ASE A1 Cylinder Head and Valve Train Service Overview
Scope of Unit B in the ASE Task List
Unit B of the ASE A1 exam specifically targets the diagnosis and repair of the cylinder head and its internal components. This section accounts for a significant portion of the total exam questions, focusing heavily on the technician's ability to interpret measurements against manufacturer specifications. You will be tested on your knowledge of both Overhead Valve (OHV) and Overhead Cam (OHC) configurations. The scope includes the assessment of the casting for structural integrity, the evaluation of the valve train’s mechanical state, and the verification of the sealing surfaces. Understanding the relationship between the valve train and the timing system is also vital, as the ASE tasks often link mechanical wear in the head to broader engine timing or driveability issues.
The ASE A1 exam assumes the candidate is familiar with the precision instruments required for engine blueprinting. You must know when to use a micrometer (outside and inside) versus a dial indicator. For example, measuring a valve stem requires an outside micrometer graduated in ten-thousandths of an inch, while checking valve seat runout necessitates a dial indicator with a specialized pilot. Other essential tools include the valve spring compressor (both overhead and bench types), the telescoping gauge (or snap gauge) for bore measurements, and the precision straightedge used in conjunction with a feeler gauge. Recognizing the correct tool for a specific task—such as using a small-hole gauge to check valve guide ID—is a frequent point of assessment.
Safety and Cleaning Procedures for Cylinder Heads
Before any measurements can be taken, the cylinder head must be chemically and mechanically clean. The ASE standards emphasize the removal of carbon deposits, old gasket material, and scale without damaging the base metal. For aluminum heads, the use of steel wire brushes is generally discouraged to prevent gouging the soft surface; instead, plastic scrapers or specialized solvents are preferred. You should understand the difference between thermal cleaning (pyrolytic ovens) and aqueous cleaning (hot tanks). It is also critical to note that all oil galleries and coolant passages must be cleared of debris. Failure to clean these passages can lead to premature failure of the camshaft bearings or localized overheating due to restricted coolant flow.
Cylinder Head Disassembly and Initial Inspection
Proper Valve Removal and Component Organization
Disassembly is a systematic process where organization is paramount. As valves are removed using a valve spring compressor, they must be kept in a numbered rack that corresponds to their original cylinder and port (intake or exhaust). This is because valves and their respective seats develop unique wear patterns, or "seats," over time. The ASE exam often tests the technician's ability to maintain this order to prevent improper geometry during reassembly. During this stage, keep the valve keepers (locks), retainers, and springs grouped together. Mixed components can lead to incorrect spring pressures or valve-to-guide clearances if used parts are reinstalled in the wrong locations.
Visual Inspection for Cracks, Burns, and Damage
A thorough visual inspection serves as the first line of defense against wasting time on a non-repairable casting. Look for "tuliped" valves—where the valve head has thinned and pulled upward—and burnt valve faces caused by localized hotspots or lean air-fuel ratios. On the cylinder head itself, inspect the combustion chamber for cracks, particularly between the valve seats where thermal stress is highest. For cast iron heads, Magnetic Particle Inspection (Magnafluxing) is the standard for finding hidden cracks, whereas aluminum heads require Dye Penetrant Testing. If you identify a crack that extends into a water jacket or a valve seat area, the head is typically deemed unserviceable or requires specialized welding and machining.
Cleaning Methods: Hot Tanks, Bead Blasting, and Solvents
Cleaning methods are determined by the material of the head and the type of contaminant. Hot tanks using caustic soda are effective for cast iron but will destroy aluminum. For aluminum components, an agitated wash with an enzyme-based or mild detergent is required. Bead blasting is often used to remove stubborn carbon from the combustion chambers and ports, but the ASE task list warns that all abrasive media must be completely neutralized and washed away. Any glass bead or walnut shell left in a blind hole can enter the lubrication system, acting as an abrasive that will quickly ruin the main and rod bearings once the engine is started.
Critical Cylinder Head Measurements and Machining Limits
Checking Head Warpage with a Straightedge and Feeler Gauge
Measurement of the cylinder head mating surface is a foundational ASE A1 task. To check for warpage, a precision straightedge is placed across the deck surface in at least six positions: longitudinally, transversely, and diagonally. A feeler gauge is then used to check for gaps between the straightedge and the head. While cylinder head resurfacing limits vary, a common general rule on the exam is that warpage exceeding 0.003 to 0.004 inches (0.076 to 0.102 mm) across the length of a four-cylinder head requires machining. Excessive warpage prevents the head gasket from sealing properly, leading to combustion leaks or coolant-to-oil intermix.
