Valves For Detroit Diesel 71 Series Inline Engines (271, 371, 471, 671): Regulating Exhaust Flow
In the Detroit Diesel Series 71 engine, the primary airflow into the cylinders is controlled by the blower, which forces fresh air into the combustion chambers. The engine operates on a two-stroke cycle, and while there are no traditional intake valves, the exhaust valves play a crucial role in expelling exhaust gases after combustion. Proper valve function is essential for efficient scavenging and maintaining the engine’s overall performance.
Key Functions of the Exhaust Valves:
- Exhaust Expulsion:
- The exhaust valves open during the exhaust phase to allow the spent gases from the combustion process to exit the cylinder. This makes way for the fresh air supplied by the blower to enter and purge the remaining exhaust gases from the cylinder, ensuring that the next combustion cycle occurs with a clean air charge.
- Sealing the Combustion Chamber:
- During the compression and power strokes, the exhaust valves seal the combustion chamber, ensuring that pressure is maintained for efficient combustion. Proper sealing of the exhaust valves is critical for maintaining compression and maximizing engine power output.
- Supporting the Scavenging Process:
- The scavenging process in the two-stroke Detroit Diesel engine relies on the blower to push fresh air into the cylinders, forcing exhaust gases out through the exhaust valves. The timely opening and closing of the exhaust valves are essential for allowing efficient removal of these gases while preventing backflow.
- The scavenging process in the two-stroke Detroit Diesel engine relies on the blower to push fresh air into the cylinders, forcing exhaust gases out through the exhaust valves. The timely opening and closing of the exhaust valves are essential for allowing efficient removal of these gases while preventing backflow.
Components of the Exhaust Valves:
- Valve Head:
- The valve head is the flat, circular part of the exhaust valve that comes into contact with the valve seat to form a tight seal when closed. It must be free of pitting, cracks, and wear to ensure proper sealing during engine operation.
- Valve Stem:
- The valve stem is the long, narrow shaft that allows the valve to move up and down within the valve guide. It is critical for maintaining smooth valve operation and alignment.
- Valve Seat:
- The valve seat is the surface on the cylinder head that the valve head closes against, ensuring a tight seal. Any wear or damage to the seat can result in poor sealing, leading to a loss of compression.
- Valve Springs:
- Valve springs ensure that the exhaust valve remains closed when it is not being opened by the camshaft. They are essential for keeping the valve in the correct position and preventing valve float, especially at high RPMs.
- Valve Guides:
- Valve guides are cylindrical bushings within the cylinder head that guide the valve stem as it moves. Proper clearance between the valve stem and guide is necessary for smooth operation and to prevent oil from leaking into the combustion chamber.
- Valve guides are cylindrical bushings within the cylinder head that guide the valve stem as it moves. Proper clearance between the valve stem and guide is necessary for smooth operation and to prevent oil from leaking into the combustion chamber.
Critical Tolerances and Measurements for Exhaust Valves:
- Valve Stem-to-Guide Clearance:
- The clearance between the valve stem and the valve guide must be precise to ensure smooth movement and prevent excessive oil consumption. For the Detroit Diesel Series 71 engine, the valve stem-to-guide clearance typically ranges from 0.0015 to 0.003 inches (0.038 to 0.076 mm).
- Measurement Process: Use a micrometer to measure the diameter of the valve stem and a bore gauge to measure the inside diameter of the valve guide. If the clearance exceeds the allowable range, the valve guides may need to be reamed or replaced.
- Valve Seat Width:
- The valve seat width determines the contact area between the valve head and the seat. Proper seat width ensures effective sealing. For the Detroit Diesel Series 71 engine, the valve seat width should be between 0.060 to 0.090 inches (1.5 to 2.3 mm).
- Measurement Process: Use a valve seat gauge to measure the width of the seat. If the seat is too narrow, it may not seal properly, and if it is too wide, heat dissipation may be inadequate. Worn seats should be recut to ensure proper seating.
- Valve Face Angle:
- The valve face angle should match the angle of the valve seat for proper sealing. Typically, the valve face angle for exhaust valves in this engine is 45 degrees.
- Measurement Process: Inspect the valve face for wear or pitting, and use a valve grinding machine to restore the correct angle if necessary.
