Auto Tensioner Assembly & Seal Repair Kit – Models 320/08759 & 333/W2655

Two service parts specified for JCB 3CX/4CX-class backhoe loaders and related wheeled excavator platforms. 320/08759 is an auto tensioner assembly (serpentine/alternator belt) replacing legacy 320/08651, while 333/W2655 is a seal repair kit (WA-series, "W" denoting washer/seal plate pattern) for hydraulic cylinder or valve-body circuits on the same chassis. Both are high-turnover service items during 3CX/4CX 2000–4000 hour major inspections.

Key Features

• 320/08759 (Auto Tensioner): Spring-loaded pivot arm with sealed bearing cartridge, die-cast aluminum housing, pulley bore 17 mm class, replaces 320/08651 / 320/08648 / 320/08649 legacy PN

  
  

• 333/W2655 (Seal Kit): WA-series seal kit, nitrile + PTFE primary seals, backup rings, wear bands, O-rings – suited to hydraulic cylinder rod/head or control-valve body reseal

  
  

• Tensioner: grease-sealed deep-groove ball bearing, 80–120 °C continuous, damping spring rated 50 N·m class pivot torque

  
  

• Seal kit: NBR primary (std.), FKM optional for biodiesel/HVO or high-temp hydraulic estates; PTFE rod seal wafer included on cylinder trims

  
  

Technical Specifications

Application Guidance

320/08759 – Automatic belt tensioner on 3CX/4CX backhoe loaders powering the alternator, water pump, and fan drive off the 4.4–4.8 L diesel. The spring-loaded arm maintains constant belt tension as the serpentine stretches — critical because 3CX/4CX run a single multi-groove belt for most engine auxiliaries. Symptoms of a failing tensioner: chirp on cold start, glazed belt ribs, wobble at the alternator pulley, or the belt walking off under load. Replace together with the belt (typical 320/086xx-series drive belts in the same family).

333/W2655 – WA-pattern seal kit for 3CX/4CX hydraulic circuits — most commonly the dipper-arm cylinder, bucket cylinder, or loader-frame cylinder rod seals, but also cross-used on the valve-body spool covers (333/Y3448 / 333/Y3449 pattern siblings). The "W" suffix in 333/ indicates washer/seal-plate family — this kit consolidates all dynamic seals for one cylinder tear-down so no piecemeal ordering is needed.

📎 Note: 320/08759 belongs to the 320/08xxx engine-aux family (belts, pulleys, tensioners), while 333/W2655 belongs to the 333/ seal-kit family. They're ordered together during a 3CX/4CX major (4000 h) service — belt + tensioner on the engine side, cylinder reseal on the hydraulic side.

Installation Notes

320/08759 (Tensioner):

1. Relieve belt tension — rotate the tensioner arm against spring (15 mm socket on the pivot nut) and slip the old belt off.

   
   

2. Unbolt the tensioner pivot boss (2× bolts, 45–55 N·m typical) from the front cover / idler bracket.

   
   

3. Swap the new tensioner — do not reuse the old pivot bolts if thread stretch is visible. Torque evenly.

   
   

4. Route the new belt per the 3CX/4CX decal (usually: crank → water pump → alternator → fan → AC → tensioner).

   
   

5. Rotate the arm, seat belt, release — the spring should hold ~5–8 mm deflection at the longest belt span.

   
   

6. Run 30 s, re-check belt alignment — the tensioner arm should sit ~45° from the boss at idle, not bottomed.

   
   

333/W2655 (Seal Kit):

1. Depressurize hydraulics to 0 bar; support the cylinder (dipper/bucket) on timber blocks before disconnecting pins.

   
   

2. Unpin rod-end / head-end, withdraw piston rod from the barrel. Inspect barrel hone & rod chrome — if scored >0.1 mm, re-chrome before reseal.

   
   

3. Strip old seals (rod seal, wipers, O-rings, backup rings) — note the stack order: wiper → rod seal (NBR or PTFE wafer) → O-ring → backup → head O-ring.

   
   

4. Fit new stack from 333/W2655 — PTFE wafer must be pre-heated (80 °C water dip 2 min) and tapped onto the rod mandrel before final seating, or use the plastic installation sleeve if supplied.

   
   

5. Reassemble, torque gland nut to spec (typical 120–150 N·m cross-pattern on large cylinders), reconnect hoses, bleed air at the furthest cylinder port.

   
   

6. Cycle the dipper/bucket 5× full stroke under no-load, re-check for weep at the rod gland after 10 min.

   
   

Quality Notes

• Tensioner bearing: double-sealed, grease-packed for 4000 h / 80 °C continuous; pivot boss forged steel, zinc-coated

  
  

• Seal kit: NBR compound 70 Shore A std., FKM 75 Shore A opt.; PTFE wafer 0.5 mm wall on cylinder rod trims

  
  

• All rubber compounds compatible with ISO HM/HV hydraulic oil & biodegradable ester (HEES) — FKM recommended for ester

  
  

Ordering

Specify 320/08759 (auto tensioner, 3CX/4CX, replaces 320/08651) and/or 333/W2655 (WA seal kit, 3CX/4CX hydraulic cylinder/valve). Provide machine serial number to confirm: 3CX vs. 4CX tensioner boss bolt pattern is shared, but seal-kit trim (cylinder diameter – 100 mm vs. 115 mm rod) varies across sub-revisions — the parts book will call the correct WA variant.

All values nominal; confirm belt routing decal and cylinder diameter (seal stack OD) against your machine's manual before installation. Tensioner pivot bolts are torque-critical — over-torque strips the front-cover thread boss on 3CX/4CX.