- Safety first
- Wear eye protection, nitrile gloves, and closed-toe shoes.
- Work on a flat, stable surface with good lighting and ventilation.
- Disconnect the negative battery cable before starting.
- Use quality jack stands rated for the car weight; never rely on a hydraulic jack alone.
- If you feel out of depth at any point, stop and hire a professional.

- Job overview (what "main bearings" work is)
- Main bearings support the crankshaft in the block and allow it to rotate on a thin oil film.
- Replacing main bearings means removing the oil pan and lower engine access (or removing the engine), removing main caps, measuring journals and bearing clearances, installing new bearing shells (or undersized bearings) and torquing caps to spec.
- Possible additional work: replacing main bearing bolts (often single-use/Torq-to-yield), rear main seal, oil pump, and possibly crankshaft machining or replacement if journals are scored or out of spec.

- Why you might need to replace main bearings
- Knocking noise from lower engine (rod/main knock).
- Low oil pressure combined with wear.
- Metal in oil or bearing material visible on oil drain magnet.
- Visual inspection shows scored/flattened bearing shells or crankshaft damage.
- If damage is present, not only bearings but crankshaft may require grinding to undersize or replacement.

- Required replacement parts (typical)
- Engine-specific main bearing set (OEM or matched oversize set if crank journals machined).
- Main bearing bolts (if torque-to-yield or manufacturer specifies single-use bolts).
- Rear main seal.
- Oil pan gasket (and possibly oil pan if damaged).
- Assembly lube (engine assembly grease).
- Engine oil and oil filter.
- Gaskets/sealant for any removed components (timing cover, front crank seal if removed).
- Optional/conditional: crankshaft (if journals damaged beyond repair) or crankshaft grinding to undersize plus matched undersize bearings; new oil pump if worn.

- Basic tools you likely already have (and how to use them)
- Ratchet and socket set (metric): for removing bolts and nuts. Use the correct socket size to avoid rounding fasteners; pull straight — don’t use cheater bars unless socket is rated.
- Combination wrenches (metric): for tight spaces where sockets don’t fit; hold the head of a bolt while you turn the nut.
- Screwdrivers (flat and Phillips): for clips and prying small parts carefully.
- Pliers (slip-joint, needle-nose): for hose clamps and small components.
- Torque wrench (click-style, metric range covering at least 10–150 Nm): sets bolts to correct torque; always torque in proper sequence and re-check after initial torque if manual instructs.
- Drain pan: to catch oil and coolant.
- Jack and jack stands: to lift and safely support the car.
- Shop rags and brake cleaner (or parts cleaner): for cleaning surfaces and metal shavings.
- Oil filter wrench: to remove oil filter.

- Additional/advanced tools required (why they’re required and how to use them)
- Engine hoist/crane (if removing engine): required if you decide or need to remove the engine for easier access. Use rated hoist, chain to lifting points, lift slowly and squarely.
- Engine stand (if engine removed): supports engine for rotation and access; mount with proper bolts into block.
- Torque angle gauge (if bolts are torque-to-angle): used with torque wrench to achieve specified angle on top of torque value; attach to bolt head and rotate specified degrees.
- Dial caliper / micrometer (outside micrometer): measure crankshaft journal diameters to determine if journal wear is within spec. Use by zeroing calibrator and measuring across journal at multiple points.
- Plastigauge: inexpensive way to measure oil clearance between bearing and journal. Place a strip of plastigauge on crank journal, install bearing cap and torque to spec, remove cap and measure flattened width against scale to determine clearance.
- Dial bore gauge (or inside micrometer): measures bearing bore inside main caps to check roundness and size if needed; used after removing caps with piston in place on stand or engine out.
- Feeler gauges: for measuring small gaps and checking thrust clearance if needed.
- Harmonic balancer puller / crankshaft pulley puller: to remove the crankshaft pulley if it obstructs access to front seal or timing cover.
- Bearing installation tools (plastic drift or wooden dowel / soft-faced hammer): to seat bearings or guides without damaging them.
- Seal puller and installer: to remove and install rear main seal without damage.
- Impact wrench (optional): speeds fastener removal but be cautious—don’t use for final torquing.

