Ordered, theory-heavy procedure for replacing a propeller-shaft strut (stern strut/cutless-bearing housing) on a small marine installation driven by Hino W04-series engines. I assume “strut” means the shaft support (cutless bearing) commonly fitted inboard of the propeller – if you meant something else (engine mount / shock strut), the same theory applies but the components differ.

1) Fault, root causes and what the repair must achieve (theory up front)
- Symptom clusters: shaft vibration/noise at speed, heat/smell from stern tube area, increased propulsion shaft runout, water ingress at stern gland/coupling, abnormal gearbox loads or thrust bearing wear.
- Typical root causes: worn/deteriorated cutless bearing (loss of rubber, grooving), fretting/corrosion or distorted strut flange, misalignment (shaft sag or axial displacement), cutless extrusion or foreign-object damage, galvanic corrosion weakening bolts/hull seating.
- What replacement must do: restore correct radial clearance/support of the shaft so the shaft line is straight and concentric through the bearing; restore secure hull attachment and sealing; transmit radial loads and allow correct axial thrust flow to the hull/engine thrust bearing; eliminate excessive friction, vibration and water intrusion. Replacement is not just swapping metal — you must restore geometry, material condition and mounting stiffness.

2) Preparations (theory and intent)
- Plan access and lifting: support shaft and propeller to remove load from bearing (you must maintain shaft axial position while removing the strut to avoid changing coupling alignment).
- Tools/consumables: engine/gearbox alignment tools (dial indicators, feeler gauges), shaft supports/cribbing, flange-spreader, jack, new strut and cutless bearing (or replacement liner), stainless fasteners or correct-grade replacement studs, anti-seize/Loctite per spec, bedding compound/sealant for flange faces, anode(s), torque wrench, torque sequence chart, hull paint/anti-foul for exposed metal.
- Safety: isolate engine, immobilise shaft, tag controls, ensure hull supports and lifting equipment rated for load.

3) Preliminary inspections and measurements (why before removal)
- Measure present shaft runout and bearing wear pattern with dial indicator while slowly rotating shaft (establish baseline). Theory: this tells you where wear has occurred, whether the shaft was rubbing the bearing wall, and the magnitude of eccentricity you must correct.
- Measure coupling face-to-face and take marks/photographs for reassembly. Theory: you must preserve coupling angular and axial relationship to ensure gearbox alignment on reassembly.
- Inspect outer flange faces, bolt condition and hull seating for corrosion or distortion. Theory: a bent flange or uneven seat will reintroduce misalignment after fitment.

4) Supporting the shaft and removing axial/radial loads
- Support the propeller shaft at multiple points (temporary bearings or jack supports) so it does not drop or shift when the strut is removed. Theory: cutless bearings usually carry a significant radial load; removing them without support changes shaft deflection and ruins alignment.
- Remove coupling halves or separate propeller coupling only as needed, keeping marks and maintaining axial position with scribed reference lines.

5) Dismantling the old strut (ordered)
- Disconnect any anodes, piping, or water-cooling lines attached.
- Loosen and remove flange bolts in an even star pattern. Theory: avoids distortion of the flange.
- Gently separate strut from hull. If bonded/grouted, use flange spreaders or heat per specification; do not pry on hull plating. Remove strut assembly. Keep hardware and note any seized bolts or fractured studs (indication of corrosion).

6) Inspection of shaft and mating surfaces (theory why critical)
- Check shaft journal where it passed through the bearing for scoring, collar grooves, diameter wear; check for fretting corrosion. Theory: a damaged shaft journal requires repair (machining or sleeve) before new bearing is fitted or the new bearing will fail quickly.
- Inspect hull seat and flange face flatness. If hull seat is distorted or corroded, re-bedding or steel patching/grouting may be required to achieve a true flange seat.
- Check prop shaft straightness (runout). If straightness is outside allowable range, address shaft before fitting strut.

7) Preparing and fitting the new cutless liner/strut (theory and steps)
- If the new strut is supplied with a replaceable cutless liner, press-fit or glue the liner per supplier instructions. Theory: correct interference fit secures the liner so it does not extrude under load and maintains concentricity.
- Fit any required thrust washers or sleeve bearings exactly per orientation marks. Theory: liners are often split and must be installed so the split gap is staggered relative to any other split bearings to avoid leakage/extrusion paths.
- Apply appropriate marine-approved sealant or bedding compound to the strut flange or hull seat only as specified — the flange must be able to be torqued flat. Theory: prevents water ingress between flange and hull and establishes a sealed surface; excessive compound can prevent proper seating.

