AERA Cylinder Head Rebuilding & Valve Train 2 — Questions and Answers
Question 1: What effect does removing material from a cylinder head gasket surface (resurfacing) have on the valve train geometry of an overhead-cam engine?
- It raises the camshaft relative to the head, reducing valve clearance and potentially increasing valve timing duration (Correct answer)
- It lowers the camshaft relative to the block, increasing valve clearance
- It has no effect on valve train geometry because the cam is self-adjusting
- It changes only the compression ratio without affecting valve timing
Correct answer: It raises the camshaft relative to the head, reducing valve clearance and potentially increasing valve timing duration
On OHC engines, resurfacing raises the cam centerline relative to the deck, which closes up valve lash or shimmed clearances and can affect valve timing; shims or adjustments must be rechecked.
Question 2: Combustion chamber volume is measured to ensure uniformity across all cylinders. What is the primary reason for equalizing combustion chamber volumes?
- To balance compression ratios and prevent uneven power output or detonation in individual cylinders (Correct answer)
- To maximize surface area for improved cooling efficiency
- To reduce the amount of valve grinding required during rebuilding
- To comply with emissions regulations for combustion efficiency
Correct answer: To balance compression ratios and prevent uneven power output or detonation in individual cylinders
Unequal combustion chamber volumes create unequal compression ratios, causing some cylinders to run leaner/hotter and be more prone to detonation while others produce less power.
Question 3: A valve spring that measures below the minimum free-length specification should be:
- Replaced, as a short free length indicates fatigue and insufficient spring pressure (Correct answer)
- Shimmed to restore the correct installed height and spring pressure
- Reused if the spring rate per inch is still within spec
- Tested for coil bind before making a replacement decision
Correct answer: Replaced, as a short free length indicates fatigue and insufficient spring pressure
A valve spring below minimum free length has taken a permanent set, indicating metal fatigue; shimming is not an acceptable repair because the spring rate may also be compromised.
Question 4: What is 'valve recession' and why is it a concern on older cast iron heads with hardened valve seats?
- It is the sinking of the valve seat into the head over time, reducing valve stem height and eventually causing loss of valve adjustment (Correct answer)
- It is the withdrawal of the valve stem from the guide caused by thermal expansion
- It is the erosion of the valve face caused by abrasive combustion deposits
- It is the inward movement of the valve keeper grooves under spring pressure
Correct answer: It is the sinking of the valve seat into the head over time, reducing valve stem height and eventually causing loss of valve adjustment
Valve recession occurs when soft iron seats wear, allowing the valve to sink deeper into the head; this shortens effective valve stem height, tightening lash on solid-lifter engines until valves stay partially open and burn.
Question 5: When selecting the correct replacement valve seat insert for a cylinder head, which dimensions are most critical?
- Outside diameter, inside diameter (seat width angle), and insert height (Correct answer)
- Insert weight, material hardness, and color code
- Only inside diameter, as the OD is a press-fit standard
- Valve stem diameter and spring retainer groove depth
Correct answer: Outside diameter, inside diameter (seat width angle), and insert height
The outside diameter determines the press-fit interference with the counterbore, the inside diameter and angle determine the seating surface, and the height must match the counterbore depth to sit flush.
Question 6: What is the correct interference fit range for pressing a valve seat insert into a cast iron cylinder head counterbore?
- 0.003–0.005 in (0.076–0.127 mm) (Correct answer)
- 0.010–0.015 in (0.254–0.381 mm)
- 0.0005–0.001 in (0.013–0.025 mm)
- 0.008–0.012 in (0.203–0.305 mm)
Correct answer: 0.003–0.005 in (0.076–0.127 mm)
A 0.003–0.005 in interference fit provides sufficient retention to keep the insert from spinning or pulling out while avoiding cracking the head casting.
Question 7: After valve seat grinding, the seat contact pattern should be verified using:
- Prussian blue or machinist's dye applied to the valve face, then lightly tapping the valve to transfer the pattern to the seat (Correct answer)
- A dial indicator measuring runout of the seat relative to the guide bore
- Feeler gauges inserted between the valve and seat to check contact width
- A vacuum tester applied to the port with the valve held closed by hand
Correct answer: Prussian blue or machinist's dye applied to the valve face, then lightly tapping the valve to transfer the pattern to the seat
Prussian blue transfers from the valve face to the seat showing the exact location and width of the contact band, allowing the machinist to verify centering and width without guessing.
AERA Cylinder Head Rebuilding & Valve Train 2
What effect does removing material from a cylinder head gasket surface (resurfacing) have on the valve train geometry of an overhead-cam engine?