Possibly the most critical component to making power in any Chevy engine is the valvetrain. The components you choose and how you assemble them will determine the valvetrain’s overall stability, durability, and, of course, how much air and fuel it allows into the combustion chambers. For our purposes, you must also consider the cylinder heads as part of the valvetrain. They determine the geometry of the system by providing the mounting points for the rocker arms and seats for the springs, and they supply the sealing surface for the valves.
Cylinder Head Machine Work
As with the block, proper machining, inspection, and cleaning of your cylinder heads is critical to a successful engine build. And also like the block, many of the machining processes require very specialized equipment that makes it extremely difficult to do the work yourself.
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Decking is the process of machining off a portion of the surface of the heads that mate with the block. This ensures a square, flat surface to provide proper sealing with the head gasket. Decking can also be used to increase compression by reducing the size of the combustion chambers.
An offshoot technique of the decking process is known as angle milling. Angle milling is the process of angle cutting the deck of the head to reduce the valve angle. By cutting more material off of the exhaust side of the deck of the heads, you can reduce the valve angle by a couple of degrees. For example, a 23-degree Chevy head can be angle milled to give a valve angle of 21 or 20 degrees without making other drastic changes. The shallower valve angle improves flow into the engine by reducing the shrouding effect around the valves.
Of course, angle milling a pair of cylinder heads does require further changes. Because the way the head sits on the block has changed, you will likely have to slot the head bolt holes for the head bolts to fit the threads in the block.
You will also need to machine the surfaces where themanifold and head mate to get them to match up again. Special care must be taken to make sure the intake runners in the manifold and head haven’t moved out of alignment. Finally, you may also need to add more clearance for the changed pushrod angles. When done well, angle milling is a good way to add power in a class where your choice of racing heads is limited. But when done poorly, angle milling can cause more harm than good.
Many aftermarket performance heads ship with bronze valveguides installed. All aluminum cylinder heads must have either bronze or iron guides. The bronze reduces the chance of the valvestem galling to the wall of the guide. If you are using stock-style heads and a high-lift cam, you may want to add bronze guide inserts.
Whether you are running bronze guides or using cast iron, the guides must be honed to the correct tolerance. For an aluminum head with bronze valveguides, that should usually be around 0.0012 of an inch on the intake and 0.0015 of an inch for the exhaust, which sees more heat. For a cast iron head with iron guides you can get by with 0.0015 of an inch of clearance on both. An additional benefit of properly honed guides is that the guide walls will have a nice crosshatch pattern just like the cylinder bores. The crosshatch helps retain a tiny bit of oil between the valvestem and the guide to reduce the chances of galling.
One of the ways high-performance springs normally get their extra strength is by being wound to a larger diameter than their stock counterparts. So when using stock or stock-replacement heads, it can often be necessary to re-cut the valvespring pockets to a larger diameter to accommodate your specific springs. Valve seats are sometimes cut deeper into the heads to increase the spring’s installed height without having to use valves with longer (and heavier) stems.
When cutting spring seats, it is critical that your machinist doesn’t cut too far and break into the water jacket. On some heads, just widening the spring seat to match the diameter of your racing springs will intrude into the water jackets. In such a case, shims must be used to raise the original seat.
Screw-In Rocker Studs
Stock heads use press-in rocker studs because they are more economical. These are fine with lightweight stock springs, but the extra force created by a high-pressure racing valvespring can pull the stud out of the head. As a general guideline, any time you are using valvesprings with more than 100 lbs of seat pressure, the heads should be tapped to accept screw-in studs.
It is possible to cut the threads with a hand tap, but it isn’t advisable. When cutting the threads by hand, there is no way to guarantee that the alignment is correct. Any error here can change valvetrain geometry and lead to part failures.
The seats for the intake and exhaust ports provide the sealing surface for the valves. They should also be considered part of the port and vital to ensuring the greatest amount of air and fuel are able to flow into the combustion chambers. The seats are, in fact, the transition between the port and the combustion chamber.
Most rulebooks allow a competition three-angle valve job on the seats. The standard angles in this situation are 45 degrees for the actual seal, with a 30-degree angle above it (the entrance to the combustion chamber) and a 60-degree angle below it (between the seat and the port). This is a vast improvement over a stock valve job that employs just a single angle. But it can still be improved.
