AERA AERA Materials, Coatings & Surface Finishing 2 — Questions and Answers
Question 1: What is the Ra (average roughness) measurement scale used by AERA and what does a lower Ra value indicate about a surface?
- Ra measures average peak-to-valley height in microinches or microns; lower Ra means a smoother surface (Correct answer)
- Ra measures hardness of the surface coating
- Ra indicates surface temperature during machining
- Ra describes the angle of the cross-hatch pattern
Correct answer: Ra measures average peak-to-valley height in microinches or microns; lower Ra means a smoother surface
Ra quantifies surface texture by averaging the absolute deviations of the surface profile from the mean line, and a lower Ra number means fewer and shallower peaks and valleys.
Question 2: Why does AERA advise against using an overly smooth (mirror-like) finish on cylinder bores for cast iron rings?
- Cast iron rings require some surface texture to retain oil and seat properly; a mirror finish causes ring scuffing and oil starvation (Correct answer)
- Mirror finishes are too expensive to produce
- Smooth bores cause excessive ring flutter
- Mirror finishes increase bore diameter beyond spec
Correct answer: Cast iron rings require some surface texture to retain oil and seat properly; a mirror finish causes ring scuffing and oil starvation
Without micro-valleys to hold oil, a mirror-smooth bore starves the ring-to-bore interface of lubrication, leading to scuffing especially during initial break-in.
Question 3: What is the purpose of phosphate coating (Parkerizing) applied to some engine components during rebuilding?
- Provides corrosion resistance and a microscopic oil-retaining surface on ferrous parts (Correct answer)
- Hardens the surface to Rockwell C 60+
- Acts as a thermal barrier coating
- Replaces chrome plating on worn journals
Correct answer: Provides corrosion resistance and a microscopic oil-retaining surface on ferrous parts
Phosphate conversion coatings create a porous iron phosphate layer that resists rust and absorbs oil, serving as a break-in lubricant carrier on parts like piston rings and camshafts.
Question 4: According to AERA material guidelines, what type of bearing insert material is commonly used in high-performance rebuilt engines for superior fatigue strength?
- Tri-metal bearings (steel back, copper-lead intermediate, babbitt overlay) (Correct answer)
- Single-layer aluminum bearings only
- Cast iron insert bearings
- Bronze bushings throughout
Correct answer: Tri-metal bearings (steel back, copper-lead intermediate, babbitt overlay)
Tri-metal bearings offer a hard steel backing for support, a fatigue-resistant copper-lead layer, and a soft babbitt overlay that embeds debris and conforms to the journal.
Question 5: What material property makes ductile iron (nodular iron) camshafts preferable over grey iron camshafts in rebuilt performance engines?
- Higher tensile strength and impact resistance due to spheroidal graphite structure (Correct answer)
- Lower melting point for easier casting
- Better thermal conductivity for heat dissipation
- Lighter weight reducing rotating mass
Correct answer: Higher tensile strength and impact resistance due to spheroidal graphite structure
The spheroidal (nodular) graphite in ductile iron interrupts crack propagation less than the flake graphite of grey iron, providing greater strength and resistance to lobe chipping under high spring loads.
Question 6: Which surface treatment does AERA recognize for improving wear resistance of valve stems in rebuilt cylinder heads?
- Chrome plating or stellite hardfacing on the stem and tip area (Correct answer)
- Shot peening the stem surface
- Anodizing the aluminum valve stem
- Thermal barrier coating on the stem
Correct answer: Chrome plating or stellite hardfacing on the stem and tip area
Chrome-plated or stellite-hardened valve stems resist the abrasive wear from valve guide contact and repeated seating impacts in high-mileage or high-performance applications.
AERA AERA Materials, Coatings & Surface Finishing 2
What is the Ra (average roughness) measurement scale used by AERA and what does a lower Ra value indicate about a surface?