One of the advantages of cylinder heads is that they are relatively easy to work on and don’t require a garage full of specialty tools to do simple maintenance and/or assembly work. The work that we do not cover in this chapter is the more specialized procedures such as valve grinding, seat preparation, and installing guides. This should be left to a professional shop with the machines that can deliver this precision work for a decent price. But this still leaves quite a bit of work that you can do yourself in your garage and save yourself some money while also ensuring that the work is performed accurately and to your satisfaction.
This Tech Tip is from the full book: HIGH-PERFORMANCE CHEVY SMALL-BLOCK CYLINDER HEADS. For a comprehensive guide on this entire subject, you can visit this link:
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Valve Guides
Let’s start with a look at important areas that may not necessarily be at the top of your priority list. Let’s say that we have a set of used aftermarket aluminum heads that have seen some rough use and will require upgrading. While porting and bigger valves may come to mind first, the initial item on the rebuild agenda should be a new set of valveguides. If you think about the job a valveguide must perform, its primary mission is to maintain the valve at exactly the same location through hundreds of thousands of cycles. This precision is essential because if the guide loses its accuracy, the valve is allowed to move around in relation to the valve seat. This quickly destroys all those precise angles that your machinist spent hours establishing for ideal airflow.
For a small-block Chevy, the generally accepted clearance for valve guides to valves is 0.001-inch for the intake and 0.0015 inch for the exhaust. The exhaust side needs slightly more clearance for additional lubrication since the exhaust valve operates at a much higher overall temperature than the intake valve. This clearance also requires the valve stems to be parallel, straight, and smooth— which is not always the case. In the case of used valves, it’s worth the effort to carefully measure the stem diameter at both the top and bottom of the valve stem travel area to check for stem taper. Lateral thrust forces from the rocker arm across the top of the valvestem can easily produce a stem wear that may not be evident if the stem is measured only in the middle. If the guides are worn, after cleaning the heads, this should be the first step in the rebuild process. Most aftermarket aluminum heads are fitted with some type of bronze or bronze alloy valve guide material. Iron heads often incorporate iron guides, but bronze inserts are a good way to upgrade the guide material.
Most aftermarket heads are equipped with bronze wall guides that are soft enough to embed dirt without damaging the guide while also doing a good job of retaining oil for lubrication.
Assembling heads means more than just bolting everything together. A quality valve job also pays close attention to overall valve tip height to ensure that all the valves are as close to the same height as possible.
Once the guides have been installed and properly honed, this offers a good time to finalize your choice of valves. Often the valve head size is the main consideration, but the selection process is actually far more complicated. As we’ve seen in the cylinder head flow test comparisons, you often have a choice in terms of valve size with the same size cylinder head. If you are working on a set of iron production heads, there is often room between the intake and exhaust valves to increase the valve size. Many of the earlier 1.94/1.50- inch iron small-block heads offered sufficient room to go up to 2.02/1.60 inches. The most obvious example of this is the test in Chapter 4 on the 492 stock replacement heads. Our flow test on those heads included a change to larger 2.02-inch intake valves to complement the existing 1.60-inch exhaust valves already in place. The larger 2.02 intake valves greatly increased intake flow in comparison to similar sized iron production heads, making this a simple yet effective modification.
Valve spacing is the distance established by the manufacturer and, while it can be changed with significant modifications to a cylinder head, let’s assume that very few will attempt it. This means there is a practical limit to the size of the valves. Often, the move to a larger valve pays off in several different areas, not the least of which is a much larger flow curtain area. Combine that with work on maximizing the flow efficiencies of the larger valve and you’d have a great combination. This takes some attention to detail, especially if you are searching for that last ounce of power on your way to horsepower heaven.
