GMC 12453265 Cylinder Head Assembly

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To begin with, take off the rocker arm covers and then the intake manifold, making sure that the cooling system is drained to stop coolant from seeping into internal engine areas when the manifold and heads are removed. After that, remove rocker arms and pushrods by detaching both exhaust manifolds. Use a new head gasket and create a template on a piece of cardboard by outlining the cylinders and bolt pattern while noting the front of the engine for reference then punch holes through at the bolt locations. In 1/4 turn increments, loosen head bolts until they can be removed by hand following a tightening sequence in reverse. To maintain original positions, store bolts in cardboard holder. Take off heads carefully from engine and avoid prying between head and block when resistance is met to prevent damage of mating surfaces but instead use wood block and hammer to free out heads if need arises. To protect gasket sealing surfaces, place heads on blocks of wood. The cylinder head and block mating surfaces should be cleaned right off using a gasket scraper to remove carbon and old gasket material followed by lacquer thinner or acetone washes. Mating surfaces should be dry as any oil may result into improper gasket sealing resulting into leakage problems. Some blemishes may call for filing while others could necessitate machining; thus inspect these surfaces for damages that might have occurred before installing new parts. Chase threads in the head bolt holes with the appropriate tap then clean threads thoroughly. Set new gaskets over dowel pins in block; apply thin coat of sealant on both sides if using steel gasket before placing it ensuring raised bead faces up. Composition gaskets shall be installed without any sealant being used at all. Now, position heads atop block without shift mortification occurring to gaskets placement itself. Just before mounting the head bolts make sure that they are coated with non-hardening sealant like Permatex no 2 and then screw into their original positions tightening them by hand. Gradually torque the bolts to the specified torque following the recommended tightening sequence. Finish the installation with the rest of the steps in reverse order.

Before removing the valves, it is important to label and store them, along with their related components, so they can be kept separate and reinstalled in the same valve guides they are removed from.

After compressing the springs, install the lower groove seal if using the flat O-ring type, ensuring it lies flat and not twisted. Position the keepers in the upper groove and release the compressor, making sure the keepers seat correctly.

If they are in good condition, reassemble the valve train components in the cylinder head without major servicing.

Thoroughly clean the cylinder head and block mating surfaces by using a gasket scraper to remove carbon and old gasket material, followed by cleaning with lacquer thinner or acetone.

The cylinder head cleaning and inspection process should be started after completing the engine disassembly procedures, if a complete engine overhaul is being done.

Install the rocker arms and pivots on the cylinder head studs, threading the nuts a few turns only.

Clean the rocker arm pivot stud threads, cylinder head, rocker arms, pushrods, valve springs, shields, keepers, and retainers with solvent and dry them thoroughly.