Rather than leaving sharp edges between the angles, use a cutter to create a radius that blends the angle changes gently to further increase air and fuel flow. A slightly different strategy is warranted for the exhaust seats in highly efficient combustion chambers. Here, the goal is to guide the spent hydrocarbons out of the chamber and into the exhaust ports. This is often best achieved by eliminating the top angle and utilizing a radius to blend the chamber roof into the 45-degree valve seat.
Of course, extensive testing on flow benches has shown that the most efficient valve angles are dependent on total valve lift. If you are racing a class that limits total valve lift or requires no more than the stock lift numbers, you can consider flattening out the valve seat. With lower valve lift there is less room for the air and fuel to get around the head of the valve. In this scenario, the valve seat angle is approximately 30 degrees with 45- and 15-degree angles above and below. This works because the natural tendency of both air and liquids is to follow a surface. The shallower angles help guide the air/fuel charge along the sides of the combustion chamber and around the head of the valve.
In high-lift valvetrains—generally above 0.600 of an inch total lift—the opposite holds true. When the valve is far away from the valve seat, it only requires a small change in direction of the air/fuel charge to clear the valve. Now, the most efficient method for getting the air/fuel charge into the combustion chamber is to use taller (numerically larger) angles, which sends the incoming charge more directly into the chamber. The actual angles often vary depending on the engine builder, but they are usually somewhere around 70, 55, and 40 degrees.
One drawback of using such extreme valve angles is that it is really hard on the seat and valve. Instead of dropping the valve onto the seat, which is the case with a 45- degree seat, the valve is now actually wedging itself into the seat. As a result, the lifespan of both the seat and the valve is significantly shortened. If you choose to use large valve angles, be prepared for frequent cylinder head rebuilds.
Valvespring Seat Versus Nose Pressures
When discussing the strength of a set of valvesprings, you will often hear the terms “on the seat” and “over the nose.” These refer to the two main positions of the spring during engine operation. “On the seat” refers to the pressure the spring is exerting on the valve when it is closed (when the valve is sitting on the valve seat). There should always be pressure on the spring even when the valve is closed, or else the valve will bounce back open when it contacts the seat. If you do have valve bounce, you not only stand the chance of damaging both the valve and the cylinder head, you also lose cylinder pressure, which costs you power.
As a general rule, the higher the RPM range, the more seat pressure the engine requires to eliminate the chance of valve bounce. Good seat pressure helps transfer heat from the valve to the head. In the case of classes requiring stock hydraulic lifters, it also exerts enough pressure on the lifter’s plunger to prevent the lifter from pumping up and holding the valve open.
“Over the nose” is the spring pressure at maximum lift, when the lifter is riding over the nose of the cam. The inertial forces acting upon a lifter as a performance cam raises it can be quite drastic. The valvespring must have sufficient nose pressure both to decelerate the valve as the cam reaches max lift and then close the valve at the speed determined by the backside of the cam lobe. If the spring isn’t strong enough, the lifter will loft, or lose contact with the cam lobe. When the spring is finally able to overcome the lifter’s inertia, it will close the gaps in the valvetrain violently, sending a damaging, powerrobbing shockwave throughout the system.
Unlike seat pressure, which is based on a specific installed height and is easy to determine, a spring’s pressure over the nose is influenced by multiple factors. These are the spring’s height at max lift, seat pressure, and spring rate.
Adjusting Spring Pressures
Properly matching a spring’s seat and nose pressures to the needs of your valvetrain is critical. Unfortunately, there is no way to tell you exactly what is needed for every possible engine package. Heavier hydraulic lifters require stronger springs. Roller cams can handle more radical lobe profiles, which also require stronger springs. Springs secured by titanium retainers require less nose pressure than springs secured by heavier steel retainers. And the list goes on.
You already know what happens if the valvesprings are too light, but too much spring pressure can also cause problems. Excessive spring pressure causes elevated wear from friction on both the cam lobes and lifters. This situation can also cause the pushrods to flex as they try to move the rocker arms against spring pressure. When this happens, it delays the timing for intake and exhaust valve opening, which hurts power.
The spring’s installed height is measured when the spring is installed on the head and the valve is properly seated. It is the dimension between the top of the spring seat on the cylinder head and the bottom of the retainer, specifically the surface that the top of the spring contacts. Spring manufacturers give the seat pressure for their springs at a specific installed height. Raising or lowering the installed height can decrease or increase the seat pressure, respectively.
