COGENG Komatsu 707-99-38720 Heavy-Duty Boom Cylinder Seal Kit

 Component Decomposition

The 707-99-38720 is a complete seal kit for a heavy-duty boom cylinder (usually for 100–250 ton·meter class excavators). Below is the list of seals and their functions:

✅ Note: This kit is for disassembly and rebuild of the boom cylinder. It does not include the cylinder tube, piston rod, or bearings.

Technical Constants & Performance Data

Exact Model Compatibility

Component Specifications Table

Specific Components Included

• Main Piston Seal (U-Cup) – Qty: 1

• Rod Seal (Step Seal) – Qty: 1

• Wiper/Scraper (Heavy-Duty) – Qty: 1

• Buffer Ring – Qty: 1

• Wear Ring (Piston) – Qty: 1

• Dust Seal – Qty: As required

• Complete O-Ring & Backup Ring Set – Qty: 1 Set (All necessary sizes for gland, port, and fitting assembly)

• Note: This kit is designed specifically for 110mm bore x 75mm rod boom cylinders  An exploded diagram is included for correct assembly.
  Support Seals & Hardware
  

• Full set of precision-molded static O-rings for all gland and port connections

• Essential wear guide band to ensure smooth piston travel and minimize side load on seals

• High-pressure nylon backup rings for critical O-ring applications

• Excluded Items: This is a seal and wear component kit only. It does NOT include the piston, cylinder rod, barrel, gland nut, bearings, bolts, or any other metal parts or hardware.
  Installation decomposition diagram
  
  

  Step 1 – Cylinder removal & disassembly

  • Support boom with safety stands (or lower boom to ground)

  • Remove hydraulic lines (label ports: Piston side vs. Rod side)

  • Remove cylinder from machine (pin at boom foot and cylinder bracket)

  • Drain oil into clean container

  • Remove cylinder head (gland) using a spanner wrench or pin wrench

  • Extract piston rod assembly from tube

  Step 2 – Disassemble piston rod & cylinder head

  • Remove piston nut (use hydraulic press or breaker bar with rod clamped)

  • Slide piston off rod

  • Remove cylinder head from rod (may require heat if stuck)

  • Clean all metal parts with solvent – no abrasives on sealing surfaces

  Step 3 – Replace piston seals

  • Remove old piston seal and backup rings

  • Install wear rings first (soak in warm oil to soften slightly)

  • Install backup rings (left and right of main piston seal groove)

  • Install main piston seal (lip facing cylinder head for double-acting cylinders)

  • Install piston static O‑ring + backup rings (inner diameter seals rod to piston)

  ⚠️ Critical: Piston seal orientation varies by cylinder design. For standard double-acting boom cylinders, the seal lip faces the cylinder head (rod side) – but always verify with original seal orientation.

  Step 4 – Rebuild cylinder head (gland)

  • Remove old O‑ring and backup rings from head outer diameter

  • Install new static O‑ring + backup rings

  • Install rod seal (lip faces internal pressure – toward piston)

  • Install buffer seal (if present – lip also faces piston)

  • Install wiper seal (lip faces outward to scrape rod)

  • Install dust seal / excluder (outermost)

  Step 5 – Assemble cylinder

  • Lubricate all seals with clean hydraulic oil or assembly grease

  • Carefully slide cylinder head onto rod (use a sleeve to protect rod seal from threads)

  • Slide piston onto rod (torque piston nut to spec – see below)

  • Insert rod assembly into cylinder tube with gentle rotation

  • Screw cylinder head into tube (hand tight, then torque to spec)

  Step 6 – Reinstall & test

  • Reconnect hydraulic lines (correct orientation: piston side = extend, rod side = retract)

  • Start engine, cycle boom slowly to purge air (full stroke 5–6 times)

  • Check for external leakage at rod and head threads

  Common Installation Mistakes (Boom Cylinder)

  Symptom	Likely Cause
  Cylinder drifts down (internal leakage)	Piston seal damaged during install or incorrect lip orientation
  External leakage at rod	Rod seal cut on rod threads (use thread protector) or wiper installed backwards
  Cylinder sticks / jerks	Wear rings too tight (did not soak), or wrong wear ring thickness
  Head threads leak	O‑ring twisted or missing backup rings
  Cylinder won't extend/retract fully	Cushion seal (if present) installed incorrectly blocking cushion valve port

  Frequently Asked Questions
  

  Q1: Which Komatsu excavator models use the 707-99-38720 boom cylinder seal kit?
  A: This seal kit is designed for specific Komatsu excavator boom cylinders that require OEM part number 707-99-38720. Please check your machine model and cylinder part number before ordering.

  Q2: Is this an OEM or aftermarket seal kit?
  A: It is an OEM-grade heavy-duty replacement kit, manufactured to meet or exceed original Komatsu specifications for sealing performance, durability, and fit.

  Q3: What makes this a "heavy-duty" boom cylinder seal kit?
  A: The kit features premium polyurethane seals and high-grade rubber compounds designed to withstand extreme hydraulic pressures, temperature fluctuations, and heavy digging applications.

  Q4: What is included in the COGENG 707-99-38720 seal kit?
  A: The kit includes all necessary rod seals, piston seals, wipers, O-rings, and backup rings for a complete boom cylinder rebuild – no missing parts.

  Q5: How do I purchase the COGENG 707-99-38720 boom cylinder seal kit?
  A: Contact us with your Komatsu machine model and cylinder details for fitment confirmation, then order directly through our website or sales team for fast shipping.

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