REMOVAL & INSTALLATION
151 Four and 250 Inline Six Cylinder Engine
- Drain the cooling system and remove the air cleaner assembly. Disconnect the PCV hose.
- Tag and disconnect the throttle linkage at the carburetor. Tag and disconnect the fuel line, vacuum lines, and any electrical connections at the carburetor.
- Remove the top radiator hose, and the battery ground strap. Disconnect the wires from the temperature sending unit, leaving the harness clear of the clips on the rocker cover.
- On the 250 six, disconnect the coil wires after tagging them, and remove the coil. Tag and disconnect the spark plug wires from the plugs.
- Remove the intake and exhaust manifolds.
- Remove the rocker arm cover. Back off the rocker arm nuts, and pivot the rocker arms so the pushrods will clear.
- Take a piece of heavy cardboard and cut 8 or 12 holes (depending on whether you are working on a four or six cylinder) in it the same diameter as the pushrod stem. Number the holes in relation to the pushrods being removed. This cardboard holder will keep the pushrods in order (and hopefully out of harms way) while they are out of the engine. Remove the pushrods one at a time.
Pushrods MUST be returned to their original locations.
- Remove the cylinder head bolts one at a time, and mark them or keep them in order, as they should go back in their original locations. You may need a flex bar on your socket, or a piece of pipe on your ratchet, as the bolts are under a lot of torque.
- Remove the cylinder head, along with the gasket. If the head seems tuck to the block, gently tap around the edge of the head with a rubber mallet until the joint breaks. NEVER pry between the head and block as you may gouge one or the other. Often it is necessary to carefully scrape the top of the engine block and the cylinder head to completely remove the gasket.
- Clean the bottom of the head and top of block thoroughly before reinstalling the head. Place a new gasket over the dowel pins in the top of the block.
Different types of head gaskets are available. If you are using a steel-asbestos composition gasket, do not use gasket sealer.
- Lower the cylinder head carefully onto the block, over the dowel pins and gasket.
- Coat the heads and threads of the cylinder head bolts with sealing compound. GM part No. 1052080 or equivalent, and install finger-tight.
- Tighten the head bolts gradually in three stages, following the sequence illustrated, to the specification listed under "Torque Specifications."
- Install the pushrods in the exact location from which they were removed. Make sure they are seated in their lifter sockets.
- Swing the rocker arms over into their correct position. Tighten the rocker arms until all pushrod play is taken up.
- Install the manifold assembly (or separate manifolds on the 1979 151 four), using new gaskets. Tighten the manifold(s) to the specified torque.
- Reverse the remainder of the removal procedure for installation. Adjust the valves, following the procedure in this section. Use a new gasket or high-temperature sealer when installing the rocker arm cover.
EXCEPT DIESEL ENGINES
- Disconnect the battery.
- Drain the coolant and save it if still fresh.
- Remove the air cleaner.
- Remove the air conditioning compressor, but do not disconnect any A/C lines. Secure the compressor to one side.
- Disconnect the AIR hose at the check valve.
- Remove the intake manifold.
- When removing the right cylinder head, loosen the alternator belt, disconnect the wiring and remove the alternator.
- When removing the left cylinder head, remove the dipstick, power steering pump and air pump if so equipped.
- Label the spark plug wires and disconnect them.
- Disconnect the exhaust manifold from the head being removed.
- Remove the valve cover. Scribe the rocker arms with an identifying mark for reassembly; it is important that the rocker assembly is reinstalled in the same position as it was removed. Remove the rocker arm bolts, rocker arms and pivots.
- Take a piece of heavy cardboard and cut 16 holes (or 8 holes if you are only removing one head) in it the same diameter as the pushrod stem. Number the holes in relation to the pushrods being removed. This cardboard holder will keep the pushrods in order (and hopefully out of harm's way) while they are out of the engine. Remove the pushrods.
Pushrods MUST be returned to their original locations.
- On models equipped with power brakes, it is necessary to disconnect the brake booster and turn it sideways to remove the No. 7 pushrod.
- Remove the cylinder head bolts, and remove the cylinder head and gasket. If the head seems stuck to the block, gently tap around the edge of the head with a rubber mallet until the joint breaks.
