HEAD PREPARATION: Once you have obtained a set of heads, look them overclosely, and clean them well. If the budget allows, take them to amachine shop. Have them cleaned and checked for cracks. If you are OK atthis point, then consider what you want done to them. After disassembly,look at the valves. If they are not burned severely or otherwisedamaged, have the shop do a valve job. This consists of grinding themating surfaces of the valve and valve seat where they touch when thevalve is closed. What is known as a three-angle valve job is standardfor performance work. Here, rather than one wide single-angle surface,three angles are used. This improves the air/fuel mixture flow at thebeginning of the valve-opening event.
If your valves are usable, goahead and use them, even if they are the stock 1.94-inch intake and1.5-inch exhaust. If you need new valves, then it might make sense tohave larger valves installed.
If your heads have never been used in aperformance application, they will most likely have press-in rocker armstuds. Screw-in-type studs are cheap insurance against valvetrainfailure when aftermarket valvesprings are used. Unless you are planningsome serious rpm for this engine, I suggest using the straight-typestuds that have no flange. These are somewhat less expensive than theflanged type. When flanged types are used, the stud boss in the headmust be milled flat as well as threaded. With the straight types, thestud bosses can be threaded with a hand tap. In doing so, the tap mustbe held exactly in line with the hole. This is best done in a drillpress using the drill press to hold the tap straight. Turning the tap byhand will reduce tap breakage.
Voice of Experience:
Mark Campbell, director of camshaft and valvetrain research anddevelopment for Crane Cams, pointed out a few pitfalls the novice enginebuilder should watch for. (Note: Casting number references are usuallymade to the last three numbers in a six-number set.) Casting headnumbers of 624 found on 350 and 305 engines may have different lengthvalves or spring pocket depth. Casting head numbers of 882 may also havethis problem. Intake and exhaust valves may be of different lengths. OEsprings might also be of different lengths. As originally assembled,these would usually have valve rotators on the top of the spring. Theseshould definitely be removed for performance purposes. If not noticed,this could cause early valve float. Your machine shop should be aware ofthese differences and shim the springs accordingly. Another tip Markpassed along concerned the valve seals. On the Vortec heads, umbrellaseals can be used when lift is less than 0.420 inch. Using O-ring-typevalve seals allows springs to be used with cams of up to 0.525-inchlift. After that point, one should look closely at valvespring clearancebetween the spring and the head casting. Machining may be necessary.
At this point you should have alreadyconsulted a cam grinder and made your selection of cam, springs, andequipment. While the heads are in the machine shop, you will need to becertain the selected valvesprings will fit the pockets in the head. Ifthey are too large, then the spring pocket will need to be machined to alarger diameter. If you were careful to tell the cam man all of yourwants and needs, your valve- springs will be a stock diameter, at leastat their base.
The machine shop will surely have checked the head gasketsurface to make sure it is flat. If it needs to be milled to correct awarp, then have them mill the maximum amount to raise the compression.Unless you have the tools, have the shop assemble your heads with valvesand springs. If possible, watch them do this so that next time you canget a spring compressor tool and do the job yourself.
ASSEMBLY: Now we are ready to mate the cylinder heads to the short-block.Surely your finished short-block is mounted to an engine stand andcovered to keep out trash. The deck of the block should be carefullycleaned to make sure there is no oil on the gasket surface. I like usingbrake parts cleaner for this as many solvents can leave an oil film. Besure the dowel pins that locate the head are in place.
I recommend usingone of the no re-torque-type head gaskets. They are coated with Teflonor something similar that will permit cold flow. This cold-flowingability allows the gasket to seal without the heat of the engine.
Cleanthe threads of the head bolts with a wire brush. The threads need to beclean to reflect the correct torque values. Also clean under the head ofthe bolt.
Some bolt holes are threaded into the water jacket. Thiscreates the possibility of a water leak. In a perfect world, one couldoil the bolt threads and install the head bolts. Since the real world isnot perfect, we may need a little insurance. Put some nonhardeninggasket sealer like Permatex No. 2 on the threads of all the bolts. Thiswill seal the threads and have little effect on torque-wrench readings.
With the heads resting on the block, drop all the head bolts in place.Notice there are three different lengths. The short ones go along thebottom edge of the head, the long ones inside, and the two mid-lengthbolts are used on the ends.
Pay close attention to the recommendedtorque values. Also pay close attention to your torque wrench. Beamtypes, while more time consuming to use, are often more accurate thanthe clicker type. Pro shops using clicker-type torque wrenches may sendthem out for a calibration check as often as every seven days.
Vortec heads are a newer style...
Vortec heads are a newer style than that of Chevrolet. They areidentified by the center hold-down bolts in the valve cover and thevertical location of the intake manifold bolts. Old-style heads havefour bolts around the perimeter of the cover. There are good and betterVortec heads. A better one is the 906 casting number found under theintake port runners. These, along with others, have a 64cc combustionchamber. The intake bolt holes can be redrilled in the Vortec heads.This will allow the use of the common Chevy cast manifold whennecessary.
This is the combustion chamber...
This is the combustion chamber of a 906 Vortec head. Notice the fillerbetween the valves and the open area near the spark plug. This headflows well and has a 64cc combustion chamber. The small chamber providesgood compression with flat-top pistons.