Engine Teardown: Manufacturing and Materials Analysis

01The brief

A $650 engine losing share to a functionally similar $250 competitor.

• Client scenario: a Canadian maker of snow-blower engines retailing at $650 faces a competing OHV engine at $250 that is now in most retail snow blowers
• Task: tear down the competitor (Powerfist 6.5 HP), benchmark it against the Honda GX200, and find where cost can come out without losing function
• Scope: the four parts that drive cost and reliability: crankshaft, press-fit gears, crankcase cover and flywheel
• Same method for every part: identify the material, infer the manufacturing route from physical evidence, explain the competitor's rationale, then recommend changes

02Method

Every claim tied to evidence on the part or in the lab data.

• Composition: EDS/XRF data from the metallurgical report (light elements like carbon are not detected, so alloys are matched on Fe, Si, Mn, Cr and Cu)
• Microstructure: etched and unetched optical micrographs at 50x to 500x to read graphite form, phases and heat treatment
• Process forensics: parting lines, flash, sand texture, lathe and hob marks, tapped threads, balancing holes and required tolerances
• Alloy matching: MatWeb composition search at ±0.5 wt%, narrowed with ASM handbook and Aluminum Association designations
• Trade-offs: load checks against the engine's torque, casting process economics at 80,000 units per year, and a Granta EduPack eco-audit

03Crankshaft

Sand-cast grey iron versus forged, heat-treated steel.

• Composition 95.78% Fe, 2.84% Si: silicon above 0.6 wt% rules out steel and falls in the 1 to 3% cast iron range
• Uniform flake graphite in a ferrite-pearlite matrix identifies grey cast iron, closest to ASTM A48; no martensite or case-to-core change, so no heat treatment
• Cast, not forged: sand-textured counterweights, an untrimmed flash seam, undercut geometry and equiaxed grains with no forging flow lines
• Then machined: ground journals with spiral marks and a sharp-edged Woodruff keyway
• Honda by contrast: fine steel microstructure with flow lines and an induction-hardening heat tint, so forged and heat treated
• Load check: the crankshaft sees about 8.2 to 9.1 lb-ft; forged steel (yield 750 to 850 MPa) is far beyond need, while grey iron adds vibration damping
• Recommendation: switch to sand-cast grey iron, removing forging energy, press tooling and heat treatment while meeting the functional requirements

Crankshaft in the crankcase; boxed area shows the casting seam

Flash seam left along the counterweight curvature

Sand-textured counterweight next to the ground journal

Crankshaft micrograph, 50x: uniformly distributed graphite

Crankshaft micrograph, 200x: flake graphite in a ferrite-pearlite matrix

Honda GX200 crankshaft with its press-fit gears

Honda crankshaft micrograph: fine pearlite-ferrite steel, no graphite

04Press-fit gears

Right material for each load, and one machining step to drop.

• Thin speed-regulating gear: ferrite-pearlite, no heat treatment, composition matches AISI 1018
• Thick torque-carrying gear: 0.94% Cr and needle-like tempered martensite, so quenched and tempered AISI 5130
• Inferred route: bar stock, blanking (thin) or hot forging (thick), lathe turning of OD, faces and press-fit bore, involute hobbing (hob marks on the teeth), Q&T for the thick gear, then grinding and deburring
• Recommendation: drop the tooth chamfer on the Honda thick gear to remove a lathe operation
• Recommendation: investment cast the lightly loaded thin gear to cut machining and material waste

Competitor crankshaft: cast counterweights, machined journals and the two press-fit gears (arrows)

Thin gear, 200x etched: ferrite and pearlite, no heat treatment

Thick gear tooth, 500x: needle-like tempered martensite

Hob marks on the thin gear teeth

Thick gear tooth finish

Honda thick gear: the circled tooth chamfer is an extra lathe operation

05Crankcase cover

Permanent mould today; die cast with one fewer CNC setup recommended.

• Function first: seal oil with a flat gasket and rotary seal, locate the crankshaft bearing, resist corrosion; a static gasket face needs Ra ≤ 3.2 µm
• 84.26% Al, 11.98% Si, 1.78% Cu with Si and Al-Fe-Si intermetallics: a hypoeutectic 3xx.x casting alloy; MatWeb narrowed it to 336.0, 339.0, 383.0 or 385.0, and copper content points to AA339.0
• Process evidence: a parting line and dull non-die-cast skin, yet lettering too fine for sand, so gravity permanent mould casting
• Machined after casting: the gasket flange, tapped holes and the bearing bore, since a press fit needs about 0.01 mm, beyond casting tolerance
• Competitor route: cast, trim, two CNC setups, hand tap the off-axis oil ports, deburr
• Casting beats machining from billet: about 10% scrap versus 10 to 60%; at 80,000 engines a year, high-pressure die casting has the lowest cost per part
• Die-cast finish already meets 3.2 µm, so the gasket flange stays as-cast and machining drops to one setup (bearing bore and output flange)
• Alloy change to AA360.0: less copper for corrosion and hot-tear resistance, less silicon for better machinability and tool life

Crankcase cover with its flat gasket and press-fit bearing

Cover micrograph: Si and Al-Fe-Si intermetallics in an aluminum matrix

Parting line on the crankcase cover

Dull as-cast skin beside the machined gasket flange and a post-tapped hole

Fine cast lettering: too sharp for sand, so a permanent mould

Bearing bore: a press fit needs about 0.01 mm, so it is machined

Off-axis oil port threads: cast to size, then hand tapped

06Flywheel

Green-sand grey iron, dynamically balanced; ADI proposed.

• Requirements: high density for rotational inertia, damping for torque ripple, wear resistance and castability
• 94.61% Fe, 2.43% Si, 0.67% Mn, flake graphite in a ferrite-pearlite matrix and a positive magnet test: grey iron, likely ASTM A48 Class 30 (about 7.15 g/cm³)
• Process evidence: parting line and sand texture (green sand casting), concentric lathe marks, a bored centre, a milled keyway and three drilled rim holes from dynamic balancing; no coating
• Mapped the full 10-step route from mould preparation and induction melting to shot blasting, CNC turning, drilling, keyway milling and balancing
• Recommendation: austempered ductile iron (ASTM A897 Grade 3, UTS above 1200 MPa, yield above 800 MPa), keeping sand casting and adding an 800 to 900 °C austenitize and isothermal salt-bath hold
• Higher strength allows thinner sections, keeping rotational inertia at lower mass and reducing load on the crankshaft

Flywheel underside: three drilled holes on the rim from dynamic balancing

Flywheel, unetched 200x: flake graphite

Flywheel, etched 200x: ferrite-pearlite matrix

Flywheel parting line and circular lathe marks

Rough green-sand surface on unmachined areas

Bored centre hole with concentric tool marks

Milled keyway that locks the flywheel to the crankshaft

07Eco-audit

Granta EduPack lifecycle comparison of the original and proposed designs.

• Material phase energy down 12.3%, mainly from casting materials and the ADI change
• Manufacturing energy up 2.3%, from heat treating the thick gear, and more than offset elsewhere
• End-of-life CO₂ offset up 5.6%
• Net: 7.2% lower lifecycle energy with the same functional performance

08Outcome

• Showed the competitor's price gap comes from deliberate choices: casting over forging and skipping secondary treatments where loads allow
• Cost-down package: grey iron crankshaft, investment cast thin gear, die-cast AA360.0 cover with one machining setup, and ADI flywheel
• Every change checked against the part's functional requirement before recommending it
• 7.2% lower lifecycle energy alongside the cost savings