Cylinder head gasket on an old lined engine: asbestos, asbestos-free and liner protrusion

The real role of the cylinder head gasket

On an old lined engine, the cylinder head gasket must not be seen as a simple sheet placed between the block and the head. It is a complex sealing component, subjected to combustion pressure, temperature, the relative movements between head, block and liners, as well as the progressive loss of load linked to the settling of the gasket.

SAE publications from the 1960s to the 1980s show that the cylinder head gasket is one of the most difficult applications in terms of sealing. It must perform several functions at the same time: contain the combustion gases, prevent the passage of water, prevent the passage of oil, all while withstanding the deformations of the engine in operation. Around the combustion chamber, the gasket must receive a high local pressure and retain a certain elastic recovery capacity, because the engine is not a perfectly immobile assembly. The head, the block and the bolts move under the effect of combustion pressure and temperature. ^[1]

A cylinder head gasket works through contact pressure. For the seal to last, the pressure applied to the critical areas of the gasket must remain above a necessary minimum, even when the engine heats up, cools down, takes load or undergoes combustion pressure peaks.

This is particularly true around the fire ring. Old gaskets often use a metal ring, a flange or a fire ring around each cylinder. The role of this area is to concentrate a high specific pressure in order to contain the combustion gases. SAE publications of the time already describe this logic: the material or element placed around the chamber must have enough stiffness to generate a high load with little crush, while retaining enough conformability to follow the imperfections and movements of the assembly. ^[1]

This explains why a cylinder head gasket cannot be judged solely by its thickness or its appearance. Two visually similar gaskets can behave very differently depending on their compressibility, their relaxation, their recovery capacity, their density, their impermeability and the nature of their armature.

Asbestos cylinder head gaskets: composition and mechanical behaviour

Old cylinder head gaskets were often made of a metal armature combined with a soft material based on asbestos, fibres and organic binders. On old European engines, one finds for example perforated metal sheets covered with impregnated asbestos board, metal meshes with asbestos, or asbestos boards combined with metal sheets.

Asbestos had several qualities that explain its massive use: good thermal resistance, chemical inertness, reinforcing capacity, ease of industrial processing and relatively low cost. Technical publications of the time stress that asbestos did not only play a filling role: it contributed to the mechanical behaviour of the gasket, to its ability to conform to the surfaces, to holding up under temperature and to its ability to maintain the seal over time. ^[2]

This is an important point for the restoration of old engines: replacing a period gasket with a modern asbestos-free gasket does not just consist of replacing one fibre with another. Sometimes the complete mechanical behaviour of the gasket is changed.

The switch to asbestos-free gaskets

From the late 1970s and early 1980s, manufacturers sought to replace asbestos for health and regulatory reasons. But SAE publications show that this transition was not simple. Manufacturers explain that no single fibre alone reproduced all the properties of asbestos. It was therefore necessary to use blends of fibres, fillers, binders, or to completely rethink the structure of the gasket. ^[3]

Reinz, for example, explains in 1982 that replacing asbestos with simple alternative fibres can degrade quality if these fibres do not recover the elastic properties needed in the gasket area. The AFM 5 solution presented by Reinz does not simply consist of replacing asbestos with another fibre: part of the mechanical functions is transferred to a more elaborate load-bearing metal structure. ^[4]

Flexible graphite represents another development. Publications from the early 1980s show that it allows low relaxation, good thermal resistance and an interesting sealing capacity at lower stress. But here again, its behaviour is not that of an old organic gasket. ^[5]

The practical consequence is simple: on an old engine, one must be careful when comparing a period asbestos gasket, a re-manufactured organic asbestos-free gasket, a graphite gasket or a modern multi-layer gasket. The geometry of the engine has not changed, but the gasket may no longer react in the same way to crush, temperature and clamping pressure.

Why liner protrusion is so important

On a wet-liner engine, the liner is not only a friction surface for the piston. It also contributes to the seating geometry of the cylinder head gasket. Its protrusion above the plane of the block determines the local pressure exerted on the gasket around the cylinder.

Liner protrusion is therefore not a secondary dimension. It is a functional sealing dimension.

If the liner does not protrude enough, the local pressure around the fire ring can become insufficient. The gasket can then lose its ability to contain the combustion gases, especially after settling, temperature rise and load cycles. The gases can start to pass through, which can cause local overheating, attack of the gasket, corrosion, acidification of the coolant or cavitation on the water side of the liners.

If the liner protrudes too much, the opposite problem appears: the load concentrates excessively on the liner and on the fire ring. The gasket can then be locally crushed, the load distribution disturbed, certain elements deformed, or even the sealing efficiency reduced on the other areas of the gasket.

SAE publications do not give a universal protrusion value, because this value depends on the engine, the type of liner, the base gasket, the cylinder head gasket, the stiffness of the head and the block. On the other hand, they clearly show that the gasket load must be thought of as a complete system: head, bolts, gasket, block and liners. In a 1980 SAE paper, the height and stiffness of the liner are integrated into the calculation of the gasket load distribution. The document even distinguishes the working thickness between head and liner from that between head and block. ^[6]

This is exactly what is observed in practice on old Renault, Simca, Peugeot, Alfa engines, or other wet-liner engines: liner protrusion directly influences the effective clamping of the gasket around the fire ring.

