Tools & supplies
- Metric socket/ratchet set, extensions, swivel joints
- Breaker bar, breaker bar socket for crank pulley
- Torque wrench (0–200 ft·lb) and smaller torque wrench for lower values
- Engine hoist or engine support bar (depending on whether engine is lifted)
- Engine stand
- Harmonic balancer / crank pulley puller and installer
- Flywheel / crankshaft holding tool
- Harmonic damper holding tool (or equivalent)
- Plastic gauge (Plastigage) and feeler calipers/ruler to read it
- Micrometer (0–3" / 0–75 mm), inside micrometer or dial bore gauge for journal and bore measurements
- Straightedge and dial indicator (for runout)
- Seal driver or appropriate size socket for rear main seal
- Engine assembly lube
- Clean rags, parts cleaner, shop vacuum
- Gasket set, new main bearings, rod bearings, thrust bearing, rear main seal, oil pump pick-up gasket, oil pump (recommended), crankshaft bolt(s) (replace if required), new timing components as applicable, oil, coolant
- Replacement fasteners if torque-to-yield (check manual)
- Plastics bags and markers to tag bolts/parts
- Jack and jackstands, wheel chocks
- Service manual / torque specs & diagrams for your Mazda3 engine variant

Safety first
- Work on a level surface, block wheels, set parking brake.
- Disconnect negative battery terminal.
- Use jackstands; never rely on a jack.
- Wear eye protection, gloves; ventilate if using solvents.
- If removing the engine, use a properly rated engine hoist and follow safe lifting procedures.
- Keep the work area clean; metal debris in oil passages will ruin a crankshaft or bearings.

Notes before starting
- “Crankshaft job” usually means remove, inspect/measure, and replace or refinish as needed, plus replace bearings/seals and related hardware. This is an engine-out or at least engine-lift job on many Mazda3 engines; check your year/engine service manual for specific steps and torque specs.
- This guide is generic for Mazda MZR / Skyactiv 4‑cyl layouts; always verify clearances and torque values from factory documentation.

Step-by-step overview (remove, inspect, replace, reinstall)

1) Prep & disassembly (engine in car unless you plan engine-out)
- Drain engine oil and coolant.
- Remove intake tubing, air box, throttle body as needed for access.
- Remove accessory drive items (alternator, power steering pump bracket, A/C compressor — hang out of the way if you keep hoses connected).
- Remove engine covers and any undercovers to access oil pan and crank pulley.
- Remove the serpentine belt.
- Remove crankshaft pulley/harmonic balancer: use a holding tool on the flywheel or a crank pulley holding tool, then remove the center bolt. Use harmonic balancer puller to extract the damper—do not hammer on the face.
- Remove front timing cover and timing components (belt or chain). If timing belt, align timing marks, then remove belt and cam gear. If timing chain, follow chain removal per manual (often requires camshaft caps removal or chain tensioner release).
- Support engine from above with an engine support bar or prepare to lift engine with hoist if needed. For many Mazda3s you can remove subframe/crossmember and lower engine slightly for access; follow manual.
- Remove oil pan: unbolt and gently pry. Clean mating surfaces.
- Remove oil pump and pickup assembly if it interferes with main cap removal.
- Remove flywheel/flexplate if interfering with rear main cap access (may require removing transmission or using access plate).
- Remove the front timing cover, and if needed the rear main cap access (on some engines the rear main seal is accessed with the engine out).

2) Removing main caps & crankshaft
- Mark main caps and their orientation/position (numbered and face towards front of engine). Keep caps in order.
- Loosen main cap bolts in sequence gradually (reverse of torque sequence) to avoid warping. Remove bolts and caps.
- Carefully lift the crankshaft out straight—have an assistant or use a small hoist. Keep the journals clean; place crank on padded surface or engine stand cradle.
- Inspect bearing shells for scoring, metal contamination, and color (overheating signs).

3) Measurements & inspection
- Clean crankshaft thoroughly with solvent. Inspect journals for grooves, scoring, pitting.
- Measure main and rod journal diameters with micrometer at multiple positions around circumference and along journal length.
- Check runout with a dial indicator mounted to a fixed point on block or on engine stand; rotate crank and read runout. Compare to factory tolerances.
- Check bores in block for roundness with a dial bore gauge.
- If journals are within spec, you can reuse crankshaft; if out of spec, crank must be ground (undersize) or replaced. If ground, you must use matching undersized bearings and replace main and rod bearings.
- Measure thrust bearing wear and endplay with dial indicator and compare to spec.