Valves
The simplistic view is that the only real important consideration for a valve is its overall head diameter. But if you spend some time on a flow bench you will learn that subtle nuances and tiny angle changes can have a profound affect on the dry flow and wet flow characteristics of any cylinder head. Let’s start with the stem and work our way toward the business end of the valve. Current engine technology is always pushing the envelope toward lighter components, and this extends to valves as well. The GEN III small-block has already moved to an 8-mm (0.313-inch) valve stem, in comparison to the standard small-block 11/32-inch (0.343) valve stem. While this is only a 0.030-inch change, it does contribute to reducing valve weight, which reduces the necessity for higher-load valve springs and produces less abuse on the valve. AFR’s Eliminator heads are the first aftermarket first-generation 23-degree heads that we know of that have made the move to the thinner valve stem diameter to reduce weight.
One place where there may be some flow hiding is with a 12-degree back angle on the intake valve. The back angle is the radius between the stem and the valve head and has a direct impact on flow. This is an area of concern for the exhaust side as well.
The back angle on a valve is the transition radius between the valve stem and the head of the valve. This is an area almost universally ignored by enthusiasts, but flow bench testing has shown that cylinder heads become very attuned to this angle. There are no hard and fast rules, but the two most common intake valve back angles offered are 10 and 12 degree. Depending upon the intake port you are working on, the taller 12-degree angle can often offer a slight flow improvement. A taller angle might be better in some instances, but this does add weight to the valve. This is where paying attention to the valves offered by the manufacturer can pay off. For example, Manley offers most of its small-block intake valves with a 10-degree back angle, but there are a few selected 2.02 and 2.055-inch Race Flo valves available with a 12- degree back angle. Again, there’s no guarantee that these 12-degree back angle heads are worth more flow. That must be tested in your application in order to make that decision,
A taller angle, tulip-style exhaust valve with a back angle of 15 degrees is actually much more popular with valve manufacturers. This works because of the direction of flow and also because weight is not quite of the same concern since the exhaust valve is substantially smaller. These valves tend to increase mid-range flow while not hurting max lift flow. While back angles are not the magic solution to every application, they represent an area you should pay attention to when investigating flow improvements.
Valve Seat Angles
We can now move on to the actual valve seat itself. While virtually all small-block valves are ground with a 45-degree sealing angle, it’s worth noting that some experiments have been done in an attempt to try a less acute angle in hopes of improving flow. For example, the 30-degree seat angle was actually used on production Pontiac engines in the 1960s, but eventually fell out of favor as valve lifts continued to escalate. The 30-degree seat offers some small advantages at valve lifts of 0.300 inch and below, but tends to drastically cut flow at higher valve lifts. On the opposite end of the spectrum, 50 to 55-degree valve seats are now finding favor in Pro Stock and IHRA Pro Mod applications that also see valve lift numbers approaching one inch. This very steep angle promotes high-lift flow and, according to the head specialists we’ve spoken to, Pro Stock cylinder head specialists rarely concern themselves with flow under 0.400 inch.
While work continues with these taller valve angles, the street small-block is almost universally equipped with 45-degree seats. As evidenced by the work on either side of 45 degrees, this angle is the best compromise while also serving to act as a very efficient seat-cleaning angle as well for minimizing carbon buildup on the exhaust side. One trick detailed in Chapter 10 is a 30-degree back cut on both the intake and exhaust valves. This small step is almost always worth a few CFM improvement with little or no penalty and is a quick way to improve the airflow on almost any small-block head.
Regardless of which cylinder head you choose, consider a 30-degree back cut on both the intake and exhaust valves to be a requirement. Dart claims that this should be a standard requirement for any performance cylinder head.
Factory valves generally come with a dip or ditch created directly behind the 45-degree seat angle. This ditch tends to hurt flow in both the intake and exhaust locations. The best solution is to replace them with higher quality aftermarket valves that create a gentler radius.