- How to use key tools (concise usage)
- Torque wrench: set to required Nm, snug bolt, then apply smooth steady pull until wrench clicks. Tighten in specified sequence and recheck per manual.
- Plastigauge: cut a 2–3 mm wide strip, lay across journal, install bearing shell and cap, torque to spec once, remove cap, measure width against gauge chart; thicker flattened width = smaller clearance. Do not reuse plastigauge.
- Micrometer: zero it, close on journal, read measurement on scale. Measure journal at multiple axial and rotational positions to detect taper or ovality.
- Dial bore gauge: set to a known size (using a micrometer), insert into bearing bore, rock to find minima reading, read dial for bore size. Used to detect out-of-round or oversize bores.
- Engine hoist: attach chain to proper lifting points, lift gradually, ensure balanced load and no shifting; lower onto engine stand.
- Seal installer: use correct diameter tool to tap seal evenly into bore without deforming sealing lip.

- Step-by-step high-level procedure (not exhaustive but covers essential stages) — follow manufacturer manual for torque specs, sequences, and clearances
- Prepare
- Drain engine oil and remove oil pan and oil pickup tube.
- Remove ancillary parts blocking lower access (exhaust headers as needed, starter, transmission if necessary — some engines allow pan access with transmission in place).
- Remove oil pump if it obstructs access (note orientation and timing marks).
- Expose mains
- Clean workspace around caps to prevent debris entering the block.
- Mark cap orientation and location (each cap is fitted to a specific position/number and direction).
- Loosen and remove main cap bolts in reverse of tightening sequence to avoid distortion.
- Remove main caps carefully; keep them in order and orientation.
- Inspect and measure
- Visually inspect bearing shells for scoring, embedded metal, discoloration.
- Remove bearing shells and inspect crank journals for grooves, discoloration, and roundness.
- Measure journal diameters with micrometer and bearing bores with dial bore gauge.
- Measure bearing clearances with plastigauge when new bearing shells are trial-fitted — use OEM bearing halves and torque caps to spec, then remove and read plastigauge.
- Compare results to service manual. Typical small-4-cylinder clearances are on the order of thousandths of an inch (check manual for exact mazda specs).
- Decide repair path
- If clearances are within spec and shells only worn, replace bearings with new OEM shells and reassemble.
- If clearance excessive but journals OK, an undersize bearing set (e.g., +0.25 mm) may be required (paired with crankshaft undersize grind if journals machined).
- If journals are scored/tapered/oval beyond limits, crankshaft machining (grinding and polishing to undersize) or crankshaft replacement is needed.
- Replace main bolts if specified single-use and always replace rear main seal and oil pan gasket when reassembling.
- Reassembly
- Clean all mating surfaces thoroughly; remove any debris.
- Apply assembly lube on bearing surfaces; install new bearing shells into caps and block.
- Install main caps in the original order and orientation.
- Torque main cap bolts in the sequence and to the torque (and/or angle) specified by Mazda.
- Reinstall oil pickup, oil pump, oil pan with new gasket/sealant, and remaining components.
- Refill engine with fresh oil and a new filter.
- Prime the oil system before first start (crank engine with fuel/ignition disabled or use priming procedure) so bearings get oil before running under load.
- Break-in and test
- Start engine and monitor oil pressure and for leaks.
- Listen for abnormal noises; run at moderate RPM for a short period and recheck torque where required by manual.
- Change oil and filter after recommended break-in mileage.

- Important tolerances and replacements to note
- Always get Mazda factory service manual specs for your model/year/engine — clearances and torque values vary by engine (e.g., 2.0 vs 2.5 vs older engines).
- Main bearing shells are typically engine-specific and must match block and crank journal sizes.
- Main bolts may be torque-to-yield (single-use) — replace if required.
- If crankshaft journals require machining, you must get correct undersize bearings to match (e.g., 0.25mm, 0.50mm oversize bearings) — if journals cannot be machined, replace crankshaft.

- Common pitfalls and warnings (short)
- Do not reuse bearing shells; always replace with new matched set.
- Do not ignore crankshaft journal condition — new bearings on a damaged journal will fail quickly.
- Incorrect torque or sequence on main caps can warp the crank and destroy the engine.
- Plastigauge is accurate for clearance checks but don’t over-torque or reuse it.

- Estimated difficulty and time
- Difficulty: high for a beginner. Requires precision measuring, careful cleaning, and correct torquing. Mistakes can lead to engine failure.
- Time: many hours to days depending on experience and whether the engine is removed.

- Final recommendation
- If you have only basic hand tools, expect to acquire at minimum: a good torque wrench, plastigauge, micrometer/dial caliper, and the correct bearing set and gaskets. For anything beyond replacing lightly worn bearings, consult a machine shop or professional mechanic.

(Do not proceed without the Mazda service manual for your exact engine — it contains the exact torque specs, clearance limits, and sequences that are critical for a successful repair.)
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