Lined Renault engines: the protrusion dimension

On Renault wet-liner engines with paper base gaskets, of the Excelnyl type, or steel, the manufacturer's documentation indicates a correct protrusion between 0.04 mm and 0.12 mm. Base gaskets exist in several thicknesses, identified by colour, in order to obtain the correct liner height.

On other assemblies with O-ring base seals, the logic is different: the O-ring only ensures the water seal. The liner rests directly on the cylinder block, and the protrusion then depends on the manufacturing dimensions of the block and the liner. One must therefore not confuse a paper base gasket, which contributes to height adjustment, and an O-ring, which does not have this function. ^[7]

Why fit the liners in a graduated (stepped) arrangement

Renault documentation also indicates a rule that is often forgotten: the liners must be positioned so that the difference in protrusion between two neighbouring liners is at most 0.04 mm, and so that the protrusion is graduated from cylinder no. 1 to cylinder no. 4, or vice versa. In other words, the four liners must form a kind of progressive staircase, from the smallest protrusion to the largest, or from the largest to the smallest. ^[7]

This rule is very important. It means that it is not enough for each liner to be individually “within tolerance”. The set of four liners must also be consistent.

For example, four liners at 0.05 / 0.06 / 0.08 / 0.09 mm form a progressive set. Conversely, an arrangement such as 0.05 / 0.11 / 0.06 / 0.12 mm can be problematic, even if all the values individually appear to be between 0.04 and 0.12 mm. This arrangement creates a high/low/high alternation, thus an irregular load distribution on the gasket.

The graduated arrangement prevents a liner that is much higher than its neighbour from locally concentrating an excessive share of the clamping load. It also limits sudden breaks in contact pressure between two neighbouring cylinders. On an old organic gasket, which works through crush, densification and partial recovery, this regularity is essential.

Liner height, a pressure dimension more than a geometry dimension

The best way to understand liner protrusion is therefore not to see it as a simple measurement with a dial gauge. It is a way of setting the pressure applied to the cylinder head gasket around the cylinder.

The gasket must remain sufficiently compressed after clamping, after relaxation, after temperature rise and during the combustion cycles. SAE publications show that the gasket is never in a perfectly static situation. It is subjected to dynamic movements due to combustion pressure and to quasi-static movements linked to temperature variations. ^[8]

This is why an engine can run for a while with a borderline liner height, then end up burning a cylinder head gasket. When cold, at assembly, the value may seem acceptable. But in operation, the local pressure can become insufficient, especially with a gasket that relaxes, a head that works, bolts that lose part of their load, or an engine that runs hotter than originally.

What to remember: protrusion, pressure, sealing

On an old lined engine, liner protrusion is a critical dimension. It determines the local pressure of the gasket around the fire ring, and therefore the seal against combustion gases.

Old asbestos-based gaskets had mechanical and thermal qualities that are difficult to reproduce. Modern asbestos-free gaskets can be excellent, but they do not necessarily behave like period gaskets. Their compressibility, their recovery, their relaxation and their contact pressure requirement can be different.

For this reason, during an engine rebuild, one must carefully check the protrusion of each liner, respect the manufacturer's tolerances, avoid excessive differences between neighbouring liners, and aim for a graduated arrangement when the documentation prescribes it.

On these engines, the success of a cylinder head gasket does not depend only on the tightening torque or the quality of the gasket. It also depends on the actual geometry of the block, the height of the liners, the cleanliness of the seating faces, the regularity of the protrusions and the compatibility between the gasket used and the original assembly.

Sources

^[1] SAE 650431 — The cylinder head gasket must ensure several different seals; the combustion zone requires high thermal resistance, high unit load and recovery; relaxation, stiffness and torque loss are central.

^[2] SAE 700025 — Describes the conventional European gaskets of the time: perforated sheet with asbestos board, metal mesh with asbestos, asbestos board with metal sheets; the chambers are surrounded by a metal flange to obtain the necessary specific pressure.

^[3] SAE 810364 — Asbestos offered thermal resistance, chemical inertness, structural reinforcement, ease of processing and low cost; no single fibre perfectly replaces asbestos.

^[4] SAE 820141 — Asbestos-free gaskets are problematic because no material takes up all the properties of asbestos; Reinz AFM 5 transfers part of the mechanical role to a load-bearing metal structure.

^[5] SAE 830214 — Flexible graphite as an asbestos-free material with low relaxation, with good temperature/pressure resistance, but different from elastomer-bonded organic materials.

^[6] SAE 800071 — Treats the gasket as a complete engine/gasket/environment system; distinguishes the fire ring/flange/body zones, integrates the block/liner stiffnesses, and defines the working thickness between head and liner surface.

^[7] Renault manual MR-MOT-C(E) — Protrusion 0.04 to 0.12 mm for the assemblies concerned, blue/red/green Excelnyl gaskets, O-rings ensuring only the seal, maximum difference between two neighbouring liners of 0.04 mm, and graduated arrangement from cylinder 1 to 4 or vice versa.

^[8] SAE 700025 — The head/block seal is not static: the minimum local pressure must never drop below the necessary value, despite the dynamic and quasi-static movements of the joint plane.