4) Replacements required (recommended)
- Always replace main bearings, rod bearings, thrust bearing and rear main seal when removing crank.
- Replace main cap bolts and rod bolts if they are torque-to-yield (check manual) or show damage.
- Replace oil pump and pick-up gasket if contaminated/damaged.
- Replace crankshaft pulley bolt (often single-use) and any small rubberized parts on harmonic balancer.
- Consider replacing timing belt/chain tensioner, water pump, and other front-end components while accessible.
- If crankshaft journals are damaged beyond spec, replace or professionally grind the crank and fit matching bearings.

5) Installing crankshaft
- Clean block oil holes and passages with solvent and compressed air (keep hands/eyes protected); do not let debris enter.
- Prime oil pump with fresh oil.
- Install main bearings into block and caps—ensure tabs align and oil grooves are oriented correctly. Apply a film of assembly lube.
- Set crank into place carefully.
- Install main caps in original order and orientation. Hand-start bolts.
- Torque bolts in correct sequence and in steps to final torque using the service manual sequence. If bolts are stretch/T-Y, replace with new and follow tightening procedure (angle/tightening steps).
- Check crank endplay/thrust clearance with dial indicator; if out of spec, address thrust bearing fit.
- If using Plastigage for clearance check: place a strip on journal, install cap and torque to spec, remove cap and measure width of crushed plastigage, compare to clearance table. Repeat per journal as needed.
- If clearances are correct, remove plastigage, clean thoroughly, apply assembly lube and reassemble permanently with proper torque.

6) Reassembly of remaining components
- Reinstall oil pump, pickup, new gaskets.
- Install new rear main seal with seal driver, ensuring flush and not cocked.
- Reinstall oil pan with new gasket or RTV as specified and torque pan bolts to spec.
- Reinstall timing components, ensuring correct timing alignment. Replace belt/chain if removed.
- Reinstall harmonic balancer using installer tool; torque crankshaft pulley bolt to spec (use holding tool).
- Reinstall accessories, intake/exhaust components removed.
- Reinstall engine mounts/subframe if removed, using proper torque.
- Refill oil and coolant, prime oil system (crank engine without fuel to build oil pressure if possible), check for leaks.
- Run engine to temperature, re-torque manifold/head bolts if required by service manual after run-in, check oil pressure and listen for abnormal noises.

How to use key tools (short, specific)
- Harmonic balancer puller/installer: thread puller bolts into damper evenly; pull straight to avoid damaging snout. For install, use installer to press damper on straight to proper depth—do not hammer.
- Flywheel/crank holding tool: engages flywheel or damper teeth to prevent turning while loosening/torquing main pulley bolt.
- Torque wrench: use correct drive size and calibrated wrench; torque in stages and sequence as specified. For angle torques, use an angle gauge.
- Plastigage: cut piece slightly shorter than journal width, place on journal, install cap and torque to spec, remove cap and compare flattened width to scale to determine clearance—do not rotate crank while plastigage is installed.
- Micrometer / dial bore gauge: measure journals and bores at multiple axial and circumferential positions to check for taper and out-of-round.

Common pitfalls to avoid
- Not replacing bearings/seals: reuse increases risk of early failure.
- Not cleaning oil passages: grit causes bearing & journal failure.
- Skipping journal and block measurements: fitting clearances are critical.
- Improper torque sequence or reusing torque-to-yield bolts: causes cap distortion and oil clearance changes.
- Using impact tools to remove/install harmonic balancer or bolts without holding crank—risk of damage.
- Installing main caps out of order or flipped: always keep caps matched and oriented.
- Forgetting to prime the oil pump or turning engine over dry—immediate bearing damage.
- Damaging the rear main seal lip on install—use correct driver and lubricant.
- Not replacing related timing components when accessible—future failure will force repeat labor.
- Not checking crank runout—an out-of-spec crank will create vibration and bearing wear even if journals measure okay.

Final checks & break-in
- Verify torque values against the factory manual.
- Start engine, monitor oil pressure, look for leaks, and listen for knocking.
- Change oil and filter after initial break‑in interval (per manual) if new bearings or shafts were installed.

Summary
Removing and replacing a crankshaft is a major job: it requires correct measurement tools, bearing replacement, attention to cleanliness, and strict adherence to torque specs and sequences. If journals are damaged beyond factory tolerances, you must have the crank reground or replace it and fit matching undersize bearings. Consult the specific Mazda3 engine service manual for exact torque figures and clearances before proceeding.
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Quick overview
- The connecting rod (conrod) links the piston to the crankshaft. It converts the reciprocating motion of the piston into the rotating motion of the crank. If a conrod or its bearings fail, the engine will make knocking noises, lose oil pressure, and often suffer catastrophic damage.
- Replacing or repairing conrods is a major internal engine job: you will open the bottom and/or top of the engine, remove pistons or the whole engine, measure clearances, replace bearings and/or rods, inspect or machine the crank, then reassemble to factory specs. Expect multiple days of work and possible machine-shop work.

