SD-240 R-Line truck engine
early/mid-1950s108 hp gross / 93 hp net in 1953 manual
Longer-stroke Silver Diamond six
TorqueList Original Technical Art
International Harvester SD-240 Six
Inline6 · N/A
International Harvester · Silver Diamond Inline-Six
International Harvester SD-240 Six
The longer-stroke Silver Diamond six that added truck torque without increasing bore.
240.3
Cubic inches
The SD-240 kept the SD-220 bore but increased stroke for greater displacement, making it a natural truck-focused upgrade in International's postwar lineup.
Key variants
All variantsSD-240 R-Line truck engine
Displacement
240.3 ci
Bore
3.5625"
Stroke
4.018"
Years
1949–1950s core Silver Diamond era
Key Takeaway
The SD-240 kept the SD-220’s 3.5625-inch bore but increased stroke to roughly 4.018 inches. That change raised displacement and torque while preserving the same simple OHV inline-six architecture and commercial-duty focus.
Technical Specifications
- Compression
- 6.5:1 in the 1953 R-Line factory service manual; later 1955 specifications are commonly listed around 7.0:1
- Horsepower
- 108 hp maximum in early factory service data; later documented 1955 ratings reached 131 hp
- Torque
- 191.9 lb-ft @ 1400 rpm maximum in the R-Line factory manual; 186 lb-ft net @ 1100 rpm
- Architecture
- OHV inline six
- Block
- Cast iron
- Cylinder heads
- Cast iron
- Valvetrain
- Pushrod OHV
- Aspiration
- Naturally aspirated
- Fuel delivery
- Carbureted
- Firing order
- 1-5-3-6-2-4
- Main bearings
- 4
The SD-240 kept the SD-220’s 3.5625-inch bore but increased stroke to roughly 4.018 inches. That change raised displacement and torque while preserving the same simple OHV inline-six architecture and commercial-duty focus.
Internal Specifications
Variant-specific values are labeled. Engine families that span multiple years or RPOs can have different heads, cams, induction and rotating parts, so the exact code/year always takes priority.
Identification & Output
- Engine codes / RPO
- SD-240 (Silver Diamond 240)
- Casting numbers
- No single universal SD-240 casting is retained here; verify SD-240 stamping, 3.5625 x approximately 4.016-inch bore/stroke, casting/date and truck application.
- Power rating
- Factory manual lists both maximum brake horsepower and net brake horsepower
- Horsepower RPM
- 108 hp @ 3600 rpm maximum; 93 hp @ 3400 rpm net in the 1953 R-Line factory manual
- Torque RPM
- 191.9 lb-ft @ 1400 rpm maximum; 186 lb-ft @ 1100 rpm net
Operating Specs
- Production notes
- The SD-240 retained the SD-220’s 3.5625-inch bore and increased stroke to roughly 4.016 inches for 240.3 cid. Factory R-Line data lists 108 maximum brake hp and nearly 192 lb-ft of peak torque.
Compatibility Notes
- SD-220 and SD-240 share family architecture, but crankshaft/stroke, pistons and application tuning differ; displacement should not be inferred from external appearance alone.
- Truck-specific governor, carburetion, clutch/bellhousing and accessory hardware vary by chassis and duty rating.
Notable Applications
International L-series trucksInternational R-series trucksInternational S-series trucks
Strengths
- Strong low-rpm torque
- Durable truck construction
- Broad IH light/medium truck use
Weaknesses / Watch-outs
- Longer stroke than SD-220
- Modest high-rpm capability
- Specialist restoration market
Common Upgrades
- Carburetor rebuild
- Electronic ignition
- Cooling and oiling refresh
Racing History & Legacy
The SD-240 was a commercial truck engine rather than a factory racing powerplant. Its reputation centered on low-rpm torque and fleet durability.
The SD-240 extended the usefulness of the Silver Diamond family and became a common restoration-era International six-cylinder because of its broad use across several truck series.
References & Image Credit
Key Factory / Build Variants
Engineering Notes Worth Knowing
01
3.5625 x about 4.018-inch bore and stroke; 240.3 cu in / 3.9 L.
02
Cast-iron pushrod OHV inline-six.
03
Early service data lists about 108 hp, while later documented versions reached roughly 131 hp.
04
Used broadly in International L-, R-, and S-series trucks.
History · context · related reading
The Story Behind International Harvester SD-240 Six
The SD-240 was a natural truck-engine evolution: keep the proven Silver Diamond layout, add stroke, and gain the torque needed for heavier work. It became one of the familiar gasoline sixes of International’s postwar truck years.
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