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1.8L 16v - BP
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1.8L 16v - BP
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Reinforced Cometic MLS head gasket for the Mazda BP-4W engine of the MX-5 NB, in an 84 or 85 mm bore.
The BP-4W is a 1.8 litre naturally aspirated petrol engine. It powers the Mazda MX-5 II (NB) listed in the compatibility table below.
The original gasket carries part number BP6D-10-271. Mazda publishes neither its bore, nor its thickness, nor its construction. The engine takes one cylinder head gasket, sized for the standard combustion pressure; when that pressure climbs, the clamping force eases, the head lifts slightly and coolant seeps into a cylinder.
Sealing does not rest on the gasket alone but also on the fasteners that press the head onto the block. Beyond 100 N/mm², a composite gasket compresses for good; the 301 stainless steel of MLS gaskets takes 517 N/mm², 5 times more, and recovers its shape at every combustion. Also examine the head bolts and change any that have stretched or deteriorated.
This page brings together 34 Cometic part numbers, from the American maker of competition gaskets, in MLS construction, multi-layer steel: 17 C4983 part numbers with an 84 mm bore and 17 C4984 part numbers with an 85 mm bore, from 0.69 to 3.56 mm thick. The engine takes only one gasket: order a single part number.
| Gasket bore | Thickness | Gasket volume | Gasket type OEM not specified | Brand | Part number OEM BP6D-10-271 |
| 84 mm | 0.69 mm | 3.82 | MLS | Cometic | C4983-027 |
| 84 mm | 0.76 mm | 4.21 | MLS | Cometic | C4983-030 |
| 84 mm | 0.91 mm | 5.04 | MLS | Cometic | C4983-036 |
| 84 mm | 1.02 mm | 5.65 | MLS | Cometic | C4983-040 |
| 84 mm | 1.14 mm | 6.32 | MLS | Cometic | C4983-045 |
| 84 mm | 1.30 mm | 7.20 | MLS | Cometic | C4983-051 |
| 84 mm | 1.42 mm | 7.87 | MLS | Cometic | C4983-056 |
| 84 mm | 1.52 mm | 8.42 | MLS | Cometic | C4983-060 |
| 84 mm | 1.68 mm | 9.31 | MLS | Cometic | C4983-066 |
| 84 mm | 1.78 mm | 9.86 | MLS | Cometic | C4983-070 |
| 84 mm | 1.91 mm | 10.58 | MLS | Cometic | C4983-075 |
| 84 mm | 2.03 mm | 11.25 | MLS | Cometic | C4983-080 |
| 84 mm | 2.18 mm | 12.08 | MLS | Cometic | C4983-086 |
| 84 mm | 2.34 mm | 12.97 | MLS | Cometic | C4983-092 |
| 84 mm | 2.49 mm | 13.80 | MLS | Cometic | C4983-098 |
| 84 mm | 3.05 mm | 16.90 | MLS | Cometic | C4983-120 |
| 84 mm | 3.56 mm | 19.73 | MLS | Cometic | C4983-140 |
| 85 mm | 0.69 mm | 3.92 | MLS | Cometic | C4984-027 |
| 85 mm | 0.76 mm | 4.31 | MLS | Cometic | C4984-030 |
| 85 mm | 0.91 mm | 5.16 | MLS | Cometic | C4984-036 |
| 85 mm | 1.02 mm | 5.79 | MLS | Cometic | C4984-040 |
| 85 mm | 1.14 mm | 6.47 | MLS | Cometic | C4984-045 |
| 85 mm | 1.30 mm | 7.38 | MLS | Cometic | C4984-051 |
| 85 mm | 1.42 mm | 8.06 | MLS | Cometic | C4984-056 |
| 85 mm | 1.52 mm | 8.63 | MLS | Cometic | C4984-060 |
| 85 mm | 1.68 mm | 9.53 | MLS | Cometic | C4984-066 |
| 85 mm | 1.78 mm | 10.10 | MLS | Cometic | C4984-070 |
| 85 mm | 1.91 mm | 10.84 | MLS | Cometic | C4984-075 |
| 85 mm | 2.03 mm | 11.52 | MLS | Cometic | C4984-080 |
| 85 mm | 2.18 mm | 12.37 | MLS | Cometic | C4984-086 |
| 85 mm | 2.34 mm | 13.28 | MLS | Cometic | C4984-092 |
| 85 mm | 2.49 mm | 14.13 | MLS | Cometic | C4984-098 |
| 85 mm | 3.05 mm | 17.31 | MLS | Cometic | C4984-120 |
| 85 mm | 3.56 mm | 20.20 | MLS | Cometic | C4984-140 |
A gasket's volume is the free space it leaves above each cylinder; it adds to the combustion chamber and lowers the compression ratio. At equal thickness, the 85 mm part number leaves slightly more volume than the 84 mm one: compare two part numbers of the same bore.
