4.1 Cylinder Head Inspection, Thickness, Warpage, Cracks, and Passages
Key Takeaways
- Remove, disassemble, and clean a light-duty diesel head using VIN-specific service information; label injectors, glow plugs, and valvetrain parts before the first hammer blow.
- Inspect head-bolt and stud threads in the head and block; chase debris with a thread chaser, not a cutting tap, and treat torque-to-yield bolts as single-use unless the manufacturer explicitly allows reuse.
- Measure remaining head thickness, fire-deck flatness in multiple directions, corrosion pits, and surface finish before any mill cut, then compare every number to that engine's specification.
- Crack-test ferrous heads with magnetic particle inspection and aluminum heads with dye penetrant plus a pressure test; verify oil and coolant passages and replace questionable core and gallery plugs.
- A head is serviceable only when thickness, warpage, finish, cracks, passages, threads, and plugs all pass; independent OpenExamPrep teaching covers these published A9 skills and is not an ASE course.
Why diesel head work is its own job
Peak cylinder pressure on a light-duty diesel is not a gasoline number with extra soot. A 6.7-liter Power Stroke, 6.6 Duramax, 6.7 Cummins, 3.0 EcoDiesel, or 2.0 Turbocharged Direct Injection (TDI) stacks a roughly 16:1 compression ratio with boost. Peak pressures commonly exceed 2,000 psi in the chamber. The fire deck—the machined face that seals to the head gasket—has to stay flat, thick enough, and crack-free with coolant and oil galleries only millimeters from injector bores and four-valve seats.
That is why A9 cylinder-head diagnosis is a measurement job. A head that “looks clean after a soda blast” can still be warped 0.008 inch across a bank, thin below the mill limit, or cracked from an injector bore into a water jacket. This section is independent OpenExamPrep teaching of published A9 head-inspection skills. It is not an ASE publication and does not claim official approval, review, or partnership.
Remove, inspect, disassemble, and clean
Removal starts in service information for that vehicle identification number (VIN), not with an impact gun on the last head bolt. Photograph and label injector connectors, glow-plug positions, rocker or cam-cap order, and which bank is left on a V-engine. Duramax and Power Stroke V8 heads are not freely swapped left-to-right on every generation. A 6.7 Cummins uses one iron head, but injector lines, the harness, and the exhaust manifold still have a sequence.
Relieve residual common-rail pressure, and on Hydraulic Electronic Unit Injector (HEUI) 6.0/7.3 Power Stroke engines relieve high-pressure oil, before you crack injector fittings. Support the head. Aluminum Power Stroke and Duramax heads crack if they drop onto a fender lip. Keep fasteners in a labeled tray by length. Most diesel torque-to-yield (TTY) head bolts are single-use. Bag them as scrap once they are out unless the current procedure for that engine explicitly allows reuse, which is uncommon on these pickups and cars.
Disassemble on a clean bench. Put valves in a numbered holder so they return to the same guide. Use the OEM injector puller; do not pry piezo or solenoid injectors out of aluminum bores. A seized 6.0 or 6.7 glow plug that snaps off inside the head is a separate repair, not a “we will drill it later” note on the repair order.
Clean carbon, EGR soot, and oil varnish from ports, the fire deck, and galleries. Diesel heads collect more carbon than gasoline heads because of exhaust-gas recirculation and regeneration events. Use plastic scrapers, approved solvent, and rifle brushes in oil and coolant passages. Do not glass-bead a fire deck that will take a multi-layer steel (MLS) gasket unless that engine’s procedure allows the media—beads embed and become leak paths under fire-ring load. Do not spin a steel wire wheel on an aluminum Duramax, Power Stroke, EcoDiesel, or TDI deck. After brushing galleries, blow them clear, confirm they are open with a light or a nylon bristle, then wash again so grit is not sitting under a gallery plug.
Inspect while you clean. Rust streaks in a coolant jacket, white aluminum oxide around a combustion ring, soot tracks from a previous fire-ring leak, and eroded injector-bore seats are findings, not cosmetics.
Threaded holes, studs, and bolts
Head-bolt clamp load is what holds diesel cylinder pressure. Inspect three places, not just the bolt you can see:
| Location | What fails | Service path |
|---|---|---|
| TTY bolt or stud shank | Permanent stretch, necking, rust under the flange, damaged washer | Replace unless the OEM procedure for that engine allows reuse; do not “run them through a die and send them back” |
| Block threads | Pulled threads, coolant in the hole, debris under the bolt | Chase with a thread chaser; repair with the specified insert or replace the block/component if the hole is pulled |
| Head accessory and clamp holes | Manifold studs, injector hold-downs, glow-plug threads | Restore threads; a loose injector clamp is a combustion leak, not a noise complaint |
Chase a dirty but intact hole with a chaser. A cutting tap removes parent metal and can drop clamp load below specification. Aluminum EcoDiesel and TDI blocks are unforgiving. Iron Cummins 6.7 and compacted-graphite-iron 6.7 Power Stroke blocks can still pull threads after a previous over-torque or a broken bolt extraction.
