Editor's Note: This is the second of two parts addressing the basics ofengine assembly.
The cylinder heads are where the power is made in an engine. Money spenthere gives a better return of horsepower per dollar than money spentanywhere else in the engine. The selection of cylinder heads is usuallysubject to the rules at the track where you run. Make sure you arespending your dollars on headwork and heads that are legal.
A lot offactors go into selecting the cylinder heads for your engine. Trackrules and your pocketbook are just the beginning. Chevrolet has mademany head castings over the years. A number of them are suitable forracing.
At the least-expensive end might be heads from a 305-cid engine.These heads can have small, 58cc combustion chambers, which give goodcompression with flat-top pistons. These don't flow as well as someothers, but in a class that is not allowed many modifications, thecompression allows them to work quite well. Among the best of these areheads that have a casting number ending in 416 or 601.
The double humpheads have long been a favorite performance part. These arecharacterized by a "double hump" casting mark on each end of the head.There are several casting numbers in this family. Among them are castingnumbers ending in 462, 461, 291, and 186. The 041 casting, while it hasno double hump mark, is about the same as those mentioned above.
Toreally be able to research and identify Chevy cylinder heads, get a copyof Chevy Small-Block Part and Casting Number Guide from Steve SmithAutosports.
The double hump heads are usually expensive. Many have beenraced out and require a lot of work. Some might have modifications notallowed in your class. These heads on a nearly stock engine may not bemuch better than a 305 head. Use these heads when you need to turn 7,000rpm and when you need to make power with a large carb, headers, and abig cam.
With the cost of double hump heads rising, an alternative,where rules allow, is the Chevy Vortec head. This head can be easilyidentified by its center bolt hold-downs on the valve covers. Also, themanifold center bolts are vertical rather than perpendicular to thehead-mating surface. This is a new design that flows well and has goodcompression. I'm told there are a few carbureted manifolds made byChevrolet. I'm also told they are hard to find. An older intake manifoldwill require redrilling of the bolt holes in the heads to install. Thisis not too difficult, but be sure it is allowed under your rules. Vortecheads will be found on many of the later model Chevy V-8-powered carsand pickups.
Cylinder head selection is a major source of power. Check your rulebook,as local track rul
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 than that of Chevrolet. They areidentified by the center ho
This is the combustion chamber of a 906 Vortec head. Notice the fillerbetween the valves a
Followthe manufacturer's torque sequence on the head bolts. This will have youworking up to final torque in three stages and working from the centerof the head toward the outside. Install a rocker arm of your choice.There are stock rockers, aftermarket rockers that look like stock,roller tip, and full rollers to choose from. If your original rockershave worn grooves on the valve stem contact face, they should bereplaced. Aftermarket stock-appearing rockers such as Comp Cams PN1212-16 would make a good replacement for the stock parts. They have thecorrect 1.5:1 ratio rather than the stock 1.46:1. This will give you alittle more valve lift. Let your pocketbook and the rules be your guide.
PUSHROD LENGTH: Since we have mixed, matched, and milled some parts, weneed to be sure to use pushrods of the correct length. With the manifoldstill off, set the pushrods in place. Turn the cam so that the lobe youare checking is on the back side of the high point (valve closed). Aninexpensive plastic gauge is available from JR Motorsports, PN prc 1038,to check pushrod length. When the gauge is slipped over the rocker armstud, it should contact the valve stem and the pushrod at the same time.Having a pushrod of the incorrect length can reduce power and lead tovalvetrain failures.
INSTALLING THE INTAKE MANIFOLD: Before you install the intake manifold,check to make sure you installed the distributor drive rod. It can stillbe installed from the top, but without the plastic sleeve that preventsit from falling out when you remove the distributor. All is not lost solong as you remember not to jerk the distributor out. Remove it slowlyand carefully and you will be OK.
The intake manifold gasket set has twogaskets and two rubber snakes. Toss the snakes. Again, clean the gasketsurfaces to make them oil free. The two gaskets can be put in place.Using a silicone sealant, smear a small dab around the water passages onthe manifold. Using this same sealer on the ends of the block, lay downa line about 1/8 inch high; don't overdo it. Sealer that is squeezed outon the inside of the engine can find its way to the oil pickup screen.When the screen becomes blocked, instant catastrophic engine failurewill occur.
Carefully lower the intake manifold in place. Don't move itaround. This avoids disturbing the gasket and sealer. With all themanifold bolts in place, begin tightening from the center workingoutward.
In the stock cast iron manifold shown in the photo, the arrow points to the bolt holes whe
This photo shows the manifold bolt holes in a Vortec head. Notice that the bolt holes are
CHECK DISTRIBUTOR DEPTH This is a good time to check theinstalled height of the distributor. Install the distributor without agasket. You may need to use a long flat-blade screwdriver to positionthe distributor drive rod so the distributor will go all the way in.With the cap and rotor off of the distributor, you will see a smallsemi-oval-shaped part. This is attached to the distributor shaft. Theother pieces are free to move around the shaft, changing the ignitionadvance.
With the distributor clamped down with no gasket, the semi-ovalpart must have some vertical slack or movement. Any amount issufficient. When the gasket is installed, vertical movement willincrease. That's OK. If there is no vertical movement, the distributoris binding up in the oil pump. This may result in destroyed distributorgears or damaged distributors.
In the case of no slack, Steve Davis ofPerformance Distributors recommends the use of the Moroso distributorshim kit, PN 26150. He advises against using multiple gaskets becausethey will compress over time. For more on this, see www.performancedistributors.com/technical.htm.
CONCLUSIONS By this point, we have covered the basics of puttingtogether your first engine. You will need a book with all the torquespecs and clearances. A good one is How to Rebuild Your Small-BlockChevy from Steve Smith Autosports. You should have a torque wrench; theinexpensive beam type will do fine.
Observe as much of the machine shopwork as possible. You will learn a lot. Take your time during assembly.There is no magic here. Diligent attention to detail and cleanliness arethe secrets to engine assembly. On your next engine, you will likely gofurther in the direction of modifications to your engine. Know thebasics and then build on them, learning as you go.
This is a low-compression "taxicab" head (i.e., low performance). ManyChevy heads have a c