8) Mounting and aligning the strut (the critical theory-heavy part)
- Bring the strut up to the hull and engage bolts hand-tight in star pattern. Do not fully torque yet.
- With shaft still supported, slowly lower shaft into the new cutless bearing by adjusting supports so the shaft sits on nominal running height. Theory: the shaft must rest in the bearing under gravity/operational load to set the correct clearance and alignment — if you bolt the strut without the shaft loaded, you may trap the shaft off-center.
- Use dial indicators to check radial runout at multiple points along the shaft (near coupling and forward of the strut). Adjust shims or packing under the strut flange if the shaft is eccentric. Theory: shimming changes the strut position relative to hull and thereby moves bearing centre; the aim is concentricity of shaft and bearing centerlines and minimal runout across coupling faces.
- Iteratively tighten bolts to progressive torque in star sequence while monitoring runout. Theory: tightening changes flange twist; progressive, even torque prevents introducing misalignment.
- Final torque bolts to manufacturer/hull spec; if studs were replaced, use appropriate anti-seize/lubricant and torque values. Theory: consistent clamping force secures the flange, prevents movement under load and maintains alignment.

9) Coupling and axial alignment checks
- Reassemble coupling and check overall shaft alignment (axial and angular) between gearbox and propeller shaft using dial indicators or alignment tools; correct any misalignment by moving engine/gearbox or adjusting shaft supports per normal engine alignment practice. Theory: the strut fixes radial support; the engine/gearbox must still be aligned to prevent undue coupling loads. Some slight axial movement of the shaft may be required by the manufacturer's design — confirm thrust load path.
- Check axial clearance in bearing if specified (some cutless bearings allow slight axial play).

10) Final assembly and commissioning
- Fit new anode(s), paint/anti-foul exposed metal, reconnect any sea-cock lines or water cooling associated with strut.
- Run slow turning test in dry dock: manually rotate shaft through bearings to ensure free rotation, no scraping, no binding; measure temperature rise after short run-in if feasible. Theory: confirms bearing seating and checks for rubbing before powering the engine.
- Sea trial: run up through operating rpm and check vibration levels, noise, bearing temperature, and any leaks. Monitor thrust bearing and gearbox loads for abnormalities.

11) How each step fixes the fault (explicit mapping)
- Removing and replacing a worn cutless liner restores uniform rubber support around the shaft journal. Effect: reduces eccentric loading and vibration, reduces shaft deflection and resulting gearbox/thrust-bearing overload.
- Correct shaft support during removal and installation prevents axial/ radial shifts that would otherwise create misalignment and premature wear. Effect: preserves coupling geometry so engine and gearbox loads remain as designed.
- Shimming and incremental torque alignment restores concentricity between shaft centerline and bearing centerline. Effect: minimizes runout, reduces vibration and propeller loading, prevents accelerated wear of rubber and metal components.
- Replacing corroded flange bolts, re-bedding the flange and fitting anodes reduces galvanic corrosion and weak fasteners, preventing movement and subsequent misalignment and leaks.
- Checking and repairing shaft journal damage prevents point-contact wear zones that would quickly abrade a new liner; a smooth, correct-diameter journal yields a uniform hydrodynamic film and longer bearing life.
- Properly installed cutless rubber liner (correct interference, staggered split) prevents extrusion and sea water pathways that cause leakage and loss of bearing support.

12) Tolerances, materials and practical notes (theory guidance — check manufacturer)
- Target runout/concentricity: as low as practically achievable; many small craft aim for under 0.25–0.5 mm total indicator reading at coupling faces — consult class or builder for exact tolerances.
- Bearing clearance: depends on bearing type and shaft diameter; follow cutless liner manufacturer instructions. Excess clearance increases vibration; too tight increases heating and wear.
- Fastener practice: replace seriously corroded studs/bolts; use correct-grade stainless or high-tensile bolts per spec and apply anti-seize or thread-lock as recommended.
- Break-in: new cutless bearings typically need a short running period with inspection for heat and leakage; avoid full-load trials before break-in.

13) Common pitfalls to avoid (short)
- Removing strut without supporting shaft — causes permanent misalignment.
- Bolting strut down with shaft unloaded — traps shaft off-center.
- Reusing badly worn shafts without machining — causes premature bearing failure.
- Overtightening bedding compound interfering with flange seating.
- Ignoring coupling marks — reassembly with wrong phase causes vibration.

14) Outcome verification (what to measure after repair)
- Static and dynamic shaft runout reduced to acceptable tolerance.
- Bearing temperature within normal range during sea trial.
- Vibration levels at helm/engine within acceptable limits.
- No water ingress at new flange or through bearing, and anodes functioning.

This ordered approach links each mechanical action with the physical reason it’s necessary. Follow component manufacturers’ specs (cutless liner, fasteners, torque) and class rules for final tolerances.
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