The 6.0 Power Stroke taught a generation of shops that overheating from a failed oil cooler or EGR cooler stretches TTY bolts until the aluminum head lifts. The OEM path is new bolts, the published torque-then-angle sequence, and a head that still meets warpage and thickness. Aftermarket studs exist in the performance catalog. They are not automatically the A9 answer. The testable skill is inspect, service or replace, and install the specified fastener the specified way.
Thickness, warpage, corrosion, and surface finish
Cylinder head thickness is measured from the fire deck to a defined reference in the manual—often a pad or the cam-cover rail. Record the number before any mill cut and again after. If remaining thickness is below the minimum, the head is not serviceable. Milling an overhead-cam (OHC) TDI or EcoDiesel head changes chamber volume and cam-drive geometry (it retards the cams relative to the crank). Milling an overhead-valve (OHV) Cummins, Duramax, or Power Stroke head raises compression, changes injector and glow-plug protrusion relative to the piston, and can create piston-to-head or piston-to-valve contact. Diesel squish (piston-to-head clearance) is a tenths-of-a-millimeter dimension. A “just a little skim to clean it up” is how a fire ring meets a piston.
Warpage and flatness require a certified straightedge and feeler gauges in the paths the OEM shows: lengthwise, crosswise, both diagonals, and across each combustion chamber. Checking only the long intake rail and calling the deck flat is a miss. Aluminum heads on 6.0/6.7 Power Stroke and Duramax engines warp from overheating. Iron Cummins heads crack more often than they banana, but they still must be measured. Compare every gap to specification. A ridge of corrosion at the fire ring, or a pitted coolant port the gasket coating cannot fill, is damage even if the straightedge barely rocks inside the warpage limit.
Surface finish is not the same as flatness. MLS gaskets on modern Power Stroke, Duramax, EcoDiesel, and TDI heads need a specified roughness—usually smoother than an old composite gasket. Too rough, and the elastomer coating shears. Too smooth on some composite iron-head applications, and the gasket may not bite. Use the roughness-average specification in the manual, not a thumbnail test.
Cracks, passages, and plugs
Typical diesel crack sites:
- Between the valves in a four-valve chamber
- From injector bore to a valve seat or into a coolant jacket
- From glow-plug bore to the fire deck
- In the prechamber boss on older indirect-injection (IDI) 7.3 IDI International/Ford and 6.2/6.5 GM heads
- At an exhaust-port EGR junction
Magnetic particle inspection finds surface and near-surface cracks in ferrous heads—many Cummins 6.7 iron heads and older IDI iron heads. Mag does not work on aluminum Power Stroke, Duramax, EcoDiesel, or TDI heads. For aluminum, use dye penetrant on the fire deck, chambers, and ports, plus a pressure test: plug or plate every opening, apply the specified regulated air, and look for bubbles at the fire deck, injector cups, and jackets.
Passages must be open: oil feed to rockers or cam journals, drain-back, coolant jackets, EGR passages in the head, and injector-sleeve water jackets where used. A “rebuilt” head with a mud nest in an oil gallery will wipe an EcoDiesel cam or starve Duramax rockers on the first hot idle.
Core plugs (freeze plugs) and oil gallery plugs are part of serviceability. Electrolysis eats steel freeze plugs. A missing gallery plug after hot-tanking is an oil geyser at first start. Replace questionable plugs, seal them as specified, and pressure-test again.
Serviceability and bay examples
Return a head to the engine only when thickness is at or above minimum after any allowed cut; warpage and finish meet spec; crack tests pass; passages are clean; and threads, injector/sleeve seats, and plugs are repairable. Valve guides and seats still have to pass in the next section. Welding aluminum diesel heads is specialist work, not a bay shortcut.
- 6.0 Power Stroke: overheat from the oil cooler or EGR cooler → aluminum warpage and stretched TTY bolts. Measure the head; do not install studs on a banana deck.
- Duramax: coolant loss and sweet white smoke is often the EGR cooler first, but a cracked injector bore still happens. Pressure-test the head if the cooler is innocent.
- Cummins 6.7: mag-check between valves and around injector bores; you will measure piston protrusion later for gasket grade.
- EcoDiesel / TDI: aluminum OHC heads; dye and pressure-test; remaining thickness matters because cam geometry moves when you mill.
Inspection traps
- Magnetic-particle testing an aluminum head
- Cutting taps in head-bolt holes
- Reusing TTY bolts because the shanks “look fine”
- Checking warpage in only one direction
- Glass-beading an MLS fire deck
- Skipping gallery plugs after a tank
- Milling past minimum thickness to erase pits
- Calling soot tracks on the deck normal without checking the last fire ring
A 2014 Chevrolet Silverado 2500 with an LML Duramax overheated. The aluminum cylinder heads are off, cleaned, and visually free of cracks. A technician wants a crack test before measuring warpage. Which method is correct for these heads?
Head-bolt holes in a 6.7 Power Stroke compacted-graphite-iron block are dirty but the threads still gauge correctly. TTY head bolts came out of an overheated engine. What is the correct fastener service?
A shop is deciding whether a Cummins 6.7 iron head and a warped 6.7 Power Stroke aluminum head can be milled and returned to service. Which inspection set matches published A9 head-measurement practice?