Head gasket

To accomplish combustion sealing with standard copper head gaskets, grooves are machined into the block or the head outside of the combustion sealing area to a width that will retain a stainless wire by friction resistance, the depth of the groove is determined by subtracting the desired protrusion (height) of the installed o-ring from the wire diameter. Stainless steel wire (most often .041" diameter) is then seated into the groove by tapping with a soft faced hammer or other tool such as plastic or wood (hard faced hammers can cause dents which create combustion leakage paths). It is advisable to begin and end the o-ring nearest a bolt location to take full sealing advantage of increased loading near the bolt upon the joint in the o-ring.  

Although copper is a relatively soft material, there is a limit to which it can be displaced by an o-ring. Generally speaking, this limit is about 25% of the gasket thickness. For instance, with a 0.032" thick gasket, you would want to limit the height of the o-ring to about 0.008" above the head or deck surface. For an 0.043" thick head gasket which is the most common thickness, set the o-ring protrusion at 0.010", for an 0.050" thick gasket about 0.012", for an 0.062" thick gasket about 0.015" and so forth. If the groove is cut into the head for the wire, a "receiver groove" can be machined into the block. If the groove is cut into the block deck for the wire, then a receiver groove can be machined into the head surface. When the head is bolted to the block, the wire pushes some of the copper up into the receiver groove and makes a very effective seal. However most street/strip applications do not require receiver grooves, receiver grooves are only required on the most extreme racing applications.  

SCE Gaskets makes copper head gaskets with coolant and oil passage seals bonded to the sealing surfaces of the head gaskets. These built-in seals eliminate the need for additional sealants; these head gaskets are installed dry and there are two versions available: one for engines with machined o-ring combustion seals and one for those without o-rings. The copper head gasket offered by SCE for engines without machined o-rings includes both coolant seals and an Integral Combustion Seal which eliminates the need to have o-ring grooves machined into the block or heads. Development of these head gaskets allow users to exploit the benefits of of copper, wide range of thicknesses, superior strength, superior combustion sealing, conformability to deck surfaces and superior heat conductivity, with none of the prior disadvantages of 'old fashioned' copper head gaskets.  

Gasket Works makes a stainless steel o-ring that is only 0.004" thick and eliminates the need to groove the block or head. See: http://www.headgasket.com/images/olympic%20rings.jpg .

====Steel (shim) gaskets====

The term, "blowing a gasket" was common for racers because they were using a very thin 10 or 20 thousandths of an inch thickness shim gasket to raise compression. Because of its thinness, the gasket would blow between two inner cylinders. They could stay in the race only as long as the gasket didn't disintegrate and expose the coolant passages to the combustion chamber. Engine builders of the day were often seen applying a couple coats of aluminum paint to both sides of a set of head gaskets to make them stick in place and seal the mating surfaces. The aluminum particles in the paint would fill in the voids in surfaces of the sand cast heads and block.

Sealant use may be required when retrofitting MLS(multi-layer steel) head gaskets to engines which were not originally produced with MLS head gaskets, or when using MLS head gaskets on engines that have not been properly prepared. For proper coolant and oil sealing, MLS head gaskets require surface finishes of 30 RA (Roughness Average) or finer, this is because the elastomeric coating on the sealing surfaces of MLS head gaskets is approximately .001" thick which is too thin to seal leak paths in the peaks and valleys of rougher (RA30+) finishes.

===Removing an old head gasket===

Stock replacement head gaskets do not generally need re-torquing. Steel shim head gaskets should be retorqued. Racing engine head gaskets should be re-torqued regardless of the type of gasket being used. If your engine calls for torque-to-yield head bolts (TTY) do not re-torque.  

The purpose of re-torquing the cylinder heads is to restore the proper stretch to the head bolts after the first heat cycle. Physics dictates that the engine assembly will expand as the engine temperature increases, this expansion will increase the compressive load on the head gaskets causing a seating effect sometimes referred to as creep relaxation in composite head gaskets.  

The seating of the gaskets and threads results in a commensurate relaxation of the head bolts when the engine cools. Re-torquing the head bolts/studs restores the proper stretch to the fasteners which will insure proper cold sealing of the gaskets as well as proper combustion sealing under full load.  

One re-torque is all that is necessary (unless the manufacturers instructions indicate otherwise), subsequent re-torquing can cause plastic deformation (stretch) of the head fasteners and damage to the head gaskets.<BR>

The process for re-torquing is as follows:<BR>

Start the engine & run with no load until operating temperature is reached.<BR>

Shut down the engine & let cool completely (overnight).<BR>

Retracing the original torque pattern, one fastener at a time, loosen slightly to overcome the friction set of the bolt or nut, then re-torque to specified torque setting.