- Valve Spring Tension:
- Proper valve spring tension ensures that the exhaust valve closes fully after the camshaft releases it. Insufficient spring tension can result in valve float, especially at higher RPMs. The spring tension should be checked with a valve spring tester and typically falls within the range of 70 to 100 pounds.
- Measurement Process: Use a spring tester to measure the compression force of the valve spring. Weak or broken springs should be replaced to maintain proper valve operation.
Exhaust Valve Inspection and Maintenance:
- Visual Inspection for Wear and Damage:
- Inspect the valve heads, stems, and seats for wear, pitting, or cracking. Worn or damaged valves can result in poor sealing, which may cause a loss of compression or improper exhaust gas expulsion. Any valves showing signs of wear should be replaced to prevent engine performance issues.
- Valve Guides: Check the valve guides for wear or damage that could cause valve misalignment or oil leakage into the combustion chamber.
- Valve Face and Seat Grinding:
- If the valve face or seat shows signs of wear, it may be necessary to grind or reface the valve. Use a valve grinder to restore the valve face to the correct angle (typically 45 degrees). After grinding, ensure the valve seats are recut to match the newly restored valve face.
- Valve Stem Wear Measurement:
- Use a micrometer to measure the diameter of the valve stem. If the stem is worn beyond the manufacturer’s specifications, it must be replaced to maintain proper clearance and prevent excessive valve guide wear.
- Replacing Valve Springs:
- Inspect the valve springs for wear or loss of tension. If the spring has weakened or shows signs of cracking, replace it to avoid valve float and ensure proper valve closure at high engine speeds.
- Valve Guide Lubrication:
- Ensure proper lubrication of the valve guides to prevent excessive wear. Check for oil leaks or build-up in the valve train and clean out any sludge that could affect valve operation.
- Ensure proper lubrication of the valve guides to prevent excessive wear. Check for oil leaks or build-up in the valve train and clean out any sludge that could affect valve operation.
Valve Removal and Installation:
- Removing the Exhaust Valves:
- To remove the exhaust valves, first remove the cylinder head from the engine. Use a valve spring compressor to compress the valve springs and remove the retainers. Carefully extract the valve from the guide.
- Inspection After Removal: Once removed, inspect the valve seat, guide, and springs for wear and damage. Replace any worn components as needed.
- Installing the Exhaust Valves:
- Before installation, ensure the valve face and seat have been machined to the correct angle, and lubricate the valve stem. Insert the valve into the guide and use a valve spring compressor to install the spring and retainers.
- Spring and Retainer Installation: Ensure that the valve springs are properly seated and that the retainers are securely fastened to prevent the valve from moving out of alignment.
- Adjusting Valve Clearance:
- After installing the valves, check and adjust the valve lash (clearance between the valve stem and the rocker arm). Proper valve lash is critical for ensuring that the valves open and close at the correct times. The valve lash should be adjusted to the manufacturer’s specifications, typically around 0.010 to 0.020 inches.
- After installing the valves, check and adjust the valve lash (clearance between the valve stem and the rocker arm). Proper valve lash is critical for ensuring that the valves open and close at the correct times. The valve lash should be adjusted to the manufacturer’s specifications, typically around 0.010 to 0.020 inches.
Common Symptoms of Exhaust Valve Problems:
- Loss of Compression: Worn or improperly seated exhaust valves can result in a loss of compression, leading to reduced power and engine performance. If the engine is difficult to start or lacks power, the exhaust valves may need to be inspected for wear.
- Excessive Oil Consumption: Worn valve guides or seals can allow oil to enter the combustion chamber, resulting in excessive oil consumption and blue smoke from the exhaust. Replacing worn guides or seals can prevent oil from leaking into the combustion chamber.
- Knocking or Tapping Sounds: A knocking or tapping noise coming from the cylinder head may indicate excessive valve lash or worn components within the valve train. Adjusting the valve lash or replacing worn parts can resolve these issues.
Conclusion To The Valve Section
Exhaust valves are vital for controlling the expulsion of gases in the Detroit Diesel Series 71 engine. Proper maintenance of valve clearances, seats, and springs is essential to ensure the engine operates efficiently and without loss of power. By maintaining the correct tolerances and replacing worn components, operators can prevent valve-related issues and extend the life of their engine.