Reinforced head gasket MG 12H Midget

The MG engines covered

The 12H is a BMC A-Series petrol engine. It powers the MG Midget listed in the compatibility table below.

The original gasket carries part number AEG226. 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 Midget.

This page brings together 18 Cometic part numbers, from the American maker of competition gaskets, for a single construction: MLS, multi-layer steel. A single bore is offered, 74 mm, in 18 thicknesses from 0.69 to 3.56 mm. 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: 74 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 Midget.

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 12H engine of the MG Midget.

Choosing bore and thickness

Gasket bore. The Cometic MLS is offered in a single bore, 74 mm (C4148).

Thickness. No thickness is published for the original gasket AEG226. A thicker gasket enlarges the combustion chamber and lowers the compression ratio; a thinner one reduces it and raises the ratio.

Before settling on a thickness smaller than the original, use a dial gauge to measure how far the pistons stand proud of the block face: that value shows the clearance left between piston and head. On this engine, measure it at each cylinder.

The brand

Founded in 1989 and based in Concord, Ohio, Cometic Gasket, Inc. is an American company that designs and builds gaskets and sealing parts for engines. Its reputation rests on made-to-measure gaskets, cut to the bore and thickness requested, all produced in the United States from American materials. For the MG Midget, its part numbers are offered here in a single bore.

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 74 mm bore 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: AEG226.

Important: the part numbers on this page suit only the 12H engine: check your MG's engine code before ordering.

Why switch to a reinforced gasket?

Once the original gasket reaches its limit, the reinforced gasket takes over: at higher combustion pressure it keeps its shape rather than settling. For the MG on this page, it is called for as soon as the engine exceeds its original output.

A turbo added, a higher compression ratio or more revs with livelier cams: in every case the cylinder head gasket becomes the weak link of the tuned engine. A reinforced gasket avoids the leak and the head removal that follows, far costlier than the gasket itself. On this MG engine it is the first thing to deal with once you exceed the original output.