Placement of the seat on the valve is also important. This is one area that separates quality machine work from the also-rans. Perhaps we should take this point to go back to the early style of valve grinding machines that used separate grinding stones to individually cut each angle in a valve seat. If the machinist planned to execute a three-angle valve job, he would first use the 45- degree stone to cut a wide seat angle. Then he might apply a 30-degree top cut that would narrow the seat from the top while adding the flatter angle that exits into the chamber. Then he would use a steep 60-degree stone to transition the lower portion of the 45-degree seat into the throat area of the port. Ideally, the sharp edge left by the bottom portion of the 60- degree stone would blend into the throat area of the port to enhance increased flow. This work required finesse and sharp attention to detail since excessive grinding could easily sink the seat and reduce flow while also changing the overall valve height at the spring. As you can imagine, this was tedious and time-consuming.
There are some who contend that there are certain advantages to grinding a seat to create sharper edges between the seat angles compared to a cutter style that can leave a tiny radius between the cut angles. This may be splitting hairs when it comes to cylinder head flow, although Dart’s work with the wet flow bench indicates that a sharp edge between valve angles improves wet flow characteristics. Since we don’t know of any back-to-back testing in this area, it sounds like it could go either way, and we’re not brave enough to hazard a guess, although if forced to choose, we would side with the valve job with a more pronounced sharp edge between the valve angles.
This Tech Tip is from the full book: HIGH-PERFORMANCE CHEVY SMALL-BLOCK CYLINDER HEADS. For a comprehensive guide on this entire subject, you can visit this link:
LEARN MORE ABOUT THIS BOOK HERE
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Most good machine shops now employ a cutter-style machine that uses a specific multi-angle cutting blade shaped to create three to five valve angles that are machined simultaneously into the valve seat. The Serdi machine was the first on the market, and since then several other manufacturers have joined this movement. The advantage of this machine is that it cuts a very precise multi-angle valve job in one operation as opposed to using several stones and several steps to create the same result. The latter demands that each different combination of seat angles or seat widths must be accompanied with its own specific cutter. This requires the machine shop to invest in perhaps dozens of cutters to accommodate all the different cylinder heads and multi-angle valve jobs. While the actual machine operation is accomplished much more quickly than the original valve-grinding effort, the cost of the machine is much greater as is the cost of the multiple cutting blades. All of this contributes to a more expensive valve job. But when you consider that the valve seat is considered to be the most flow-critical area in the cylinder head, it makes sense to spend a little more money here in search of more airflow, and therefore more power.
This is a typical valve seat-cutting tool that was first popularized by the Serdi Company. The machine tool is shaped to cut all seat angles simultaneously. This also ensures that each seat in both heads is machined exactly the same.
The more traditional way of creating a multi-angle valve job was with valve grinding tools such as these. Each angle required a dedicated stone that creates that angle in the valve seat. This is a much more time-consuming process.
A good machinist knows to cut as little off the seat as possible during machining. As the seat is machined, it sinks into the port, creating a tighter radius that reduces flow. Any seat work also affects the height of the valve on the spring side of the head.
Springs, Seats and Retainers
We touch on just a few highlights on this section since this is covered in much more detail in the High-Performance Chevy Small-Block Cams & Valvetrains book, which really should be in your small-block reference library if it isn’t already! Valve springs should be carefully matched not only to the camshaft that you’ve chosen, but also to the entire valvetrain. The classic approach is to choose a very stiff spring in order to control the valvetrain. While this does work, the excessive loads also take a toll on every valvetrain component. For a street engine this is especially harmful and annoying. The majority of small-block Chevy street engines run hydraulic flat tappet camshafts that do not need high valve spring pressures. Seat loads of around 100 to 110 pounds are commonplace and easily achievable with a single spring with a damper. Max valve lift pressures on larger cams may require a dual spring with loads as high as 300 to 310 pounds.
Aluminum heads require a steel spring seat to protect the soft aluminum from the valve springs. The outside diameter style (left) cups the spring around the outside circumference, while the inside diameter style (right) contacts the spring on the inboard side of the spring.
Many aluminum roller rocker arms use a thick arm to create adequate stiffness in the rocker. With larger diameter springs, this can create a clearance problem between the outside diameter of the retainer and the underneath radius of the rocker arm. Always check all 16 rockers for adequate clearance. Steel rocker arms often offer more clearance than aluminum rockers.
