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BY REFERENCE COMETIC - Reinforced cylinder head gaskets
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BY REFERENCE COMETIC - Reinforced cylinder head gaskets
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Reinforced Cometic head gasket, in copper. Sold individually, one gasket per engine.
The petrol engines 9CG (0.9 litre) and 10CC (1.1 litre) power the Austin-Healey Sprite Mk I, Mk II and Mk III listed below.
This original gasket carries the part number AJM302. Its bore is 68 mm; the manufacturer publishes neither its construction nor its thickness. It is sized for standard combustion pressure: as that pressure rises, the clamping load relaxes, the cylinder head lifts and coolant gets into a cylinder.
The head gasket does not make the seal on its own: it holds because the fasteners press the cylinder head against the block. Copper that is 99.9% pure brings good corrosion resistance, high ductility and high thermal conductivity, without becoming brittle under heat.
This page offers the part number from Cometic, an American maker of racing gaskets, in a single construction: copper. Its bore is 67.5 mm, against 68 mm for the original gasket, and its thickness is 1.09 mm. As a general rule, a gasket bore at least 0.5 mm larger than the actual cylinder bore is chosen. The part number is sold individually, one gasket per engine.
| Gasket bore OEM 68 mm | Thickness | Gasket volume | Gasket type OEM not specified | Brand | Part number OEM AJM302 |
| 67.5 mm | 1.09 mm | 3.90 | Copper | Cometic | C4145-043 |
Gasket volume is the space the gasket leaves free above each cylinder. Added to the combustion chamber, it lowers the compression ratio, the more so as it grows. Values can only be compared at the same gasket bore.
| Fasteners | Engines | Tightening torque |
| Original cylinder head bolts | 9CG, 10CC | not published by the manufacturer |
Before fitting, check the flatness of the cylinder head and block with a straight edge and have them resurfaced if necessary. Cometic precision-tests the flatness of its copper gaskets.
| Cometic copper A sheet of oxygen-free C110 copper, 99.9% pure. The metal resists corrosion well, deforms without breaking, conducts heat well and does not become brittle under heat. Cometic precision-tests the flatness of each gasket and recommends, for optimal results, fitting it with a crimped sealing ring. |
Gasket bore. The Cometic copper gasket is offered in 67.5 mm (C4145), in that bore only.
Thickness. The manufacturer does not state the thickness of the original gasket. A thicker gasket enlarges the combustion chamber and lowers the compression ratio; a thinner one reduces it and raises the ratio.
| Your goal | Thickness to choose |
| Keep the original compression ratio | Original thickness not published by the manufacturer: measure the one on the removed gasket and set it against the only gasket offered, 1.09 mm |
| Skimmed cylinder head, make up the lost height | Only one thickness is offered, 1.09 mm: set it against the removed gasket plus the material taken off |
| Forced induction build, lower the ratio | 1.09 mm, if the thickness measured on the original gasket at removal is smaller |
| Naturally aspirated build, raise the ratio | 1.09 mm, if the thickness measured on the original gasket at removal is greater |
As the original thickness is not published, use a dial gauge to measure how far the pistons stand proud of the block, especially if the removed gasket is thicker than 1.09 mm: that figure sets the clearance left between piston and head.
 | Cometic Gasket, Inc. is an American maker of gaskets and engine sealing solutions, created in 1989 in Concord, Ohio. It became known for the made-to-order gasket, cut to the bore and thickness requested. Its whole production stays in the United States, using American raw materials. |
| Criterion | Original gasket AJM302 | Reinforced gaskets |
| Construction | Construction not stated by the manufacturer | Pure copper, 99.9% (C110 grade) |
| Gasket bore | 68 mm | 67.5 mm |
| Thicknesses available | thickness not published by the manufacturer | 1.09 mm |
| Compression ratio adjustment | None | Through the thickness chosen |
| Resistance to overpressure | Limited to standard output | Designed for forced induction |
| Model | Engine code | Power | Years |
| Austin-Healey Sprite Mk I 0.9 | 9CG | 46 hp | 1960-1963 |
| Austin-Healey Sprite Mk II 0.9 | 9CG | 46 hp | 1961-1964 |
| Austin-Healey Sprite Mk II 1.1 | 10CC | 57 hp | 1962-1964 |
| Austin-Healey Sprite Mk III 1.1 | 10CC | 56 hp | 1964-1966 |
OEM part number: AJM302.
Important: the part number on this page fits only the 9CG and 10CC engines: check your vehicle's engine code before ordering.
The copper gasket is designed to hold where the original gasket reaches its limit: pure copper is ductile and does not turn brittle under heat, which keeps the seal between cylinder head and block.
As soon as a turbo or supercharger is added, or the compression ratio is raised, the head gasket becomes the weak link of the top end. Choosing a reinforced gasket is then a matter of safety more than comfort: it avoids a failure and the cylinder head removal that goes with it.