Resurfacing Limits and Milling/Shaving Consequences
When a head is warped beyond tolerance, it must be milled or "shaved." However, there are strict limits to how much material can be removed. Removing too much metal increases the compression ratio by reducing the combustion chamber volume, which can cause engine knock (detonation). Furthermore, on OHC engines, milling the head brings the camshaft closer to the crankshaft, which retards the valve timing and can cause the timing belt or chain to become slack. Technicians must check the manufacturer's "minimum head height" or "maximum material removal" spec. If a head is milled, the ASE task list may require the use of a shim or a thicker head gasket to restore the original geometry and timing alignment.
Spark Plug Hole and Cam Bore Inspection
The inspection extends beyond the deck surface. Spark plug threads must be inspected for stripping or cross-threading; repair often involves installing a threaded insert like a Heli-Coil. On OHC heads, the camshaft bores or bearing saddles must be checked for scoring and alignment. If the head was severely warped, the cam bores may no longer be concentric, which will cause the camshaft to bind. This is verified using a mandrel or by measuring the bores with a dial bore gauge. If the bores are out of alignment, the head may require line boring—a complex machining process—or replacement.
Valve and Valve Guide Inspection and Service
Measuring Valve Stem Wear and Margin Thickness
Valves must be measured to determine if they are fit for reuse. Use an outside micrometer to measure the valve stem diameter at the top, middle, and bottom of the travel area. This identifies taper or "hourglass" wear. Equally important is the valve margin, which is the thickness of the edge of the valve head between the face and the top of the valve. If the margin is too thin (typically less than 1/32 inch), the valve will not be able to dissipate heat effectively, leading to pre-ignition and eventual melting of the valve edge. Valves with insufficient margin must be replaced rather than ground.
Valve Guide Wear Measurement Techniques (Ball, Dial Gauge)
Accurate valve guide wear measurement ASE standards involve two primary methods. The first is the direct method: use a small-hole gauge to measure the inside diameter (ID) of the guide at both ends and the center, then subtract the valve stem diameter to find the total clearance. The second is the "drop-dial" or wobble method: with the valve slightly off its seat, a dial indicator is placed against the stem, and the valve is moved side-to-side. This lateral movement indicates the guide's condition. Excessive clearance leads to high oil consumption (as oil bypasses the seals) and prevents the valve from seating squarely, which can cause erratic vacuum readings on a diagnostic gauge.
Guide Repair Methods: Knurling, Inserts, and Replacement
When valve guides exceed wear limits, they must be restored. Knurling is a displacement process where a tool raises a pattern of ridges inside the guide to reduce the ID, but this is often considered a temporary or light-duty repair. A more robust method is the installation of thin-wall bronze liners (inserts), which provide excellent lubrication and durability. In many modern aluminum heads, the guides are integral or pressed-in. Pressed-in guides are replaced by heating the head and using a driver to push the old guide out and the new one in. After any guide repair, the guide must be reamed to the final specified size to ensure the correct stem-to-guide clearance.
Valve Seat Reconditioning Knowledge for ASE
Valve Seat Angle Specifications and Multi-Angle Cuts
Reconditioning the valve seat is necessary to ensure a gas-tight seal and proper heat transfer. The ASE A1 exam requires knowledge of valve seat angle and width specs. Most engines use a 45-degree interference angle for the actual seating surface. However, a high-performance or modern seat often utilizes a three-angle grind: a 30-degree top cut to narrow the seat from the top, a 45-degree seat cut, and a 60-degree bottom cut to transition into the bowl. This multi-angle approach improves airflow (volumetric efficiency) by smoothing the path the air takes around the valve head. Understanding these angles is critical for diagnosing flow restrictions or sealing issues.
Measuring and Correcting Valve Seat Runout
Valve seat runout refers to the concentricity of the seat in relation to the center of the valve guide. If the seat is not perfectly centered, the valve will hit one side of the seat first, causing the stem to flex and eventually break. To measure this, a pilot is inserted into the guide, and a dial indicator is rotated around the seat. ASE standards generally allow for a maximum runout of approximately 0.002 inches (0.05 mm). If runout exceeds this, the seat must be recut or ground using a stone that is perfectly centered on a pilot that fits snugly in the guide.
