Once the decks are clean, always check each block and head deck for flatness with a precision machinist’s straightedge and a feeler gauge. Perform the check from front to rear, and diagonally across opposing corners. While old-school warpage limits for the length of the deck may be about 0.003 inch for V6 heads, 0.004 inch for V8 heads and as much as 0.006 inch for straight-six heads, most late model aluminum heads sealed with MLS (multi-layer steel) gaskets require tighter tolerance ranges.
Generally speaking, any deviation (gaps) of more than about 0.002 inch along deck length or diagonal measurements is unacceptable, requiring the offending deck(s) to be re-machined flat. Don’t make assumptions. Always refer to the service manual for the tolerance specification for any given engine. Also check for any isolated scratches or gouges on the deck surfaces, which can create pressure-loss leaks between the deck and gasket. Each engine design will specify a roughness average (Ra) in terms of surface finish. A specific range of surface finish is required for any given engine, in order for the head gasket to properly seal as designed.
Never attempt to re-surface any deck unless your shop has a qualified engine machinist on hand. Otherwise, farm the resurfacing out to a local engine machine shop.
Always select the proper type of head gasket for the application (composite or MLS). MLS head gaskets are commonly used in late model engines. This type of head gasket features multiple layers of material, with formed sealing beads that are designed to crush to seal. An additional sealant imprint is commonly featured as well to aid in fluid sealing.
When installing the new head gasket, pay close attention to gasket orientation. In many cases, the cooling passages on the block decks may not be symmetrical (not a mirror image front to rear). Refer to both the factory service manual and to the head gasket maker’s instructions for proper gasket placement in order to prevent blocking off a critical coolant passage. Many high-quality cylinder head gasket makers will print the word “FRONT” at one end of the gasket, as a reminder of gasket orientation.
Cylinder head guide dowel sleeves or pins will be present on the block deck. Never install a gasket and head without these locating dowels. The dowels precisely locate both the gasket and head on the block in order to align all bolt locations and all passages. If the original dowels are damaged or missing, always obtain and install new dowels.
Follow the service manual (or bolt/stud maker’s instructions) with regard to thread treatment.
Depending on the application, either a clean engine oil or a moly-type thread lubricant must be applied to the bolt threads (and the underside of the bolt heads). When dealing with head studs, this lubricant must be applied to the upper threads and to the bottom of the nuts. The type of lubrication has a direct bearing on the applied torque. Moly reduces friction between the threads, more-so than oil. If aftermarket head bolts or studs are being used, the manufacturer will likely provide two different torque specs, one for oil and one for moly. If you use moly but follow the torque spec for oil, you will over-tighten the fasteners. If you use oil but follow moly spec, you may under-tighten. Pay attention to the specified thread lube and the appropriate torque spec.
Both fastener clamping load and tightening sequence are absolutely critical to achieve a proper head gasket seal. NEVER guess at the tightening sequence. Refer to the OE repair manual (or aftermarket cylinder head instructions) for the tightening pattern and tightening phases. Tighten cylinder head fasteners in steps. For example, if a torque specification calls for a final 70 ft.-lbs., begin (in the correct sequence) with 20 ft.-lbs., followed by 40 ft.-lbs., etc., until final torque is achieved. Never fully tighten one fastener at a time. Uneven clamping load distribution can easily deform (warp) a cylinder head, resulting in a gasket leak, which will be no fault of the gasket itself.
Many of today’s engine components are sealed with the use of a formed silicone/rubber/elastomer bead seal. Providing the mating surfaces are flat and clean, there is usually no need to apply any additional chemical sealant to this style of gasket or seal.
Avoid re-using original formed-in place bead seals. Considering the relatively low cost of the seals, it makes sense to always install a new seal, even if the original appears to be in good shape. It’s better to spend a couple of extra bucks in order to avoid the need to perform the job again, should a leak occur due to a damaged or aged original seal.
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