One reason for these conservative pressures is that a hydraulic lifter can withstand only so much valve spring pressure before the load overcomes the piston in the lifter and pushes the oil out (of the lifter). This creates a lost motion device that reduces lift and kills power. Then as the engine returns to idle, the gap created by the pumped down lifter creates a horrific noise from all that clearance. The engine will also run rough, stall, and generally be no fun to drive until the lifters reestablish their proper position. This reinforces why matching the valve spring pressure to the camshaft and the valvetrain is so important. The camshaft companies make this easy for you by offering camshaft kits that come complete with the springs, lifters, retainers, springs, and keepers. If you are relatively new to the art of valvetrain component matching, it’s generally a good idea to let the cam company pick out the proper springs.
A 0.100-inch longer valve produces additional lift clearance, but keep in mind that this additional stem length also increases the valve weight, which is more mass that the valve spring must control. This also gives the retainer more leverage over the guide.
Bigger is sometimes better, but 10- degree locks are also heavier. Adding weight, especially at the very top of the valve, is not a good idea. Only consider going to larger 10-degree locks if you have had retainer and/or locks problems in the past.
This also extends to the retainers. It is critical that the retainers be included in the decision-making process and matched to the springs in order to get maximum benefit from each. Improper spring location on a retainer can cause all kinds of dynamic problems that could extend to broken springs and retainers and possibly a dropped valve. None of those situations are something that anyone wants to experience, especially at high RPM. The results are generally expensive.
Weight is another critical factor when choosing valvetrain components. Large-diameter dual springs, monster steel retainers, and big valves all have mass, or weight. At even a mild 6,000 rpm, these components have to accelerate during the lift cycle, then slow down to change direction at maximum valve lift and then accelerate back toward the closing side and stop again as the valve hits the seat. Anything you can do to lighten this process (without sacrificing strength) will increase durability, making the valvetrain happier. Titanium retainers have become very popular, even with street engines, because they reduce the mass of the retainer without sacrificing strength. Keep in mind that a small-diameter spring, or at least a small spring wire diameter, also reduces the mass of the spring that must itself accelerate and slow down at very high rates of speed.
From left to right are a single spring with a damper, a dual spring, and a conical or beehive spring. Conical springs are more difficult to manufacture and more expensive, but offer many advantages including a variable spring rate.
Often you may have to grind flats on a couple head bolt washers in order to fit them between large diameter valve springs on a small-block Chevy head. The best way to accomplish this is to place the washers in the bolt-hole recesses before the springs are installed.
The latest generation of conical, or beehive, valve springs is a move in that reduced mass direction where the small-diameter top of the spring is lighter and uses an exceptionally light retainer. This dramatically reduces the amount of spring/ retainer mass that the spring must control. This allows more of the valve spring’s pressure to be used to control the valve instead of the spring itself. If there is a disadvantage to these conical or beehive springs, it is that they are more expensive than normal valve springs, and also that these springs do not need or use a damper. While spring breakage has not been a problem with the high quality of valve spring wire, if the spring should break, there is little short of good luck that would stop the retainer from releasing the valve into the piston. The beehive springs tend to work best in hydraulic roller applications where the lifter contributes additional weight to the valvetrain. We’ve seen tests in which a Rat motor picked up 20 hp just with the installation of a set of conical valve springs over a more traditional, large diameter single spring with a damper.
Locks are also important since they are what keep the spring and the valve tied together. Spend the extra money for machined locks rather than stamped ones. They are far better and cost only a few dollars more. It’s also important to mention that the small tabs that locate the locks on the valve are not what hold the locks in place. The pressure angle created by the 7 or 10-degree taper between the locks and the retainer are actually what locks the retainer to the valve. The tiny notches on the locks are there only to position the retainer on the valve. The 10-degree locks are not necessary for most street engines. They offer a greater surface area, but are also heavier. Most 10-degree locks also offer a cutout that is used to position a lash cap, which is also something most street engines don’t need.