Increasing the installed height means either lowering the seat location on the head (by machining away material) or raising the location of the spring retainer. This can be done either by switching to valves with longer stems, using offset valve locks that change the retainer’s position relative to the valvestem, or using different retainers that locate the top of the spring higher. Its best to avoid switching stem lengths to influence spring heights, as this also changes the rocker arm height and requires changing pushrod lengths, which can lead to geometry problems.
Decreasing the installed height to raise the seat pressure can be accomplished by opposite measures. You can raise the spring seat on the cylinder head by inserting shims between the spring and seat, and you can lower the retainer height with offset locks or by using a different style retainer. Again, changing the valvestem length without checking how it will affect the rocker arm geometry is a mistake.
Valvespring Coil Bind
When changing either the installed height or max lift, you should always check to make sure the spring won’t go into coil bind. Coil bind is when the spring is compressed so far that its coils touch one another and it can be compressed no further. Uncontrolled coil bind will lead to a broken spring or bent pushrod. This is a concern when changing to a higher lift cam or higher ratio rocker arms.
Most manufacturers provide minimum spring heights at maximum valve lift for every spring, but it is a good idea to check this yourself. With a spring installed on a cylinder head, use a spring-compressing tool to compress the spring until it goes into full bind. Now, measure the distance between the bottom of the retainer to the spring seat. The recommended safety margin is 0.060 of an inch, so add this to the height of the spring at coil bind. Subtract this number from your installed height. This is your maximum spring travel—which is also your maximum valve lift for this setup.
If you desire more valve lift than your maximum spring travel will allow you need to increase your installed height. Often, this change will make your seat pressure too low, which means new springs will be required. When you are checking the valvesprings for coil bind issues, it is also a good time to check the entire valve assembly to make sure there also won’t be clearance issues between the retainer and valve seal when the valve is at full lift.
Installing the Cam Bearings
The first step in installing any camshaft is properly installing the cam bearings. This is not difficult, but it does require a special cam installation tool. A cam installation tool uses a chuck to hold the bearing firmly while it is pressed into the cam bore. The chuck is normally wrapped with a thick rubber band to protect the bearing from scratches. A long bar extends from the chuck, allowing you to use a dead blow hammer to drive the bearing into position.
A cam-bearing installation tool is necessary because it is the only way to make sure that the cam bearings are installed perpendicular to the bore. The tool is a bit of an investment, but it is money well spent by the time you’ve built your second engine.
Installing the Cam
Installing the camshaft can get a little bit messy. Begin by lubricating the cam bearings with assembly lube. Beginning at the rear of the cam, lubricate it all the way up to the center cam journal. Use engine-assembly lubricant on the journals as well as the lobes if you are running a roller cam. If you are using a flat-tappet camshaft, lubricate the lobes with moly lube, which provides better protection against the sliding friction produced by the flat-tappet lifters. Use a moly lube or white-lithium paste on the cam’s distributor gear.
Install the cam into the block, being extremely careful not to bang the edge of the cam journals or the lobes into the bearings or the housing bores. If you chip the edge of a lobe, the camis ruined. Once the third journal is into the block, stop and finish lubricating the rest of the camshaft. Do not forget the fuel pump eccentric. Now, finish installing the cam into the block. Before moving on, make sure the camshaft spins in its bores without binding.
Several companies offer “cam handles” which screw into the front of the cam and give you a nice, solid handle to hold on to. These are nice, but not absolutely necessary. One trick is to attach the camtiming gear looselywith a bolt or two, which can provide a better grip than trying to hold on to the very end of the camshaft.
Determining Pushrod Length
If you are used to working with stock Chevy small-blocks, finding the perfect pushrod length isn’t as much of a priority as it is for a purpose- built race engine. That’s because stock-style, non-roller rockers have a large flat that makes contact with the valvestem tip. Missing the pushrod length by a little bit isn’t much of a problem because the flat on the rocker tip covers for the error.
Today, almost every racecar in every class can run roller-tip rocker arms. With the roller, the contact point between the rocker and the valvestem is very narrow. Miss the mark even by a little and you will get accelerated valveguide and valvestem wear. Big-lift camshafts and high-ratio rocker arms only exacerbate the problem. The pushrod length controls the location where the rocker tip contacts the valvestem, so getting that dimension correct is absolutely critical.
