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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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Cometic copper reinforced gaskets, sold individually: one gasket per cylinder bank, so two per engine.
The M100 petrol engine of 6.3 litres (6,332 cc), a V8, codes M100.980, M100.981 and M100.982, powers the Mercedes-Benz 300 SEL 6.3 and 600 detailed below. It takes two cylinder head gaskets, one per cylinder bank.
Mercedes-Benz gives neither the part number, nor the bore, nor the thickness, nor the construction of the original gaskets of this V8. Sized for the standard combustion pressure, they no longer hold when it rises: the clamping load drops, the head lifts and coolant passes into a cylinder.
The gasket does not seal on its own: it also depends on the fasteners that clamp the cylinder head onto the block. Copper at 99.9% resists corrosion, deforms without breaking and carries heat away well, with no brittleness at high temperature. On this V8, torquing in several passes keeps the pressure even.
This page offers two part numbers from the American maker Cometic, a specialist in racing gaskets, in one construction only: copper. Their bore is 104.14 mm and their thickness 1.60 mm. The V8 takes one gasket per cylinder bank: order both part numbers, H4338063C for the left bank (LHS) and H4339063C for the right bank (RHS).
| Gasket bore | Thickness | Gasket volume | Gasket type OEM not specified | Brand | Part number |
| 104.1 mm | 1.60 mm | 13.62 | Copper | Cometic | H4338063C |
| 104.1 mm | 1.60 mm | 13.62 | Copper | Cometic | H4339063C |
A gasket's volume is the space it keeps free above each cylinder. That space joins the combustion chamber and lowers the compression ratio; only gaskets of the same bore are compared. Here, both gaskets have a 104.14 mm bore.
| Fasteners | Engines | Tightening torque |
| Original cylinder head bolts | M100.980, M100.981, M100.982 | not published by the manufacturer |
Before fitting, run a straight edge over the cylinder head and block to check they are flat, and have them resurfaced if needed. Cometic checks the flatness of each copper gasket it makes. This check holds for the V8 of 6.3 litres as for any other engine.
| Cometic copper A plate of pure oxygen-free copper, grade C110 (99.9% copper), which resists corrosion, crushes without cracking, carries heat away well and stays supple at high temperature. Cometic checks the flatness of every gasket and advises, for the best result, fitting it with a sealing ring crimped into the head or the block. |
Gasket bore. The Cometic copper gasket is offered in 104.14 mm (H4338063C and H4339063C), with no other bore.
Thickness. The manufacturer gives no thickness for the original gaskets of this engine. 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: measure a removed gasket and compare its size with the only gasket offered, 1.60 mm |
| Skimmed cylinder head, make up the lost height | Only one thickness is offered, 1.60 mm: compare it with that of the removed gasket, plus the metal taken off when resurfacing |
| Forced induction build, lower the ratio | 1.60 mm, if the thickness measured on the original gasket at removal is lower |
| Naturally aspirated build, raise the ratio | 1.60 mm, if the thickness measured on the original gasket at removal is higher |
With no published original thickness, measure with a dial gauge how far the pistons stand proud of the block's gasket face, above all if the removed gasket is thinner than 1.60 mm: this reading gives the free space between piston and head.
 | Cometic Gasket, Inc. is an American maker of gaskets and sealing parts for engines, born in 1989 in Concord, Ohio. It made a name with the custom gasket, cut to the bore and thickness wanted, made entirely in the United States from American raw materials. The part numbers H4338063C and H4339063C are its own. |
| Criterion | Original gaskets, part number not published | Reinforced gaskets |
| Construction | Construction not specified by the manufacturer | Pure copper, 99.9% (grade C110) |
| Gasket bore | not published by the manufacturer | 104.14 mm |
| Thicknesses available | thickness not published by the manufacturer | 1.60 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 |
| Mercedes-Benz 300 SEL 6.3 (W109) | M100.981 | 250 hp | 1968-1972 |
| Mercedes-Benz 600 (W100), Pullmann | M100.980 | 250 hp | 1964-1979 |
| Mercedes-Benz 600 (W100), Pullmann | M100.982 | 250 hp | 1964-1979 |
OEM part number: not published by the manufacturer.
Important: the part numbers on this page suit the Mercedes-Benz M100.980, M100.981 and M100.982 engines of 6.3 litres: check your vehicle's engine code before ordering. The M100.985 engine of 6.9 litres is not covered: its 107 mm bore is larger than that of the gaskets offered.
When the original gasket reaches its ceiling, this copper gasket takes over: pure copper stays ductile when hot and does not turn brittle, which preserves the seal between head and block. The V8 of 6.3 litres finds here two gaskets with a 104.14 mm bore.
With a turbo, a supercharger or a raised compression ratio, the head gasket becomes the first breaking point of the top end. A reinforced gasket rules out the leak and the cylinder head removal that follows, a job far costlier than the gasket. This holds for the V8 of 6.3 litres as for any hard-driven engine.