Beehive or conical springs (left) are dramatically lighter than a typical dual spring both in spring mass and also in retainer weight. The spring not only controls the mass of the valve and retainer, but its own mass as well. By reducing spring and retainer weight, the spring can offer more spring pressure to control the valve and the entire valvetrain is happier and more durable.
One last component with aluminum heads is a set of valve spring seats. Besides creating a steel barrier between the very hard steel valve spring and the soft aluminum head material, valve spring seats also serve as locators for the springs. There are two basic types, either an inside or outside diameter locator. Spring engineers tell us there is no real difference in terms of performance between the two. It’s more a matter of preference, although with large-diameter springs on a small-block Chevy, space comes at a premium so the inside diameter locators are easier to work around.
This Tech Tip is from the full book: HIGH-PERFORMANCE CHEVY SMALL-BLOCK CYLINDER HEADS. For a comprehensive guide on this entire subject, you can visit this link:
LEARN MORE ABOUT THIS BOOK HERE
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Installed Height
Even if you never plan to assemble a set of cylinder heads, it’s important to know the procedure and why extra time is spent carefully assembling the cylinder heads so that the valvetrain can perform its tasks efficiently for thousands of trouble-free miles. We’ve already established the valve-to-guide clearance, so the next step is to measure the valve spring installed height. This is the distance from the spring seat in the head to the bottom side of the retainer where the spring seats. The stock dimension for a small-block Chevy is 1.780 inches. This establishes the starting point, if you will, for the valve spring. This compresses the valve spring slightly, which creates its seated pressure. It’s also necessary to measure all 16-valve installed heights because the distance from the spring seat to the retainer will vary by perhaps 0.030 inch or more. This is due to variations in the height of the valve as well as different positions of the valve seat.
Installed height is the actual height of the spring between the retainer and the seat. The easiest way to measure this is with a height mic that quickly reads out the distance.
Different manufacturers make retainers that appear to be the same, but in reality create much different installed heights. One trick to tighten or loosen an installed height figure is to try a different manufacturer’s retainer.
Another choice for altering installed height is valve locks. Crane offers locks in standard, plus 0.050 and minus 0.050-inch heights that make quick work of tuning the installed height. Ideally, you want to use these same locks on all the valves. If you use them only on selected valves, it is imperative to keep an accurate record of their location so they don’t get mixed in with other, standard locks.
The machinist determines the shortest installed height of all 16 valves and uses that dimension as his reference point for the test. Let’s say his shortest installed height is 1.760 inches and the tallest is 1.800. The machinist can then install a 0.040- inch thick shim on top of the spring seat for the tallest installed height valve, which reduces that distance to the 1.760-inch height. This creates the exact same load (within perhaps 10 pounds) on each valve. This ensures that the valve spring pressures are equal and each valve has the same control load governing its performance.
Retainer-To-Seal Clearance
Next we get into checking retainer-to-seal clearance. Here, our dimension is the distance from the bottom portion of the retainer just underneath the locks to the top of the valve guide seal. We need to have adequate clearance between the retainer and the seal so that valve lift does not push the retainer into the seal. This damages the seal and causes oil control problems, especially on the intake valve side of things. The minimum clearance between the retainer and the seal is 0.050 inch, and even this is tight. The point is that we don’t want the retainer smacking the seal at high RPM when perhaps a small amount of valve loft or float kicks the lift up slightly. This can happen in over-rev situations when the spring loses control of the valve for a short period of time.
There are several ways to improve retainer-to-seal clearance should you discover an interference or tight condition. One of the easiest ways to increase the installed height of the spring is by raising the retainer. Crane, for example, offers machined locks in 0.050-inch steps where the retainer can be raised or lowered by 0.050 inch. By raising the retainer this amount, this also reduces the valve spring seat load and overall pressure on the valve. With a 360- pound-per-inch spring, increasing the installed height by 0.050 reduces the load by approximately 18 pounds. Raising the retainer also moves it closer to the inside of the rocker arm. Raising the retainer has the same effect as sinking the valve stem tip in the retainer, so you need to be careful to ensure there is adequate clearance between the roller tip and the retainer.