Analogy
- Think of the piston as your hand that pushes, the conrod as your forearm, and the crank journal as your elbow. Bearings are like thin socks between the arm and elbow so the arm can pivot smoothly. If the socks wear away or the elbow is damaged, the arm bangs on bone (rod knock), and things quickly go badly.

Detailed description of every component you’ll deal with
- Piston: the cylinder-side component that the combustion force pushes on. Usually has piston rings and a wrist pin (gudgeon pin).
- Piston rings: seal combustion and control oil. Often removed/inspected when doing rod work.
- Wrist pin (gudgeon pin): the pin that connects piston to the small end of the rod. Can be floating (retained by circlips) or pressed.
- Small end of rod: the top end that connects to the wrist pin; may have a bushing or be plain.
- Connecting rod body: forged or cast steel/aluminum piece transmitting force.
- Rod cap: the bottom half of the big end of the rod, bolted to the rod body. The cap and rod are matched and must remain with their original position unless replacing as matched sets.
- Rod bearings (big-end bearings): two thin shell halves (upper/lower) that sit between rod/rod cap and the crank journal. They are replaceable wear items.
- Rod bolts/studs and nuts: fasteners holding cap to rod. Many are torque-to-yield (single use) and must be replaced.
- Crankshaft journal (big journal): the round shaft surface the rod bearings ride on; precise diameter and hard finish.
- Oil passages and holes: supply pressurized oil to rod bearings; block and rod have holes and grooves.
- Block (cylinder block) lower area / main bearings: support the crank; you’ll remove oil pan and possibly the lower main caps.
- Oil pump and pickup screen: supply oil to bearings; must be clear and in good condition.

Why this repair is needed (theory)
- Bearings provide a minute oil film between the rod and journal. That hydrodynamic film prevents metal-to-metal contact. If oil supply is inadequate, bearings overheat and wear, reducing clearance and then allowing metal contact — causing a “rod knock.”
- Causes of rod/ring/piston failures: low oil pressure, dirty oil/contamination, bearing fatigue from high mileage, detonation, hydrolock (water in cylinder), thrown timing components, or high RPM abuse.
- If bearing wear progresses, the bearing shells can collapse or get chewed away, the rod can score or bend, the crank journal can be damaged — often requiring crank grinding or replacement.

Symptoms that indicate rod/rod-bearing problems
- Persistent deep knock from the engine that changes with rpm or load (often louder at idle and decreases with higher rpm or vice versa depending on failure).
- Sudden low oil pressure.
- Metal shavings in oil filter or on drain plug.
- Engine will not rev properly, misfires, or runs rough; in catastrophic cases, seized engine or hole in the block.
- Excessive crankcase noise, vibration, or smoking if oil is burning.

Tools, parts, and supplies you’ll need (basics)
- Factory service manual for your Mazda3’s specific engine (essential for torque specs, sequences, and clearances).
- Engine hoist (if removing engine).
- Engine stand (if working with the engine out).
- Basic hand tools: metric sockets and wrenches, breaker bar.
- Torque wrench (accurate range covering rod bolt torque).
- Feeler gauges, micrometers (0–100 mm), dial bore gauge, and/or inside mic for journal diameter and bores.
- Plastigage (useful if you don’t have gauges to check bearing clearance).
- Piston ring compressor, hammer, assembly lube.
- Bearing sets (new rod bearings), new rod bolts (strongly recommended), new wrist pin clips if applicable.
- Clean rags, solvent, oil, gasket set, new oil and filter.
- Shop manual, service data.
- Optional: micrometer for rod bolts (for measuring stretch), torque-angle gauge if bolts are torque-to-yield.
- Machine shop access (for crank grinding, line-boring, rod alignment) if journals or bores are out of spec.

High-level procedure (general workflow — adapt to Mazda3 engine in manual)
Note: This is a general guide. Consult the factory manual and obey torque/clearance specs exactly.

1) Preparation and safety
- Work on a clean level surface. Disconnect battery.
- Drain oil and coolant (if you remove the engine).
- Label everything and take photos for reassembly.
- Remove intake/exhaust/mount components as needed to access head/oil pan.

2) Decide access method
- Two common approaches:
a) Engine out: remove engine from car, put on an engine stand, disassemble. Gives best access.
b) Engine in car, but remove cylinder head so pistons can be pushed up and out through top, and access rod caps via oil pan removal. On many Mazda3 engines you must remove the head to get pistons out; check manual.

3) Disassembly basics
- Remove oil pan and oil pickup/screen.
- Remove rod bearing caps: mark each rod and cap with cylinder number and orientation. Caps are matched to rods; keep their orientation.
- With caps off, you can rotate crank to bring each piston to bottom to remove wrist pin or, if head removed, push piston up and out of bore.
- Remove pistons from rods (retaining clips if present) or remove entire piston/rod assemblies together if replacing rods only.