The problem with custom, handbuilt engines is that it is almost impossible to predict the correct pushrod length before the engine is actually assembled. Factors that can affect pushrod length include, but aren’t limited to:
• block deck height
• cylinder head deck height
• camshaft base circle
• lifter design and height
• valvestem length
Even the design of the rocker arms and their mounting system impacts pushrod length. Changing any one of these variables can require significant pushrod length changes, so it is often difficult to predict exactly what you will need ahead of time. Instead, the easiest and most reliable plan is to wait until the engine is assembled and then measure to see what length pushrods you will need. You can then order them from your performance parts distributor and even have them overnighted to your location, if necessary.
Improperly sized pushrods in an engine equipped with roller-tipped rockers can be disastrous. When the valvetrain geometry is correct, the tip of the rocker makes contact with the top of the valvestem just north (the intake side) of the center of the valve tip. It then moves across the center of the valve tip at mid-lift, and to the exhaust side of the top at maximum lift, and then returns as the valve closes. The distance the roller tip travels on either side of the center should be equal. This maximizes the downward force the rocker places on the valve and minimizes side loading on the stem.
When the pushrod length is incorrect, the rocker arm tip won’t be centered over the valvestem. In this case, when the camshaft activates the valvetrain, part of the force created by the motion of the rocker arm will be used to push the valvestem laterally as well as vertically. The valvestem then scrubs against the valveguide in the head, causing excessive wear on both components. This wear will open up the valveguides and allow the valve to flop around. If you are lucky, the only problems caused will be poor valve control (engine power will drop sooner at the higher RPM levels), a loss of compression, and an engine that mysteriously starts burning oil. If it isn’t caught, however, it can lead to a broken valve head and even a junked engine.
There are several ways of finding proper pushrod length, but when it comes to stud-mounted rocker systems, by far the simplest way is with a Sharpie and a wrench to turn the crankshaft. Begin by setting up the components for one cylinder. You can use an adjustable-length checking pushrod designed especially for this task.
Different manufacturers sell checking pushrods, but most operate on the same principle. They are typically a two-piece pushrod that is threaded to allow it to expand in length. You can adjust the checking pushrod’s length until you have what you need, then use that as a guide to order the real set.
Some, such as Comp Cams’ Hi- Tech Checking Pushrods are threaded so that each revolution is equal to 0.050 of an inch of length. The pushrod’s collapsed length is marked, so after you find the correct size, simply count the number of revolutions the pushrod has been expanded and you will know the right size. Keep in mind, however, that a checking pushrod is significantly weaker than a standard pushrod. A weaker checking spring must always be used with variable-length pushrods because standard racing valvesprings will bend them.
Whether you are using a checking pushrod or a standard piece that you think might fit correctly, begin by coloring the valve tip with either a Sharpie or machinist’s dye. Then, install the pushrod and rocker arm in place while the lifter is on the base circle of the cam. If you are using a solid lifter, set the valvetrain to zero lash. If you are running a hydraulic lifter, tighten down the rocker adjuster to your normal preload.
Using a wrench on the nose of the crank, spin the motor over several times, then remove the rocker and check the mark left on the valve tip. The roller tip on the rocker should have left a shiny spot where it wore away the ink you placed on the valve tip.
If the pushrod length is correct, this mark should be centered across the top of the valvestem. If it is too high (closer to the lifter valley), try a slightly longer pushrod or lengthen the checker. If it is too low (closer to the exhaust ports), try a slightly shorter pushrod. Now simply repeat the process, adjusting the pushrod length each time, until you have the wear mark centered on the valve tip. Sometimes the low-tech methods really are the best.
If you are using a checking pushrod, you have two options for determining correct pushrod length. First, you can count the revolutions the pushrod has been expanded, calculate the extra length, and then add that to the pushrod’s base length to find out what length pushrods you should order. Or, you can simply wrap a piece of tape around the threads so the length won’t change, package the checking pushrod up, and send it off to your preferred pushrod manufacturer. They will match your checker against their stock and ship back to you the correct pushrods (along with your checker).
Attempting to measure the pushrod with a set of calipers is problematic because the oiling holes on either end make it difficult to find the true “tip” of the pushrod. Most manufacturers use a custom measurement tool that most of us don’t have access to, so it’s really better not to even try.
When you do receive your new pushrods, it’s not a bad idea to repeat the checking process with the new rods. This is the easiest way to ensure a mistake hasn’t been made and that you haven’t been sent the wrong length pushrods. It’s easy insurance.
