder Head (64134/81208/81536). Always match piston size (Gr. I/II).

Crankshaft and Piston Assembly - Exploded View

Diagram positions referenced (Crankshaft diagram):

  • Pos. 10 — Left Crankshaft Bearing (0625623204)
  • Pos. 15 — Roller Bearing (56530123200)
  • Pos. 19 — Connecting Rod Kit (54830015244)

Preparatory Work

Raise bike on lift stand See Owner Manual (p. 1). Drain gear oil (hot): remove drain plug (58030021100, Pos. 3) with new copper seal (58038022000), torque 20 Nm See Owner Manual (p. 1). Check level via monitoring screw (M6, 8 Nm). Remove exhaust, fuel tank, airbox, seat, radiators if needed. Disconnect battery, wiring. Clean oil screen (p. 135-136 manuals) using hose clamp pliers (60029057000).

Engine Removal

  1. Remove subframe screws (M8 30 Nm Loctite 2701, M8 35 Nm Loctite 243) See Owner Manual (p. 1).
  2. Disconnect throttle body, shift linkage, coolant hoses.
  3. Unbolt engine mounts (torque reverse on reinstall: check manual p. 67 for bearings).
  4. Lift engine out; place on stand.

Disassembly: Top End

Crankshaft Seal (0760385273)
  1. Remove cylinder head nuts (M10 studs, torque 10.5 Nm reverse), O-rings (0770107020, Pos. 3), (0770078020, Pos. 4).
  2. Lift cylinder; inspect Z-dimension (50.2-50.5 mm). Remove piston (match rings).
  3. Clean ports; replace base gasket (A44030035020, Pos. 14).

Disassembly: Bottom End

Engine Crankcase Assembly - Exploded View

Diagram positions referenced (Crankcase): Pos. 3 — Drain Plug (58030021100); Pos. 26 — Clutch Gasket (A44030025000).

  1. Pull crankshaft using tool; inspect conrod (54830015244).
  2. Press bearings: left (0625623204), roller (56530123200). Replace seals (0760385273).
  3. Disassemble transmission; inspect gears/shafts (A46033501000). Clean shift mechanism, replace seals (0760142460).
  4. Balancer shaft: seals (47030076100), bearings.
  5. Clutch cover gasket (A44030025000); retaining bracket (75030085100).

Reassembly: Bottom End

  1. Press new bearings/seals with lube. Install crankshaft, conrod; torque nuts (check manual).
  2. Reinstall transmission, balancer (torque per manual). Use bearing/seal kit (54830018488).
  3. Full gaskets (A44030099000). Torque case bolts progressively (M6 10-12 Nm).

Reassembly: Top End

  1. Install base gasket, cylinder (lube). Piston on conrod (arrow forward), rings (gaps staggered).
  2. Head with O-rings, combustion insert; torque studs M10 nuts 10.5 Nm crisscross.
  3. Power valve covers with gasket (A44037006000).

Engine Installation and Finishing

  1. Mount engine; torque mounts (manual p. 67 bearings play check).
  2. Reattach peripherals. Fill gear oil (0.80 L 15W/50) to level screw; torque 8 Nm See Owner Manual (p. 1).
  3. Prime oil pump (wake-up connector, throttle full). Check leaks; run engine ventilated See Owner Manual (p. 1).
  4. Test ride; read errors with KTM tool.

Post-rebuild: Check gear oil level cold See Owner Manual (p. 138). Minor service next: piston, O-rings, Z-dim.

References

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