Head gasket
(→Head gasket repair additives) |
(→Replacing blown head gaskets) |
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*If you can, do a leak down test. | *If you can, do a leak down test. | ||
*If you can, pressure test the coolant system. If you pressurize the system, and you lose pressure, you may have a blown head gasket. | *If you can, pressure test the coolant system. If you pressurize the system, and you lose pressure, you may have a blown head gasket. | ||
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| + | ==How to choose a head gasket== | ||
| + | Will depend on desired quench, and engine performance characteristics. | ||
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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 they don't throw you out of spec on the squish dimension (piston crown to under side of cylinder head with the piston at TDC). Generally speaking, this figure is 0.035" to 0.045". 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, with the piston coming in close proximity of the underside of the cylinder head. This will "squish" the otherwise dead mixture out of the area and jet it towards the spark plug, thus fully mixing the mixture, contributing to more complete combustion and the elimination of detonation. The best piston 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 ring around the perimeter of the piston to accomplish squish. Absolutely flat-top pistons will work best, such as the ones offered by Keith Black. Even the dished pistons offered by them have the flat area opposite the dish. | ||
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| + | This is probably one of the most important areas 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. I'll use a 350 Chevy for this explanation. If the piston deck height is, for instance, 0.025", then a gasket with a thickness of 0.015" (Fel-Pro FPP-1094) would put the squish at 0.040". If the piston deck height is, for instance, 0.012", then a gasket with a thickness of 0.028" (GM 10105117) would put the squish at 0.040". Mix and match the piston deck height with the gasket thickness in your particular application to achieve this ideal 0.035" to 0.045" squish. | ||
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| + | Another consideration is the type of engine, naturally aspirated, supercharged and so on. It's been said that the COMETIC mls (multi-layered steel)gaskets, although relatively expensive, negate the need to "O" ring the block to accommodate higher boost numbers. I use COMETIC's in my blown 383 SBC with 10 pounds of boost and will be increasing boost to 12 - 15 pounds for this racing season, and have had no problems. | ||
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| + | ==Head gasket sealants== | ||
| + | Current technology OEM and racing head gaskets are designed to be used without additional sealant, however there are cases where head gasket sealants are required or helpful. Head gaskets sealants are specifically designed for the application; due to extremely high pressure within the combustion chamber which must be contained by the combustion seal, generic fluid sealants such as silicone should not be used for head gasket sealing. Unlike RTV silicone, head gasket sealants do not cure, they remain pliable indefinitely thereby allowing the head/gasket/block interface to achieve metal-to-metal contact under the compressive load of the tightened head bolts. Metal-to-metal contact insures a proper combustion seal. Conversely, curing sealants such as RTV silicone can form a 'rubber layer' upon which the head/gasket/block interface are separated allowing combustion pressure to eventually form a leak path to the coolant system or to the outside of the engine. Differential expansion rates of bi-metal engines are also accomodated by use of proper head gasket sealants which will not shear because they do not cure.<BR> | ||
==Replacing blown head gaskets== | ==Replacing blown head gaskets== | ||
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Don't use wire brushes -- the wires can break off, get into the oil pump, and lock it up. ''(What about 3M "hair brush" pads? What are those? Safe to use?)''. | Don't use wire brushes -- the wires can break off, get into the oil pump, and lock it up. ''(What about 3M "hair brush" pads? What are those? Safe to use?)''. | ||
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| − | ====MLS | + | ====MLS head gaskets==== |
| − | Sealant use may be required when retrofitting MLS 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. | + | Sealant use may be required when retrofitting MLS 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. |
====Steel Shim Head Gasket==== | ====Steel Shim Head Gasket==== | ||
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