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2.5L 12v - M30B25
Manufacturers
2.5L 12v - M30B25
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Reinforced Cometic head gasket, in MLS. Sold individually, one gasket per engine.
The BMW M30B25 and M30B28 are naturally aspirated twelve-valve inline-six petrol engines, of 2.5 litres for the M30B25 and 2.8 litres for the M30B28. They are fitted to the BMW E3, E9, E12, E23, E24 and E28 models listed below.
This original gasket carries the part number 11 12 1 730 745, which replaces 11 12 1 730 746. BMW lists it as an asbestos-free cylinder head gasket without specifying its construction; its bore is 88 mm. BMW does not publish its thickness in the standard size: its repair size, 11 12 1 730 748, measures 2.07 mm (+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.
The head gasket does not guarantee the seal on its own: the seal depends on the fasteners, which clamp the cylinder head against the block. Beyond 100 N/mm² of admissible stress, a composite gasket is crushed for good, 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 88 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 88 mm | Thickness | Gasket volume | Gasket type OEM not specified | Brand | Part number OEM 11 12 1 730 745 |
| 88 mm | 0.69 mm | 4.20 | MLS | Cometic | C4475-027 |
| 88 mm | 0.76 mm | 4.62 | MLS | Cometic | C4475-030 |
| 88 mm | 0.91 mm | 5.53 | MLS | Cometic | C4475-036 |
| 88 mm | 1.02 mm | 6.20 | MLS | Cometic | C4475-040 |
| 88 mm | 1.14 mm | 6.93 | MLS | Cometic | C4475-045 |
| 88 mm | 1.30 mm | 7.91 | MLS | Cometic | C4475-051 |
| 88 mm | 1.42 mm | 8.64 | MLS | Cometic | C4475-056 |
| 88 mm | 1.52 mm | 9.24 | MLS | Cometic | C4475-060 |
| 88 mm | 1.68 mm | 10.22 | MLS | Cometic | C4475-066 |
| 88 mm | 1.78 mm | 10.83 | MLS | Cometic | C4475-070 |
| 88 mm | 1.91 mm | 11.62 | MLS | Cometic | C4475-075 |
| 88 mm | 2.03 mm | 12.35 | MLS | Cometic | C4475-080 |
| 88 mm | 2.18 mm | 13.26 | MLS | Cometic | C4475-086 |
| 88 mm | 2.34 mm | 14.23 | MLS | Cometic | C4475-092 |
| 88 mm | 2.49 mm | 15.14 | MLS | Cometic | C4475-098 |
| 88 mm | 3.05 mm | 18.55 | MLS | Cometic | C4475-120 |
| 88 mm | 3.56 mm | 21.65 | MLS | Cometic | C4475-140 |
Gasket volume is the room the gasket leaves free above each cylinder. That volume adds to the combustion chamber: the larger it is, the lower the compression ratio. The comparison only makes sense between gaskets of the same bore.
| Fasteners | Engines | Tightening torque |
| Original cylinder head bolts 11 12 0 621 144, M12 x 156 | M30B25, M30B28 | 60 Nm, 20 min pause, 80 Nm, hot engine 25 min, then 35° (new bolts, washed and oiled; TIS procedure in force) |
Before assembly, run a straight edge across the cylinder head and the block to check they are flat, and have them machined if necessary. These gaskets need no sealing paste: Cometic supplies them already coated.
| Reinforced ARP cylinder head studs BMW M30 → |
 | Cometic MLS An assembly of stamped stainless steel sheets riveted together. The outer sheets have 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 versatile solution, from the tuned naturally aspirated engine to moderate forced induction. |
Gasket bore. The Cometic MLS comes in 88 mm (C4475), and in that bore only.
Thickness. BMW does not publish the thickness of the original gasket in the standard size; the repair size (+0.30 mm) measures 2.07 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 | Original thickness not published by the manufacturer: measure the removed gasket and take the closest value offered |
| Skimmed cylinder head, make up the lost height | The offered value closest to the measured thickness of the original gasket + what was removed |
| Forced induction build, lower the ratio | A thickness greater than that of the original gasket measured at removal |
| Naturally aspirated build, raise the ratio | A thickness smaller than that of the original gasket measured at removal, down to 0.69 mm, the thinnest offered |
If the chosen thickness is below the original one, use a dial gauge to read how far the pistons stand above the block: it determines the clearance left between piston and head.
 | Cometic Gasket, Inc. is an American manufacturer of engine gaskets and sealing solutions, created in 1989 in Concord, Ohio. Its reputation rests on the gasket made to order, in the bore and thickness wanted. All manufacturing remains in the United States, using American materials. |
| Criterion | Original gasket 11 12 1 730 745 | Reinforced gaskets |
| Construction | Construction not stated by the manufacturer | Multi-layer stainless steel (MLS) |
| Gasket bore | 88 mm | 88 mm |
| Thicknesses available | thickness not published by the manufacturer, 2.07 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 |
| 2500 (E3) | M30B25 | 150 hp | 1968-1977 |
| 2.8 Bavaria, 2.8 L, 2800 (E3) | M30B28 | 170 and 173 hp | 1968-1977 |
| 2.5 CS (E9) | M30B25 | 150 hp | 1974-1976 |
| 2800 CS (E9) | M30B28 | 170 and 192 hp | 1967-1974 |
| 525 (E12) | M30B25 | 150 hp | 1975-1981 |
| 528, 528 i (E12) | M30B28 | 165, 170, 171, 177 and 184 hp | 1975-1981 |
| 725 i (E23) | M30B25 | 150 hp | 1977-1986 |
| 728, 728 i (E23) | M30B28 | 170 and 184 hp | 1977-1986 |
| 628 CSi (E24) | M30B28 | 184 hp | 1979-1987 |
| 525 i (E28) | M30B25 | 150 hp | 1981-1987 |
| 528 i (E28) | M30B28 | 184 hp | 1981-1987 |
OEM part number: 11 12 1 730 745, which replaces 11 12 1 730 746; repair size 11 12 1 730 748.
Important: the part numbers on this page fit only the M30B25 and M30B28 engines: check your vehicle's engine code before ordering.
When the original gasket reaches its limit, the reinforced gasket still resists: under higher combustion pressure it keeps its shape instead of being crushed.
When power goes up, through a supercharger, a turbocharger or a raised compression ratio, the head gasket is the first part to give way in the top end. Choosing a reinforced gasket therefore puts reliability ahead of comfort: you avoid the failure, and the cylinder head removal that follows it.
