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Thread: Valve Clearance

  1. #1
    Member JUNIOR BUILDER
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    Valve Clearance

    Was going to ask on this forum, the best way to check valve to piston clearances, but started reading elsewhere on the net and found this. There are three different articles, I'll try and post them seperately for anyone else's benefit. I think I would prefer #1.....

  2. #2
    Member JUNIOR BUILDER
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    Method #1

    Overview Piston to valve clearance deals with the space between the valve and the piston during a critical point in the cam timing knows as the "overlap period". At no other point in the four stroke cycle does the valve get closer to the piston. Thus it is imperative that enough clearance exist in order to avoid a collision between the piston and the valve.The overlap period occurs near the end of the exhaust stroke and at the beginning of the intake stroke, when both valves are open for a short During Overlap both valves are open as the piston rises.
    period of time. Overlap is a critical period in the cam cycle, as the movement of exhaust gases out of the combustion chamber "draws" in a fresh charge through the intake valve. Many people have even erroneously measured clearance at the point of maximum valve lift, thinking that is the most likely point at which interference would occur. However when a valve reaches its maximum lift, the piston is actually the furthest away since this occurs during the intake stroke, when the piston is moving down the cylinder bore.Minimum clearances between the valve and piston should be 0.080" on the intake valve, and 0.100" on the exhaust valve. If you run aluminum rods, the clearances should be even greater. In this article we'll show you the two methods most commonly used to measure piston to valve clearance.Clay Method
    There are a couple ways to check for clearance. One way is to place a 1/4" thick strip of clay on the piston, and turn the engine over through one full cycle (two rotations past TDC) allowing the valves to make an impression in the clay. The downside to clay is that the heads must come off to do the check. Clay is also cumbersome, it tends to stick to the piston, and requires a lot of practice to get reliable measurements. The clay is then carefully peeled off the piston, cut in various locations, and the thickness is measured to determine how much clearance exists. This method works well in that it gives you a three dimensional view of not only how much clearance there is, but also where the interference is occurring. This helps determine if the problem is too shallow of a valve pocket in the piston, or if the pocket is not wide enough for the valve head.

    Spring Method
    This involves using a light weight valve spring and a feeler gauge. The procedure is to replace the intake and exhaust valve springs for one cylinder with light tension "checking" springs. They can be bought at any hardware store for under a dollar. They must fit squarely between the seat and retainer and be just stiff enough to hold the valve closed.Adjust the rockers to zero lash, irregardless of whether it is a hydraulic or a solid lifter. It is extremely important if you are using a hydraulic lifter, to AVOID preloading the lifter. Instead you want to remove all the slack in the pushrod, without forcing the plunger in the lifter downward. Be sure to make the lash adjustments when the cam lobes for that cylinder are on the base circle. (Piston is at Top Dead Center of the compression stroke.) A quick way to set the lash is to adjust the intake rocker when the exhaust valve just starts to open. Then adjust the exhaust rocker when the intake valve is just beginning to close (coming back up after full lift.) The benefit to this approach is you don't have to take the heads off, but you do need to remove the valve springs with some sort of spring removal tool. With the test cylinder ready, set your feeler gauges to 0.100". Starting with the piston at top dead center of the compression stroke, rotate the crank one full cycle in direction of normal rotation.As the piston travels back up the bore the exhaust valve opens and the exhaust stroke begins. Keep an eye on the valve train, and you will notice the exhaust valve spring slowly compress as the valve fully opens. As you approach TDC you will notice the intake will begin to open before the exhaust valve has fully closed. This period, roughly 10-15 degrees before and after TDC is the overlap period. Both valves are slightly open and piston is near the top.It is precisely during this overlap period where you'll want to use the feeler gauge to measure the distance between the valve stem and the rocker arm tip or roller. The piston is so close to the valves at this point that you can push down on the checking spring and feel the valve contact the piston. It is this distance which is critical, and needs to be a minimum of 0.080" on the intake side, and 0.100" on the exhaust. It may take several cycles to get the hang of where the overlap period is and how to take the measurements.
    ConclusionYou will notice that the overlap period does not last very long, and in fact it is very easy to miss. In terms of degrees of crankshaft rotation, the total overlap period may be as little as 40 degrees for a mild cam, in other words 20 degrees before and 20 degrees after TDC. At some point within this range the piston and valve will be the closest. Therefore it is imperative to check the clearance at least every two degrees during the overlap period. Take a few measurements, then rotate the crank a few times and check again until you get the same number each time you measure. A helpful trick is to push down on the retainer with your thumb so that the valve a contacts the piston. Use your index finger to keep the rocker arm taught against the pushrod. As you rotate the crank the valve will "ride" the piston, and you will be able to see the space between the valve stem and rocker tip get smaller and smaller during the overlap period. Use your feeler gauge to measure the point where the gap is the smallest. If you determine that you have at least .080" on the intake and .100" on the exhaust side then you have sufficient clearance to run that cam. In the event you don't have enough clearance, there are several options. The easiest option is to not use that cam. Most people don't want to hear that, especially since they've already paid for the cam. This leaves a couple options in order to achieve the necessary clearance.If your clearances are relatively close, within 0.010", you can attempt to retard or advance the camshaft. Retarding the cam effectively opens the exhaust valve earlier and the intake valve later. This translates to a few thousandths of an inch more clearance on the intake side, and roughly the same amount less on the exhaust side. In the case where your intake valve clearance was running close, and your exhaust side had plenty to spare, you could retard the camshaft 2 to 6 degrees and gain some clearance. Conversely, advancing a cam will bring the intake closer and the exhaust slightly further from the piston during overlap. Realize, however, that when you advance or retard a cam you change the performance characteristics and power-band that was designed by the manufacturer. The other option is to replace the pistons with aftermarket types which accommodate larger valves. Replacing pistons means a significant amount of work and money in tearing down and rebuilding the motor. However in the end an aftermarket piston with a deep valve pocket will ensure proper clearance.The final solution is to notch the existing pistons. The following steps detail how to accomplish this. Gaining Clearance
    Put a dab of white latex paint on the valve face, and then with the head on the block and the piston at TDC, force the valve down until it contacts the piston. This results in the paint transferring on to the piston in precisely the location which needs clearancing. In this case, to the right of each intake valve relief.

