Head gasket
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===O-ring sealing=== | ===O-ring sealing=== | ||
| − | 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. | + | [[File:O-ringdrawing.jpg|thumb|right|450px|O-ring diagram]] |
| + | 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. <br style="clear:both"/> | ||
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. | 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. | ||
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=="Blown" head gaskets== | =="Blown" head gaskets== | ||
| + | {| | ||
| + | |[[File:Morris Marina Blown head gasket.jpg|360px]] | ||
| + | |[[File:Boosted pontiac 400 engine+blown head gasket.jpg|350px]] | ||
| + | |[[File:Blown into cooling.jpg|350px]] | ||
| + | |} | ||
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===Reasons why head gaskets fail=== | ===Reasons why head gaskets fail=== | ||
*Too much outward pressure that overcomes the clamping force of the head to the block | *Too much outward pressure that overcomes the clamping force of the head to the block | ||
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==How to choose a head gasket== | ==How to choose a head gasket== | ||
| − | ===Quench=== | + | ===Quench vs. head gasket thickness=== |
| − | The head gasket thickness | + | The head gasket thickness depends on the desired quench measurement and desired compression ratio. Piston to valve clearance must be verified. |
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| + | Although static compression ratio should be determined by the piston configuration and the volume of the combustion chamber, small compression ratio adjustments are possible by altering gasket thickness as long as this doesn't cause the '''[[quench]]''' (or "squish") dimension (piston crown to under side of cylinder head with the piston at TDC) to be out of spec. Generally speaking, this figure is no less than 0.035" to 0.045" for engines built with steel rods. There will be some flex in the crankshaft, rods and pistons as they whip around at speed and this clearance will be diminished as a result\. | ||
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| + | The turbulence caused by the piston coming in close proximity of the underside of the cylinder head will "squish" the unburned mixture out of the area and jet it towards the spark plug, thus fully mixing the mixture. This action contributes to a more complete combustion, more power, less emissions and suppresses detonation. | ||
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| + | [[File:Round dish sbc piston.jpg|thumb|left|340px|This stock type SBC piston is the least desireable design as far as quench action is concerned]][[File:KB P-N 135 SBC 383 PISTON.jpg|thumb|330px|The D-cup design is much better for quench action if a dish is required]] | ||
| − | + | The best piston design to use for this is one which has a dead flat area where it will meet the cylinder head. Stock Chevy pistons, for instance, have only a thin band around the perimeter of the piston to accomplish squish. Flat top pistons having minimal valve reliefs will work best. The D-cup pistons offered by various manufacturers also work well (better than a round dish) when a dish is needed. <br style="clear:both"/> | |
This is a very important area of engine building and should be considered carefully before ever buying any parts. You must know what the piston deck height (distance from the crown of the piston to the block deck surface with the piston at top dead center) is before continuing. | This is a very important area of engine building and should be considered carefully before ever buying any parts. You must know what the piston deck height (distance from the crown of the piston to the block deck surface with the piston at top dead center) is before continuing. | ||
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