It’s a common analogy to refer to the camshaft as the “brain” of the engine. And that’s pretty accurate to think of the cam that way. After all, it’s the camshaft that controls when the valves open, as well as how fast, how far, and for how long. If the events of the camshaft controls are not properly timed to piston position and spark timing, power will definitely suffer. The process of dialing in the cam’s position relative to the crank is called “degreeing in” the cam, and its importance in building a racing engine cannot be overstated.
Often, the cam timing will be correct, and once you verify that you can move on. But if it isn’t, you can use an adjustable timing set to move the cam’s position forward or back to get results that match the specs provided by the camshaft’s manufacturer.
Frequently, when cam timing is found to be incorrect, the manufacturer is immediately blamed for grinding the cam incorrectly. While this is a potential reason for incorrect cam timing, it’s not the only one. More common reasons are a mistake made when machining the block or heads, extreme core shift, an incorrectly marked cam or crank gear for the timing chain, an incorrectly machined cam or crank gear keyway, or simply an accumulation of incorrect machining tolerances.
Even if everything is correct, there are times when you may want to purposely alter the cam timing, and being able to measure how much you’ve changed it is the only way to do it accurately. By advancing or retarding cam timing, you can influence where in the RPM range the engine makes power.
For most Chevy race engines, advancing the cam—or moving the point of maximum intake valve lift closer to TDC—will move the peak power lower in the RPM range. Retarding the cam, or going in the opposite direction, will move peak power higher in the RPM range.
Intake Centerline Method
Begin by installing your camshaft “straight up.” This means the timing set should be indexed so that the marks on the cam and crank gears are pointing directly at each other.
The most commonly used method for degreeing in a cam is based around finding the intake centerline and then measuring everything based off of that. Thismethod is popular because it doesn’t require a lot of specialized equipment, it can be performed with the cylinder heads on or off of the engine (which does require a few different tools), it requires few complex calculations, and it is fairly straightforward.
The centerline method does, however, require a few specialized tools that you can either assemble yourself or purchase as a kit. In the illustrations here, I am using an onhead cam degreeing kit from Powerhouse Tools. As its name implies, this kit allows you to degree in a camshaft with the cylinder heads and valvetrain intact. It measures camshaft lift at the rocker arm, so if you want to determine lobe lift you must remember to divide out the rocker ratio. An on-head kit is useful if you will be performing camshaft swaps on an already-built engine.
A more standard cam degreeing kit works with the cylinder heads off. This type is typically more useful during engine assembly because the cam is normally degreed in as soon as the crankshaft, cam, and timing chain are in place. This style measures movement at the lifter so that you can see actual lobe lift. With either style, the methods are virtually the same.
The first step is to make sure your camshaft and timing set are properly installed. It’s okay if you have all of the valvetrain assembled, but it is better to degree the cam during the pre-fitting process so you can catch any potential problems early. Begin by installing the timing set “straight up,” which is neither advanced nor retarded. The tick marks on the cam gear and crank gear should be pointed directly at each other. In other words, the cam gear tick mark should be pointing straight down while the tickmark on the crank gear points straight up.
You will notice in the photo illustrations that I am using an adjustablelength checking pushrod.When these photos were taken this engine was still in the pre-fitting stage, and I had not yet ordered the correct length pushrods. I am also using the lightweight checking springs included in the kit from Powerhouse.
If you choose to check the cam during the pre-fitting stages, just install one of the rocker arms you plan to run on the intake valve on the number-one cylinder. This is the second valve back on the right bank of cylinders. Use a checking pushrod at the proper length to set the valve at zero lash. If you are using a checking pushrod, make sure to always use a lightweight checking spring with it. Your race springs are too strong and will bend the checking pushrod. Finally, you should also have your cam card handy in order to check your findings against the manufacturer’s specs.
Finding Piston TDC
Install your degree wheel on the nose of the crank and use a piece of wire to form a pointer. You can mount the degree wheel by using the crank bolt, but this isn’t the best idea because you will need to turn the crank both clockwise and counterclockwise. This can loosen the bolt and allow the wheel to slip. And anytime the degree wheel slips, it’s time to start over.
The degree wheel can also usually be bolted directly to the harmonic damper, but the best method is to use a crank socket. This socket from Powerhouse not only secures the degree wheel but also allows you to use a ratchet with a 1/2-inch drive to easily turn the crank in either direction. Next, loop one end of the wire around a bolt and secure the wire to one of the cylinder heads. Bend it to form a pointer as close as possible to the wheel without touching it.
