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The Legacy of the Red Diamond Series
The International Harvester Red Diamond engine series was introduced in the late 1940s as a heavy-duty gasoline powerplant for trucks, military vehicles, and industrial equipment. Built by International Harvester (IH), a company founded in 1902 and known for its agricultural and commercial machinery, the Red Diamond engines were designed to bridge the gap between light-duty inline sixes and massive diesel platforms. Among the most recognized variants was the RD406—a 406 cubic inch inline-six that powered medium and heavy trucks throughout the 1950s and early 1960s.
These engines were known for their durability, simplicity, and torque-rich performance. While IH eventually shifted toward diesel power in the 1960s, the Red Diamond series remained in service for decades, especially in rural fleets and municipal vehicles. Though exact production numbers are hard to pin down, estimates suggest tens of thousands of RD-series engines were built, with the RD450 and RD501 also gaining popularity in fire trucks and military transports.
Terminology Clarification
The RD406, when paired with the Holley 885 FFG carburetor, requires a fuel pressure of approximately 5 PSI. This specification ensures consistent fuel delivery without overwhelming the needle and seat assembly in the carburetor. Using an electric fuel pump is common in restorations, but it must be regulated to avoid flooding or lean conditions.
For mechanical pumps, suction pressure must exceed 15 inches of mercury to maintain adequate flow. If suction drops below this threshold, fuel starvation can occur, especially under load. Rubber hoses on the suction side should be replaced with ethanol-resistant lines, and the tank should be inspected for rust flakes or water contamination.
Ignition Timing and Cylinder Reference
Initial ignition timing for the RD406 is set at 5 degrees BTDC. Interestingly, some IH engines—particularly in the MV and VS series—used cylinder #8 as the timing reference, leading to confusion among mechanics unfamiliar with the brand’s quirks. The RD406, however, typically uses cylinder #1 for timing, though documentation from the era occasionally references alternate cylinders depending on distributor orientation.
Point gap should be set at .019 inches, with a dwell angle of 32 degrees. These settings ensure optimal coil saturation and spark delivery, especially important in high-compression gasoline engines like the RD406.
Field Anecdote and Restoration Insight
In upstate New York, a retired quarry mechanic restored a 1953 International R-series truck equipped with an RD406. After installing a modern electric fuel pump, he noticed intermittent flooding and hard starts. The issue was traced to excessive fuel pressure—nearly 9 PSI—overwhelming the Holley carburetor. After installing a pressure regulator and replacing the standpipe, the truck ran smoothly.
He also discovered that the ignition timing had been set using cylinder #8, a holdover from his experience with MV-series engines. Once corrected to cylinder #1 and reset to 5 degrees BTDC, the engine idled cleanly and pulled strong under load.
Common Issues and Solutions
While the RD406 is no longer in production, its legacy lives on in vintage truck communities and industrial restorations. Some enthusiasts have adapted the engine for propane or natural gas, citing cleaner combustion and easier cold starts. Others have retrofitted electronic ignition systems to eliminate point wear and improve reliability.
Modern fuel pumps, pressure regulators, and ignition modules can be integrated without compromising the engine’s original character. However, care must be taken to preserve the mechanical integrity and historical accuracy of the build.
Conclusion
The International RD406 remains a testament to mid-century American engineering—simple, powerful, and built to last. With proper fuel pressure, accurate timing, and attention to detail, these engines can still perform reliably in modern conditions. Whether powering a vintage truck or serving as a museum piece, the Red Diamond continues to shine in the hands of those who understand its quirks and respect its heritage.
The International Harvester Red Diamond engine series was introduced in the late 1940s as a heavy-duty gasoline powerplant for trucks, military vehicles, and industrial equipment. Built by International Harvester (IH), a company founded in 1902 and known for its agricultural and commercial machinery, the Red Diamond engines were designed to bridge the gap between light-duty inline sixes and massive diesel platforms. Among the most recognized variants was the RD406—a 406 cubic inch inline-six that powered medium and heavy trucks throughout the 1950s and early 1960s.
