Reinforced head gasket MG 18G 18GA MGB 1.8

The MG engines covered

The 18G and 18GA are 1.8 litre petrol engines. They power the MG MGB convertible and GT models listed in the compatibility table below.

The original gasket carries part number AJM377. MG publishes neither its bore, nor its thickness, nor its construction. An engine takes a single 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.

Reinforced cylinder head gaskets

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 check the condition of the head bolts and replace any that are stretched or damaged. Do this for the head bolts of your MGB.

This page brings together 22 Cometic part numbers, from the American maker of competition gaskets, for a single construction: MLS, multi-layer steel. Three bores are offered: 83 mm (C4147, in 18 thicknesses from 0.69 to 3.56 mm), 83.5 mm (C14048-040, H0794SP7030S and H0794SP7040S) and 84.5 mm (H0794SP1040S). The 83.5 mm bore comes in 0.76 mm and 1.02 mm; the 84.5 mm bore comes in 1.02 mm only. An engine takes a single gasket: order one part number. The thicknesses offered let you 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 only gaskets of the same bore: 83, 83.5 or 84.5 mm here.

Preparation and fitting

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. Also clean the gasket faces thoroughly before fitting. Do this on the head and on the block of your MGB.

Technical guide

The construction offered

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. All the part numbers on this page are offered in this construction. Here it serves the 1.8 litre engine of the MGB.

Choosing bore and thickness

Gasket bore. The Cometic MLS is offered in 83 mm (C4147), 83.5 mm (C14048-040, H0794SP7030S, H0794SP7040S) and 84.5 mm (H0794SP1040S).

Thickness. No thickness is published for the original gasket AJM377. A thicker gasket enlarges the combustion chamber and lowers the compression ratio; a thinner one reduces it and raises the ratio.

To choose a thickness below the original, first use a dial gauge to read how far the pistons stand above the block face: that figure gives the clearance left between piston and head. On this engine, take that measurement at each cylinder.

The brand

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 these MG engines are offered here in three bores.

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 to 84.5 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: AJM377.

Important: the part numbers on this page suit only the 18G and 18GA engines: check your MG's engine code before ordering.

Why switch to a reinforced gasket?

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 MG models on this page, it is the answer as soon as the engine goes beyond its original output.

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 these MG engines, the reinforced gasket is the first item to address once you go beyond the original output.