- Install in the reverse order of removal. NEW head gasket(s) should be used. Gaskets on the 260 V8 gaskets do not have a stripe. On the 265 and 301 engines, coat all rocker stud lower threads, the cylinder head bolt threads, and the underside of the bolt head with thread sealer. On all engines, the head bolts should be dipped in clean oil before installing. Tighten all head bolts in sequence to 60-70 ft. lbs., then again in sequence to the specified torque (see Torque Specifications chart in this section). Re-tighten the bolts after the engine is warmed up.
When installing the intake manifold remember to use new gaskets and O-ring seal, if so equipped.DIESEL ENGINES
- Remove the intake manifold, using the procedure outlined earlier in this section.
- Remove the rocker arm cover(s), after removing any accessory brackets which interfere with cover removal.
- Disconnect and label the glow plug wiring.
- If the right cylinder head is being removed, remove the ground strap from the head.
- Remove the rocker arm bolts, the bridged pivots, the rocker arms, and the pushrods, keeping all the parts in order so that they can be returned to their original positions. It is a good practice to number or mark the parts to avoid interchanging them.
- Remove the fuel return lines from the nozzles.
- Remove the exhaust manifold(s), using the procedure outlined above.
- Remove the engine block drain plug on the side of the engine from which the cylinder head is being removed.
- Remove the head bolts. Remove the cylinder head.
- Clean the mating surfaces thoroughly. Install new head gaskets on the engine block. Do NOT coat the gaskets with any sealer. The gaskets have a special coating that eliminates the need for sealer. The use of sealer will interfere with this coating and cause leaks. Install the cylinder head onto the block.
- Clean the head bolts thoroughly. Dip the bolts in clean engine oil and install into the cylinder block until the heads of the bolts lightly contact the cylinder head.
- Tighten the bolts, in the sequence illustrated, to 100 ft. lbs. When all bolts have been tightened to this figure, begin the tightening sequence again, and torque all bolts to 130 ft. lbs.
- Install the engine block drain plugs, the exhaust manifolds, the fuel return lines, the glow plug wiring, and the ground strap for the right cylinder head.
- Install the valve train assembly. Refer to the "Diesel Engine Rocker Arm Replacement" in this section for the valve lifter bleeding procedures.
- Install the intake manifold.
- Install the valve covers. These are sealed with RTV (Room Temperature Vulcanizing) silicone sealer instead of a gasket. Use GM # 1052434 or an equivalent. Install the cover to the head within 10 minutes, while the sealer is still wet.
CLEANING & INSPECTION
Any diesel cylinder head work should be handled by a reputable machine shop familiar with diesel engines. Disassembly, valve lapping, and assembly can be completed by following the gasoline engine procedures.
Once the complete valve train has been removed from the cylinder head(s), the head itself can be inspected, cleaned and machined (if necessary). Set the head(s) on a clean work space, so the combustion chambers are facing up. Begin cleaning the chambers and ports with a hardwood chisel or other non-metallic tool (to avoid nicking or gouging the chamber, ports, and especially the valve seats). Chip away the major carbon deposits, then remove the remainder of carbon with a wire brush fitted to an electric drill.
Be sure that the carbon is actually removed, rather than just burnished.
After decarbonizing is completed, take the head(s) to a machine shop and have the head "hot tanked." In this process, the head is lowered into a hot chemical bath that very effectively cleans all grease, corrosion, and scale from all internal and external head surfaces. Also, have the machinist check the valve seats and re-cut them if necessary. When you bring the clean head(s) home, place them on a clean surface. Completely clean the entire valve train with solvent.
CHECKING FOR HEAD WARPAGE
Lay the head down with the combustion chambers facing up. Place a straight-edge across the gasket surface of the head, both diagonally and straight across the center. Using a flat feeler gauge, determine the clearance at the center of the straight-edge. If warpage exceeds .003 in. in a 6 in. span, or .006 in. over the total length, the cylinder head must be resurfaced (which is akin to planing a piece of wood). Resurfacing can be performed at most machine shops.
When resurfacing the cylinder head(s) of V6 or V8 engines, the intake manifold mounting position is altered, and must be corrected by machining a proportionate amount from the intake manifold flange.