| Fasteners | Engines | Tightening torque |
| Original cylinder head bolts B660-10-135A | BP-4W | None |
Before reassembly, check the flatness of the head and block mating faces with a straightedge and have them resurfaced if needed. Cometic gaskets arrive already coated: do not apply any sealing paste. Before fitting, clean the gasket faces with care.
 | Cometic MLS Several stainless steel sheets, stamped and riveted together. Each outer sheet carries active beads tracing the outline of every cylinder and fluid passage. A moderate clamping load is enough to seal, and the bore distorts very little. This versatile gasket suits anything from a tuned naturally aspirated engine to light boost. The 34 part numbers on this page are offered in this construction. |
Gasket bore. The Cometic MLS is offered here in 84 mm (C4983) and in 85 mm (C4984).
Thickness. No thickness is published for the original gasket BP6D-10-271. 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: read it on the removed gasket and keep, among the offered values, the nearest one |
| Skimmed cylinder head, make up the lost height | The offered value nearest the figure read on the removed gasket, increased by whatever the machinist took off the head or block |
| Forced induction build, lower the ratio | A thickness greater than that of the removed gasket |
| Naturally aspirated build, raise the ratio | A thickness smaller than the removed gasket's, down to the 0.69 mm minimum |
To go below the original thickness, first read with a dial gauge the height by which the pistons stand above the block face: it sets the clearance that remains with the head.
 | Cometic Gasket, Inc. is an American company founded in 1989 and based in Concord, Ohio, which designs and builds gaskets and sealing parts for engines. It earned its name with made-to-measure gaskets, cut to the bore and thickness required, all made in the United States from American materials. Its part numbers for this Mazda engine are offered here in 84 mm and in 85 mm. |
| Criterion | Original gasket BP6D-10-271 | Reinforced gaskets |
| Construction | Construction not stated by the manufacturer | Multi-layer stainless steel (MLS) |
| Gasket bore | not published | 84 mm (C4983) and 85 mm (C4984) |
| Thicknesses available | not published by the manufacturer | 0.69 to 3.56 mm (17 values per bore) |
| Compression ratio adjustment | None | Through the thickness chosen |
| Resistance to overpressure | Limited to standard output | Designed for forced induction |
| Model | Engine code | Power | Years |
| MX-5 II (NB) 1.8 i (NB8C) | BP-4W | 154 hp | 2002-04 à 2005-10 |
OEM part number: BP6D-10-271.
Important: the part numbers on this page suit only the BP-4W engine: check your Mazda's engine code before ordering.
When the original gasket is no longer enough, the reinforced gasket replaces it: under higher combustion pressure it keeps its shape instead of collapsing. For the Mazda on this page, it is the answer as soon as the engine is built beyond its original configuration.
Whether you add a turbo, raise the compression ratio or rev higher with livelier cams, the cylinder head gasket becomes the weak link of a high-output engine. A reinforced gasket prevents the leak and the head removal that goes with it, far costlier than the gasket. On this naturally aspirated engine, the reinforced gasket matters most when the compression ratio or the output goes up, or when a turbo is added.