Valve Seat Width Importance and Adjustment
The width of the valve seat is a compromise between sealing and cooling. A seat that is too narrow provides an excellent seal but cannot transfer enough heat from the valve to the cylinder head, leading to burnt valves. A seat that is too wide provides great cooling but allows carbon to build up, preventing a tight seal. Typical specs are roughly 0.060" to 0.080" for intake valves and slightly wider for exhaust valves. Technicians adjust the seat width and its position on the valve face by using the top (30°) and bottom (60°) stones to "move" the 45° seat up or down the valve face.
Valve Spring, Retainer, and Seal Service
Testing Valve Spring Pressure and Squareness
Valve springs are the most overlooked component in the valve train. They must be tested for tension using a spring pressure tester. You must measure the pressure at both the "installed height" (valve closed) and the "open height" (valve fully open). Weak springs lead to valve float at high RPM, where the spring cannot close the valve fast enough, potentially leading to piston-to-valve contact. Additionally, springs must be checked for squareness by standing them on a flat surface against a square; if the spring leans more than 1/16 inch, it is distorted and will apply uneven side-loading to the valve stem.
Measuring and Setting Valve Spring Installed Height
A critical step in ASE A1 valve train diagnosis is performing a valve spring installed height check. This is the distance from the spring seat (on the head) to the underside of the retainer when the valve is closed. If the head or valve seats have been ground, the valve sits deeper in the head, which increases the installed height and reduces spring tension. To correct this, technicians use hardened steel shims placed under the spring. The formula for shim selection is: (Measured Height - Specified Height) = Required Shim Thickness. Failure to maintain this height results in insufficient seat pressure and poor high-speed performance.
Valve Stem Seal Types, Installation, and Leak Diagnosis
Valve stem seals control the amount of oil that lubricates the valve stems. There are two main types: umbrella seals, which move with the valve, and positive-stop seals, which are fixed to the valve guide. On the ASE exam, remember that blue smoke from the tailpipe upon startup or after a long period of idling is a classic symptom of leaking valve stem seals. When installing positive-stop seals, a protective sleeve must be placed over the valve lock grooves to prevent the sharp edges from cutting the new seal's lip. Incorrectly installed seals are a leading cause of high oil consumption and carbon-fouled spark plugs.
Assembly, Lash Adjustment, and Final Verification
Proper Valve Lubrication and Assembly Sequence
During reassembly, all moving parts must be coated with high-pressure assembly lube to prevent dry-start wear. This includes the valve stems, cam lobes, and lifters. The assembly sequence must be followed precisely to ensure that no components are stressed. For engines with Hydraulic Lash Adjusters (HLAs), it is often recommended to bleed the air out of the lifters or soak them in oil before installation. In contrast, solid lifters must be installed with the specific lash (clearance) required by the manufacturer. Ensuring that the valve keepers are fully seated by lightly tapping the top of the valve stem with a mallet is a safety step that prevents the valve from dropping into the cylinder upon initial startup.
Valve Lash Adjustment Procedures (Mechanical vs. Hydraulic)
You must know how to check valve stem height and its impact on lash. On OHC engines with mechanical buckets, lash is adjusted by changing shims. On OHV engines, it is usually adjusted via a nut on the rocker arm stud. The adjustment must be made when the lifter is on the "base circle" of the cam lobe (valve fully closed). For hydraulic systems, the goal is to set the "plunger travel" or "preload," typically 1/2 to 1 full turn past zero lash. Incorrect lash—too tight—will prevent the valve from closing fully, leading to burnt valves; too loose will cause excessive noise and accelerated wear on the cam lobes.
Checking for Valve-to-Piston Clearance After Service
The final verification step, especially after milling the head or installing a high-lift camshaft, is checking the valve-to-piston clearance. This is often done using the "clay method," where a piece of modeling clay is placed on the piston crown, the head is temporarily installed, and the engine is rotated by hand. The thickness of the compressed clay is then measured. ASE standards emphasize that this is a critical check to prevent catastrophic engine failure. If the clearance is insufficient, the technician must use a thicker head gasket or fly-cut the piston notches to ensure the engine can operate safely across all RPM ranges and thermal conditions. This final check ensures the integrity of the ASE A1 cylinder head and valve train service and the long-term reliability of the engine repair.