Now, remove the rocker arms (if installed) and install a piston stop through the spark-plug hole for the number-one cylinder. There are many different versions of piston stops, but the only requirement is that it must physically stop the piston from reaching TDC without damaging anything. If you didn’t remove the rocker arms earlier the valves could hit the piston stop before the piston does, giving you a false reading.
Slowly turn the crank until the top of the piston comes into contact with the piston stop. Spin the degree wheel (without turning the crank) to make this the zero point on your pointer, and tighten the wheel down. Now, turn the crank the opposite direction until the piston contacts the stop again. Make a note of, or mark the location of, the degree wheel indicated by the pointer with a pencil. Divide this number by two, giving you the piston TDC. For example, on this engine the point was 52 degrees. That means that the piston TDC is at the 26-degree mark.
Remove the piston stop and spin the crank until the pointer is at your TDC mark (26 degrees in this example). Without moving the crank, loosen the degree wheel and spin it until the zero mark is underneath the pointer. Now you know that piston’s TDC for your checking cylinder is at zero degrees on your degree wheel.
Measuring Valve Movement
Now you are finally ready to get started. Go ahead and reinstall the rocker arms, setting the lash to zero. Mount the dial indicator on the head and position it so that it touches the outside edge of the intake valve retainer opposite the rocker. Make sure the indicator is parallel to the valvestem. If it isn’t, your measurements will be incorrect.
Rotate the crankshaft clockwise until you reach the valve’s maximum lift. You will know you have reached maximum lift when the pointer on the indicator begins to move back in the other direction. With the valve at max lift, set the dial indicator to zero.
Rotate the engine counterclockwise until the indicator reads 0.100 of an inch. Turning the crank clockwise again, turn the engine until the dial reads 0.050 of an inch. You always want to hit your points by spinning the crank in the same direction it will rotate when the engine is running. This will remove variables created by slack in the timing chain. If your rings provide a lot of friction, it can be easy to go too far. If this happens, back up and try again. Just do not set the dial to 0.050 of an inch by turning the crank counterclockwise.
Once you have the indicator on 0.050, record the degree-wheel reading. Continue to rotate the wheel until the indicator goes to 0.050 on the other side of maximum lift. Again, record the number the indicator is pointing to on the wheel.
You can find your point of maximum lift in terms of crank degrees by taking the two numbers and averaging them. In this example, I came up with 80 and 140 degrees on the wheel. The average is 110 degrees, which means the intake valve for the number-one cylinder achieves maximum lift 110 crank degrees after the piston has reached TDC. It is also exactly where the cam card provided for this camshaft says it should be. Repeat the process on the exhaust valve to find the exhaust lobe centerline.
Many engine builders like to check the crankshaft location at 0.050 of an inch of lobe lift tomake sure the lobe’s opening ramp is correct. When doing this with an on-head cam checker, remember that you are measuring valve lift and not just lobe lift. It can still be done, but you have to know that your valve lift is at 0.050 of an inch lobe lift. For example, if you are using 1.7:1 ratio rockers, then when the valve reaches 0.085 lift you know the lobe is at 0.050 of an inch. The equation is
Total Lift = Lobe Lift x Rocker Ratio.
Now is also a great time to check your piston-to-valve clearances. With racing cams, we know that the intake valve is closest to the piston at 10 degrees ATDC (after Top Dead Center). For the exhaust valve, the piston is closest to the valve at 10 degrees BTDC (before Top Dead Center). Though this can sometimes vary by a degree or two, it is usually safe to check piston-to-valve clearance at these two points.
To check intake valve clearance, turn the crank until the number-one piston is at 10 degrees ATDC. Position the dial indicator on the outside edge of the valve retainer with the checking spring in place, as described in the section on degreeing in a cam. Now, push the end of the rocker down until the valve comes in contact with the top of the piston. Check the total movement on the dial indicator.
Now measure exhaust valve clearance by turning the crank until the number-one piston is at 10 degrees BTDC and repeat the process. A good rule of thumb for a race motor is to always have at least 0.080 of an inch of clearance on the intake and 0.120 of an inch clearance on the exhaust. The exhaust requires more clearance because it is closing as the piston is coming up, and if the exhaust valve has lofted there is more potential for the piston to make contact.
Written by Tony Huntimer and Posted with Permission of CarTechBooks