These engines were known for their durability, simplicity, and torque-rich performance. While IH eventually shifted toward diesel power in the 1960s, the Red Diamond series remained in service for decades, especially in rural fleets and municipal vehicles. Though exact production numbers are hard to pin down, estimates suggest tens of thousands of RD-series engines were built, with the RD450 and RD501 also gaining popularity in fire trucks and military transports.
Terminology Clarification
- RD406: A 406 cubic inch inline-six gasoline engine from International Harvester’s Red Diamond series.
- Holley 885 FFG Carburetor: A downdraft carburetor used in heavy-duty applications, known for its large throat and fuel delivery capacity.
- BTDC (Before Top Dead Center): A timing reference indicating how many degrees before the piston reaches the top of its stroke the spark plug fires.
- Dwell Angle: The number of degrees the distributor cam rotates while the ignition points remain closed, affecting coil saturation and spark strength.
- Standpipe: A vertical fuel pickup tube inside the tank, often vulnerable to rust at the air-liquid interface.
The RD406, when paired with the Holley 885 FFG carburetor, requires a fuel pressure of approximately 5 PSI. This specification ensures consistent fuel delivery without overwhelming the needle and seat assembly in the carburetor. Using an electric fuel pump is common in restorations, but it must be regulated to avoid flooding or lean conditions.
For mechanical pumps, suction pressure must exceed 15 inches of mercury to maintain adequate flow. If suction drops below this threshold, fuel starvation can occur, especially under load. Rubber hoses on the suction side should be replaced with ethanol-resistant lines, and the tank should be inspected for rust flakes or water contamination.
Ignition Timing and Cylinder Reference
Initial ignition timing for the RD406 is set at 5 degrees BTDC. Interestingly, some IH engines—particularly in the MV and VS series—used cylinder #8 as the timing reference, leading to confusion among mechanics unfamiliar with the brand’s quirks. The RD406, however, typically uses cylinder #1 for timing, though documentation from the era occasionally references alternate cylinders depending on distributor orientation.
Point gap should be set at .019 inches, with a dwell angle of 32 degrees. These settings ensure optimal coil saturation and spark delivery, especially important in high-compression gasoline engines like the RD406.
Field Anecdote and Restoration Insight
In upstate New York, a retired quarry mechanic restored a 1953 International R-series truck equipped with an RD406. After installing a modern electric fuel pump, he noticed intermittent flooding and hard starts. The issue was traced to excessive fuel pressure—nearly 9 PSI—overwhelming the Holley carburetor. After installing a pressure regulator and replacing the standpipe, the truck ran smoothly.
He also discovered that the ignition timing had been set using cylinder #8, a holdover from his experience with MV-series engines. Once corrected to cylinder #1 and reset to 5 degrees BTDC, the engine idled cleanly and pulled strong under load.
Common Issues and Solutions
- Fuel Delivery Problems
Use a pressure regulator to maintain 5 PSI. Inspect tank for rust and replace standpipe if corroded.
- Ignition Confusion
Confirm timing cylinder via distributor cap orientation. Use a timing light and verify BTDC setting.
- Carburetor Compatibility
Ensure Holley 885 FFG is properly jetted for the RD406. Rebuild kits are available but may require modification.
- Rubber Hose Degradation
Replace all fuel lines with ethanol-resistant hose. Inspect for soft spots or cracking.
- Electrical Wear
Replace ignition points and condenser. Check coil resistance and replace if below spec.
While the RD406 is no longer in production, its legacy lives on in vintage truck communities and industrial restorations. Some enthusiasts have adapted the engine for propane or natural gas, citing cleaner combustion and easier cold starts. Others have retrofitted electronic ignition systems to eliminate point wear and improve reliability.
Modern fuel pumps, pressure regulators, and ignition modules can be integrated without compromising the engine’s original character. However, care must be taken to preserve the mechanical integrity and historical accuracy of the build.
Conclusion
The International RD406 remains a testament to mid-century American engineering—simple, powerful, and built to last. With proper fuel pressure, accurate timing, and attention to detail, these engines can still perform reliably in modern conditions. Whether powering a vintage truck or serving as a museum piece, the Red Diamond continues to shine in the hands of those who understand its quirks and respect its heritage.