4) Inspection & measurement
- Inspect rod bearings: look for scoring, smearing, discoloration (overheating), embedded particles.
- Check crank journals for scoring or out-of-round. Measure journal diameter with micrometer at multiple positions and compare to spec.
- Measure bearing clearances: either with plastigage or by micrometers and bore gauges. Plastigage: place a strip on journal, install bearing cap torque to spec, then remove to measure flattened width against plastigage chart. Typical gasoline engine clearances are small (hundreds of thousandths of an inch ≈ 0.02–0.09 mm). Use factory spec.
- Check rod big-end bore and cap mating surfaces — measure roundness. Rods can be stretched or distorted; check for runout.
- Check wrist pins and piston small ends for wear or scoring.
- Inspect block bores and cylinder walls.
- Check oil pump pressure and pickup screen for debris.

5) Decide repairs based on findings
- Worn bearings only: replace bearing shells, rod bolts if specified, clean everything, reassemble to spec.
- Damaged crank journals: require machine shop to grind journals undersize and fit oversized bearings or regrind and replace crank.
- Distorted rods or scored rod bores: may need rod replacement or line-boring.
- Bent or cracked rods: replace the rod (use OEM or high-quality aftermarket; ensure correct weight/balance if replacing one rod).

6) Reassembly basics
- Clean all mating surfaces thoroughly. Debris kills bearings.
- Install new bearing shells into rods and caps; align oil holes and tangs.
- If using new rod bolts, lubricate per manual (some require oil/loctite).
- Use assembly lube on bearing surface.
- With piston/rod in bore (if installing from top), align piston orientation mark toward front of engine as specified.
- Torque rod bolts in stages to specified torque/angle. If bolts are torque-to-yield (TTY), use new bolts and follow angle spec; do not reuse.
- After torquing, rotate engine by hand several full rotations to seat bearings; check for binding.
- Reinstall oil pickup, pan, filter, fill with oil, prime oiling system if possible.
- If crank was machined, balance rods/pistons as needed. Replacing one rod may require balancing the set.

7) Break-in and testing
- Prime oil system before first start (turn engine with starter disabled to build oil pressure for a few seconds).
- Start engine and monitor oil pressure and noises. Keep revs low for initial break-in.
- Change oil and filter after short first-run interval (consult manual or machine-shop advice) because metal particles from reassembly may appear.

Specific measurement and tolerance notes (guidance)
- Bearing clearance: small gasoline engines commonly have bearing clearances roughly 0.0015–0.0035 in (0.04–0.09 mm), but you must use Mazda factory specs for your engine.
- Rod bolt replacement: many modern rods use TTY bolts—replace them. If using reusable bolts (ARP aftermarket), follow torque specs.
- Plastigage is okay for a quick check but not a substitute for proper micrometers if you suspect marginal clearances.

Common things that go wrong (and how to avoid them)
- Reusing rod bolts that are torque-to-yield — they can fail. Always follow manual and replace if required.
- Incorrect bearing orientation or installing wrong-size shells — bearings have oil-hole/notch orientation; wrong install leads to oil starvation.
- Dirt/metal debris in oil passages — thoroughly clean; change oil and filter early after reassembly.
- Not matching rod caps to rods or mixing up orientation — mark and keep matched parts.
- Incorrect torque or skipped torque steps — use torque wrench and follow sequence.
- Not checking crank condition — installing new bearings on a damaged journal guarantees early failure.
- Incorrect piston orientation or ring gaps — lead to loss of compression or piston damage.
- Failing to prime oiling system before first start — immediate bearing failure can occur.

When you need a machine shop
- If crank journals are scored, out-of-round, or undersize beyond bearing limits.
- If rods are bent or caps/bores are misaligned and require line-boring/honing.
- If you need undersize bearings after grinding the crank.
- For balancing assemblies if you replace pistons/rods and need to maintain engine balance.

Practical tips for a beginner mechanic
- Get the factory service manual for your Mazda3 engine model — it contains exact torque specs, sequences, clearances, and special procedures.
- Keep everything extremely clean. Bearings and journals survive only in clean oil; dirt will kill them quickly.
- Label every rod and cap and reassemble in original locations unless replacing matched sets.
- If this is your first internal engine job, consider removing the engine and working on an engine stand rather than contorting under the car.
- When in doubt, get crank/rod work done by a reputable machine shop — they have the measuring tools and experience.
- Budget for surprises: if journals are damaged, costs go up quickly (grinding, new bearings, possible crank replacement).

Final words
- Replacing or repairing connecting rods is not a minor job: it requires careful measurement, correct parts, and attention to cleanliness and torque procedures. If done incorrectly, the result is rapid re-failure and often catastrophic engine damage. Use the Mazda factory manual for all specs and consider machine-shop help for any journal/rod misfit.
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