Cylinder head

01

Choose the head alloy and check thermal fatigue

The cast aluminium head alloys in the reviewed research are A356 types. A Chalmers University and Volvo Cars study states that A356 is chosen for its castability, strength and toughness and its lower susceptibility to casting defects in complex structures such as cylinder heads, that A356 + 0.5% Cu gives improved strength in the 200–250 °C range, and that the alloy is employed in an overaged T7 state for a more stable microstructure. Cylinder heads experience temperatures up to 250 °C in operation, and A356 undergoes ageing above 150 °C. [1] A Hochschule Offenburg paper models A356-T6 heads: the alloy is optimised for peak hardness from the quenched state, but service temperatures over-age it with a significant loss of strength, which the authors include in thermomechanical fatigue (TMF) life predictions. [2]

The European designation for a comparable sand-cast alloy is EN AC-42100 (EN AC-AlSi7Mg0,3) to DIN EN 1706: Si 6.50–7.50%, Mg 0.25–0.45%, Fe 0.19% max. and Cu 0.05% max.. Minimum properties from separately cast sand test bars are 140 MPa tensile strength, 80 MPa proof strength and 45 HBS as cast (F), rising to 230 MPa, 190 MPa and 75 HBS after solution treatment and full artificial ageing (T6), with 2% elongation in both. The foundry notes that properties must be agreed for the actual part geometry before contract. [3] Ask which designation and temper the drawing calls for; the 0.5% Cu addition described above is outside the 0.05% Cu limit of EN AC-42100. [1] [3]

Test bars cut from the valve bridge, which the Chalmers authors identify as the zone of a head with the greatest sensitivity to thermo-mechanical fatigue cracking, showed tensile strength falling from 211 MPa at room temperature to 73 MPa at 300 °C and a thermal expansion coefficient of (25–30) × 10⁻⁶ /°C between 25 and 360 °C. Engine start-stop cycles drive this fatigue through steep temperature gradients, and downsizing, pressure charging and hybrid start-stop operation raise the load. [1]

Editorial calculation: 73 ÷ 211 ≈ 0.35, so the tested head alloy kept about 35% of its room-temperature strength at 300 °C. Editorial calculation: 230 ÷ 140 ≈ 1.64, so T6 raises the AlSi7Mg0,3 minimum tensile strength by about 64% over the as-cast state. [1] [3]

Iron heads are in series production for heavy-duty diesels: SinterCast lists compacted graphite iron (CGI) cylinder heads in series production for Hyundai 5.9 to 12.7 litre engines, the Traton 13 litre engine, MAN 10.5 and 12.4 litre engines and the DAF 12.9 litre engine. [7] ISO 16112:2017 provides five CGI grades from GJV 300 to GJV 500 (minimum tensile strength 300 to 500 MPa), with nodularity of 0–20% and no flake graphite permitted; the compacted graphite inhibits crack initiation and growth compared with grey iron. [6]

Give the supplier the engine model or drawing, alloy designation and standard (A356, EN AC-42100 or other; grey iron or CGI grade), heat treatment, casting process, the peak metal temperature and start-stop duty, required test-bar or cut-up properties and from which location (for example the valve bridge), bare casting or assembly, and quantities.

02

Check deck flatness, water-jacket leak tests and head bolting

Service limits come from the engine maker. GM's bulletin PIP5016A for Duramax diesel head gasket replacement says the head must be measured for warpage with a straightedge and feeler gauge, that a head with warpage in excess of 0.1 mm (0.0039 in) must be replaced, and that the head surface must not be machined. [4]

The same bulletin requires new M12 cylinder head bolts at every head service (the M8 bolts may be reused), clean bolt threads and holes, gasket surfaces cleaned without power sanders, wire brushes or chemical agents, and a paint-pen mark on each M12 bolt for each of the 4 torque steps so that no step is skipped or repeated. It also notes that the left and right head gaskets are not interchangeable and that a wrongly placed gasket blocks coolant and oil passages. [4]

Water-jacket integrity is checked by pressure testing. US patent 4,860,575 describes the method: the head is removed, the openings linking its coolant cavities to the block are plugged, and compressed air or hot water under pressure is introduced to find cracks or leaks between the coolant cavity and the other passages; some leaks appear only with coolant at elevated temperature and service-level pressure. Its test stand pressurises the head with air while soapy water is sprayed on it to reveal escaping air. [5]

Editorial note: ask each supplier to state the deck flatness tolerance on the drawing, the water-jacket and oil-gallery test medium, pressure, temperature and hold time, and the leak acceptance criterion, so that quotations can be compared on the same basis.

03

Heads offered as assemblies, bare castings, die-cast aluminium and HT250 iron

Listings range from engine cylinder head assemblies and bare heads for vehicle engines to heads for construction-machinery diesel engines, custom die-cast and machined aluminium heads, and a grey-iron head casting from a foundry.

04

Casting, valve bridge, water jacket, deck face, bolt holes and valve seats

The values here come from one diesel (GM's Duramax bulletin) and one alloy study (Chalmers); valve seats, guides and ports are not covered by the sources reviewed.

05

Head drawing data: chamber, seats, deck, alloy and leak test

Five of the nine fields have no value in the sources reviewed; the flatness figure is a GM service limit for one diesel, not a production tolerance.

06

Send the engine, material and test data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Engine: [model; drawing; bare head or assembly]
Material: [alloy or iron grade; standard; heat treatment]
Duty: [peak temperature; start-stop cycles]
Deck: [flatness; finish; gasket type]
Tests: [leak test medium, pressure, hold time; test bars]
Quantity: [units]

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