Reinforced head gasket Honda Prelude III 2.1 B21A1

The Honda engines covered

The B21A1 is a 2.1 litre (2056 cc) petrol engine with four in-line cylinders, 16 valves and double overhead camshafts. It powers the Honda Prelude III (BA) 2.1 listed below.

The original gasket carries the part number 12251-PK3-A01. Honda publishes neither its bore, its thickness nor its construction. An in-line four uses a single head gasket. Sized for the standard combustion pressure, it tires as soon as that pressure climbs: the clamping load fades, the head lifts and coolant enters a cylinder.

Reinforced cylinder head gaskets

Sealing depends on the fasteners that clamp the head to the block as well as on the gasket. A composite gasket crushes permanently beyond 100 N/mm²; the 301 stainless steel of MLS gaskets holds 517 N/mm², 5 times more, and springs back at every combustion.

This page brings together 36 part numbers from Cometic, the American competition gasket maker, for a single construction: MLS, a multilayer steel design. Eighteen gaskets (C4585) have an 83 mm bore and eighteen (C4586) an 84 mm bore; 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.

Technical specifications

A gasket's volume is the free space it leaves above each cylinder: it adds to the combustion chamber and lowers the compression ratio. Compare it only between gaskets of the same bore, here 83 mm for the C4585 and 84 mm for the C4586.

Preparation and fitting

Before fitting, check the flatness of the head and the block with a straight edge and have them resurfaced if needed. These Cometic gaskets arrive coated: do not apply sealant.

Technical guide

The construction offered

Cometic MLS Several stainless steel sheets, embossed and riveted together. The outer sheets carry active beads around each cylinder and each fluid passage. Moderate clamping is enough to seal and the bore distorts little: a versatile construction, from the tuned non-boosted engine to moderate boost.

Choosing bore and thickness

Gasket bore. The Cometic MLS is offered in 83 mm (C4585) and in 84 mm (C4586).

Thickness. No thickness is published for the original gasket 12251-PK3-A01. A thicker gasket enlarges the combustion chamber and lowers the compression ratio; a thinner one reduces it and raises the ratio.

Before choosing a thickness below the original, read with a dial gauge how far the pistons stand above the block's gasket face: it gives the clearance left between piston and head.

The brand

Cometic Gasket, Inc., an American company founded in 1989 in Concord, Ohio, designs and manufactures gaskets and sealing parts for engines. Its speciality is the custom gasket, cut to the bore and thickness wanted and made in the United States from American raw materials.

Advantages over the original gasket

When a reinforced gasket becomes necessary

• Adding forced induction, or raising boost on an already turbocharged engine
• Cylinder rebored beyond what the 83 mm and 84 mm bores of these gaskets covers
• Looking for a compression ratio different from standard, higher or lower
• Engine that has already lost its gasket, or is consuming coolant
• Competition use, where the gasket goes through repeated temperature rises

Vehicle compatibility

OEM part number: 12251-PK3-A01.

Important: the part numbers on this page suit only the B21A1 engine: check your vehicle's engine code before ordering.

Why switch to a reinforced gasket?

The reinforced gasket succeeds the original one when that has run out of margin: under higher combustion pressure it holds its shape instead of sagging.

Once the engine build goes further, with livelier cams, a higher compression ratio or a turbo added, the head gasket becomes the weak point of the top end. A reinforced gasket rules out the leak and the head removal that follows, a job much dearer than the part.