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2.3L 12v - M20B23
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2.3L 12v - M20B23
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Reinforced Cometic head gasket, in MLS. Sold individually, one gasket per engine.
The BMW M20B20 and M20B23 are twelve-valve inline-six petrol engines, of 2.0 litres for the M20B20 and 2.3 litres for the M20B23. They are fitted to the BMW E12, E21, E28, E30 and E34 models listed below.
This original gasket carries the part number 11 12 1 722 728, now discontinued. BMW does not specify its construction; its bore is 81 mm and its thickness 1.88 mm, or 2.08 mm in the repair size (+0.30 mm). The engine takes only one. It is intended for standard combustion pressure: as that pressure rises, clamping load fades, the cylinder head lifts and coolant passes into a cylinder.
A head gasket does not provide the seal alone: it depends on the fasteners, which press the head against the block. A composite gasket is permanently crushed beyond 100 N/mm² of admissible stress, whereas the 301 stainless steel of MLS gaskets withstands 517 N/mm², 5 times more, and regains its shape at every combustion.
This page gathers the part numbers of Cometic, an American maker of racing gaskets, in a single construction: the multi-layer steel MLS. Every gasket has an 81 mm bore, identical to that of the original gasket, and thicknesses run from 0.69 to 3.56 mm, enough to adjust the compression ratio without touching the pistons or the cylinder head. As a rule, the gasket bore chosen is at least 0.5 mm larger than the actual cylinder bore. Each part number is sold individually, one gasket per engine.
| Gasket bore OEM 81 mm | Thickness OEM 1.88 mm | Gasket volume OEM 9.69 cm³ | Gasket type OEM not specified | Brand | Part number OEM 11 12 1 722 728 |
| 81 mm | 0.69 mm | 3.56 | MLS | Cometic | C4393-027 |
| 81 mm | 0.76 mm | 3.92 | MLS | Cometic | C4393-030 |
| 81 mm | 0.91 mm | 4.69 | MLS | Cometic | C4393-036 |
| 81 mm | 1.02 mm | 5.26 | MLS | Cometic | C4393-040 |
| 81 mm | 1.14 mm | 5.87 | MLS | Cometic | C4393-045 |
| 81 mm | 1.30 mm | 6.70 | MLS | Cometic | C4393-051 |
| 81 mm | 1.42 mm | 7.32 | MLS | Cometic | C4393-056 |
| 81 mm | 1.52 mm | 7.83 | MLS | Cometic | C4393-060 |
| 81 mm | 1.68 mm | 8.66 | MLS | Cometic | C4393-066 |
| 81 mm | 1.78 mm | 9.17 | MLS | Cometic | C4393-070 |
| 81 mm | 1.91 mm | 9.84 | MLS | Cometic | C4393-075 |
| 81 mm | 2.03 mm | 10.46 | MLS | Cometic | C4393-080 |
| 81 mm | 2.18 mm | 11.23 | MLS | Cometic | C4393-086 |
| 81 mm | 2.34 mm | 12.06 | MLS | Cometic | C4393-092 |
| 81 mm | 2.49 mm | 12.83 | MLS | Cometic | C4393-098 |
| 81 mm | 3.05 mm | 15.72 | MLS | Cometic | C4393-120 |
| 81 mm | 3.56 mm | 18.34 | MLS | Cometic | C4393-140 |
Gasket volume is the free space left above each cylinder. It is added to the combustion chamber, so a larger volume lowers the compression ratio. Compare these volumes only between gaskets of the same bore.
| Fasteners | Engines | Tightening torque |
| Original cylinder head bolts | M20B20, M20B23 | not published by the manufacturer |
Before fitting the gasket, check with a straight edge that the cylinder head and block are flat, and have them machined if needed. No sealant is required: Cometic supplies these gaskets already coated.
 | Cometic MLS Stamped stainless steel sheets joined by riveting. The outer sheets carry active beads around each cylinder and each fluid passage. A moderate clamping load is enough to seal, and the bore distorts little. It is the all-round solution, from the tuned naturally aspirated engine to moderate boost. |
Gasket bore. The Cometic MLS comes in 81 mm (C4393), and in that bore only.
Thickness. The original gasket is 1.88 mm thick, and 2.08 mm in the repair size (+0.30 mm). 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 | 1.78 mm or 1.91 mm, the two values closest to the original 1.88 mm |
| Skimmed cylinder head, make up the lost height | 1.78 mm or 1.91 mm + what was removed |
| Forced induction build, lower the ratio | A thickness above 1.91 mm |
| Naturally aspirated build, raise the ratio | A thickness below 1.78 mm, down to 0.69 mm, the thinnest offered |
If the chosen thickness is below the original, measure with a dial gauge how far your pistons protrude from the block: this sets the clearance remaining between piston and head.
 | Cometic Gasket, Inc. is an American company founded in 1989 in Concord, Ohio, which designs gaskets and sealing solutions for engines. It built its name on the gasket made to order, in the bore and thickness requested. All of its production stays in the United States, with American materials. |
| Criterion | Original gasket 11 12 1 722 728 | Reinforced gaskets |
| Construction | Construction not stated by the manufacturer | Multi-layer stainless steel (MLS) |
| Gasket bore | 81 mm | 81 mm |
| Thicknesses available | 1.88 mm, 2.08 mm in the repair size | 0.69 to 3.56 mm, 17 values |
| Compression ratio adjustment | None | Through the thickness chosen |
| Resistance to overpressure | Limited to standard output | Designed for forced induction |
| Model | Engine code | Power | Years |
| 320/6 (E21) | M20B20 | 122 hp | 1977-1982 |
| 320 i, 320 i Touring, 320 i Cabriolet (E30) | M20B20 | 125, 126 and 129 hp | 1982-1993 |
| 520/6, 520 i/6 (E12) | M20B20 | 122 hp | 1976-1981 |
| 520 i (E28) | M20B20 | 125, 129 and 150 hp | 1981-1987 |
| 520 i (E34) | M20B20 | 129 hp | 1987-1990 |
| 323 i, 323 i Baur TC (E21) | M20B23 | 143 hp | 1978-1982 |
| 323 i, 323 i Cabriolet (E30) | M20B23 | 139 and 150 hp | 1982-1986 |
OEM part number: 11 12 1 722 728, repair size 11 12 1 722 729.
Important: the part numbers on this page fit only the M20B20 and M20B23 engines: check your vehicle's engine code before ordering.
Where the original gasket reaches its limit, the reinforced gasket carries on holding: at higher combustion pressure it keeps its shape instead of being crushed.
As soon as power rises, with a supercharger, a turbo or a higher compression ratio, the head gasket is the first part to fail in the top end. Moving to a reinforced gasket therefore means betting on reliability rather than comfort: it spares you the breakdown and then the removal of the cylinder head.
