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BY REFERENCE COMETIC - Reinforced cylinder head gaskets
Manufacturers
BY REFERENCE COMETIC - Reinforced cylinder head gaskets
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Reinforced Cometic head gasket in CFM-20 for the Jaguar XK6 straight-six engines of 3.4 and 3.8 litres, with the water holes at the rear. Sold individually.
The Jaguar XK6 straight-six engines, of 3.4 and 3.8 litres, power the Jaguar 340, the Mark 1, Mark 2, Mark VIII, Mark IX and Mark X, the XK150 and the XKE, listed below.
No original gasket part number is confirmed for these engines in our sources, so the original bore, construction and thickness remain unstated. The original gasket is sized for standard combustion pressure; as soon as it rises, the head loosens, lifts and coolant finds its way into a cylinder.
The gasket alone does not guarantee sealing: the bolts or studs that clamp the head to the block weigh just as much. The CFM-20 combines a galvanized steel core with a soft asbestos-free compound that fills small flaws in the sealing faces.
This page offers one part number from the American maker Cometic, a specialist in racing gaskets: the C4126, in CFM-20, with an 89 mm bore and a thickness of 1.50 mm, with the water holes at the rear. It is sold individually.
| Gasket bore | Thickness | Gasket volume | Gasket type OEM not specified | Brand | Part number |
| 89 mm | 1.50 mm | 9.33 | CFM-20 | Cometic | C4126 |
A gasket's volume is the empty space it leaves over each cylinder; it adds to the combustion chamber and lowers the compression ratio. The comparison only makes sense between gaskets of the same bore.
| Fasteners | Engines | Tightening torque |
| Original cylinder head bolts | XK6 3.4 L, XK6 3.8 L | not published by the manufacturer |
Before fitting, check the flatness of the head and block with a straightedge, then have whichever part needs it resurfaced. Cometic checks the flatness of every CFM-20 gasket before it ships.
| CFM-20 Cometic This construction layers a galvanized steel core with, on both faces, a soft asbestos-free compound blending aramid fibres, mineral fillers and elastomer binders. Between metal and fibre, it combines the strength of steel with the softness of the compound. A single model is offered here, with an 89 mm bore and a thickness of 1.50 mm, with the water holes at the rear. |
Gasket bore. The Cometic CFM-20 is offered in 89 mm (C4126), with no other bore.
Thickness. We have no confirmed original thickness for these engines. 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 unknown: measure the removed gasket and compare it with the only gasket offered, 1.50 mm |
| Skimmed cylinder head, make up the lost height | Only one thickness exists, 1.50 mm: weigh it against that of the removed gasket, plus whatever resurfacing took off |
| Forced induction build, lower the ratio | 1.50 mm, if the thickness measured on the original gasket at removal is smaller |
| Naturally aspirated build, raise the ratio | 1.50 mm, if the thickness measured on the original gasket at removal is greater |
With no confirmed original thickness, read with a dial gauge how far the pistons protrude above the block's gasket face, above all if the removed gasket is thinner than 1.50 mm: that figure gives the clearance left between piston and head.
 | Cometic Gasket, Inc., an American company founded in 1989 in Concord, Ohio, makes gaskets and sealing solutions for engines. It is known for the made-to-measure gasket, cut to the bore and thickness wanted, and makes everything in the United States from American raw materials. |
| Criterion | Original gasket not confirmed | Reinforced gaskets |
| Construction | Construction not confirmed | Galvanized steel core and asbestos-free compound (CFM-20) |
| Gasket bore | not confirmed | 89 mm |
| Thicknesses available | thickness not confirmed | 1.50 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 |
| Jaguar 340 | XK6 3.4 L / 3.8 L | not published | 1967-1968 |
| Jaguar Mark 1 | XK6 3.4 L / 3.8 L | not published | 1958-1959 |
| Jaguar Mark 2 | XK6 3.4 L / 3.8 L | not published | 1960-1968 |
| Jaguar Mark VIII | XK6 3.4 L / 3.8 L | not published | 1958 |
| Jaguar Mark IX | XK6 3.4 L / 3.8 L | not published | 1959-1961 |
| Jaguar Mark X | XK6 3.4 L / 3.8 L | not published | 1962-1964 |
| Jaguar XK150 | XK6 3.4 L / 3.8 L | not published | 1958-1961 |
| Jaguar XKE | XK6 3.4 L / 3.8 L | not published | 1961-1964 |
OEM part number: no confirmed original part number.
Important: the part number on this page is listed by Cometic for the models in the table above and its profile has water holes at the rear: check the 89 mm bore and the position of the water passages before ordering.
When the original gasket has given all it could, this CFM-20 replaces it: its steel core withstands combustion pressure without deforming, and the soft compound on each face follows the small flaws of the head and block.
When an engine is run close to its limits, the head gasket is often the first thing to go. The reinforced gasket prevents the leak and costs far less than the head removal it would force, without keeping the car off the road for long.