Be sure to measure guide diameter to ensure you get the right positive seal diameter. Positive seals come in two different diameters of 0.500 and 0.531-inch guide diameters.
Often, heads like the Vortec iron production head have tall valve guides that must be machined to increase retainer-to-seal clearance. You can purchase these cutting tools from COMP or others and do the work yourself to save some money.
Always measure retainer-to-seal clearance, as this is often the tightest clearance in the valvetrain. This is very easy to measure with a dial caliper.
Often, the published coil bind figure and the actual measured coil bind are different. Generally, the spring compresses tighter than the published length, which adds to the clearance to bind.
Another alternative, although more expensive, is to increase the length of the valve by 0.100 inch. A longer valve automatically generates an additional 0.100 inch of room between the retainer and the seal. It also allows room for more valve lift. Generally, you want to have all this thought out ahead of time so your engine plan accounts for a 0.650-inch lift camshaft that would quickly overcome a set of stock-length valves. This is why pre-planning your engine on paper first is so important—so you do not have to face purchasing two sets of valves for your engine. Also keep in mind that a longer valve and a taller valve spring both contribute additional weight to the valvetrain, increasing the amount of weight that the valvetrain must control at high engine speeds. Should you decide to go with longer valves, purchase a complete set. Since a small-block Chevy uses a standard valve height for both the intake and exhaust valves, you must use 0.100- inch longer exhaust valves as well, even if they are not needed. This keeps the valve stem heights all the same, which keeps all the pushrods the same length. See how complicated this can get?
Coil Bind
We also need to check for valve spring coil bind. This is the height of the spring fully compressed with the coils stacked solid. This measurement, subtracted from the installed height, is the amount of room the spring will allow for valve lift. As an example, let’s say we have a 1.800-inch installed height with a given valve spring that has a coil bind dimension of 1.150 inches: 1.800 – 1.150 = 0.650 inch. With a 0.590- inch valve lift, we have a coil bind clearance of 0.060 inch. Depending upon whose specs you read, either 0.050 or 0.060 inch is the minimum clearance to coil bind. Generally, this clearance will create 0.010 to 0.015 inch of clearance between each of the coils. Running valve springs near coil bind while still maintaining clearance from coil bind does not seem to affect spring durability.
We’ve also discovered that published coil bind numbers don’t always agree with actual coil bind dimensions. Generally, the cam and valve spring companies will be conservative with their specs, which means that a given spring may have a coil bind spec of 1.210 inches for example, but will actually coil bind at a much lower number, perhaps something like 1.180 inches. This is why you must check all 16 springs for coil bind anytime you are assembling heads. A shorter coil bind number just might give you the extra clearance you need without having to resort to taller valves.
Conclusion
There are many advantages to understanding how to assemble and tune up your own cylinder heads. This requires investing in a specialty spring compressor (both for on and off the engine) and you’ll be surprised at how often you will use these tools once you own them. Cylinder heads have the greatest potential effect on engine performance, so it just makes sense to have the knowledge and the tools to work on these components yourself. Once you’ve gone through the procedure a couple of times, assembly and blueprinting your own heads becomes almost second nature.
Sharp edges on the intake valve help to shear fuel as it enters the combustion chamber and tend to improve distribution within the combustion chamber. While difficult to see, the valve on the left has a wider margin and no radius to the chamber side face of the intake valve. Often, the published coil bind figure and the actual measured coil bind are different. Generally, the spring compresses tighter than the published length, which adds to the clearance to bind.
On the exhaust side, it appears that a slight radius on the face of the valve may also help flow, although this is not a universal truth. You will need to test this on each specific head application since not all exhaust ports respond to this modification.
Written by Graham Hansen and Posted with Permission of CarTechBooks
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