    With all the pistons marked, put a grinding stone on the Dremel® tool or die-grinder and carefully extend the valve relief's. Stop often to install the head and to remeasure the clearance. Try not to over do it, as removing weight results in upsetting the balance of the motor, and also weakens the piston. Be sure to take all the necessary precautions to keep the grindings out of the motor. We covered up all the adjacent cylinders and used a shop vac to suck the grindings up before we moved on. The "professional" way to do this is to buy a piston notching tool, such as the Isky Cams product shown here. The tool is simply a valve head with "teeth" on the face. This head attaches to an arbor, and is then installed in place of the valve in the cylinder head. A drill is attached to the arbor and used to turn the tool, and as a result the teeth grind the piston exactly where the valve would contact. The tool comes in various valve and guide diameters to work with basically any popular engine and head combo.

  3. #3
    Member JUNIOR BUILDER
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    Method #2.....
    There are two ways to get it wrong. The first is usually catastrophic: If you have insufficient clearance, the pistons hit the valves, followed by the predictable parts damage. On the other hand, if you have too much clearance, the engine will run, but it won’t achieve anything close to its performance potential. In either situation, you’ve wasted time and money that could have been saved by checking piston-to-valve clearance properly.
    Here is the method we use at X-Brand Racing Engines. Not everyone may agree with our technique, and that’s all right. I am convinced, however, that it is the best way to achieve repeatable, accurate results.
    First, put away your clay and your light-tension checking springs. The time-honored practice of mocking up a motor and putting clay in the valve pockets to measure clearance introduces too many variables to be trustworthy. The amount of clay on the piston top, the density of the clay, the effect that the clay has on the relative positions of the valve and piston, and the difficulty of measuring the thickness of the compressed clay accurately are just a few of the sources of potential error with this method. You don’t use clay to measure piston-to-wall clearance and bearing clearance, and you shouldn’t use it to measure valve clearance either.
    You must use the same components when checking valve clearance that you intend to use when you assemble the engine. This includes the same lifters, the same pushrods, the same rocker arms, and the same valve springs. Light-tension checking springs simply can’t duplicate the load and deflection that the valvetrain experiences with stiff race springs. The difference in actual valve clearance between checking springs and race springs is typically .020 to .030-inch. If you set up your engine with checking springs with .075-inch intake valve clearance, the actual clearance with race springs will be closer to .100-inch.
    The first step in the X-Brand method is to determine whether the valve pockets are located properly. A discarded valve that fits your cylinder head makes an ideal tool. Cut off the head of the valve and turn the stem to a point. Preassemble the engine with your bare cylinder heads (remember to use a previously compressed head gasket), put masking tape on the ring lands to center the piston in the bores, a bring the piston to 10 degrees before or after Top Dead Center (it doesn’t matter which at this point) and drop your homemade punch into the intake and exhaust guides. Give the punch a gentle tap to mark the valve stem centerline on the piston and then remove the head.
    To check the valve pocket location, remove the cylinder head and set a pair of calipers to the radius of the valve head (for example, for a 2.500-inch diameter valve head, set the calipers at 1.250-inch). With one point centered on the punch mark, swing the other point around the valve eyebrow. If the caliper hits the edge of the valve pocket, so will the valve. I recommend a minimum of .050-inch radial clearance between the edge of the valve and the pocket.
    If the valve relief is located properly, you must then check its angle. Again, a discarded valve with the proper stem diameter makes an excellent checking tool. Weld or epoxy a small steel ball onto the edge of a steel valve. Mark the tip of the valve stem with a notch in line with the ball as a reference point. If your engine has two different valve angles – a big-block Chevy or Cleveland Ford, for example – you will need to make intake and exhaust checking valves.
    Insert the checking valves into the bare head and install the head on the preassembled short block. Bring the piston to 10 degrees before or after TDC. Put a dial indicator on the tip of the valve stem and slowly rotate the valve with your fingers. If the stem rises and falls as the ball travels around the valve notch, the angle of the relief is incorrect. You can draw a “road map” by noting the position of the reference notch as you turn the valve. For example, if the valve stem rises near the top of the dome and falls at the bottom of the valve notch, then the angle of the valve relief is too steep. Using this technique, you can precisely determine how much material must be machined to correct the angle of the valve relief.
    After you have established that the valves have enough radial clearance in their respective notches and that the angles of the valve reliefs are correct, you are finally ready to check piston-to-valve clearance. Assemble the short-block and cylinder heads with the valvetrain components you intend to use. Adjust the valve lash, set up a dial indicator on the valve spring retainer so that its plunger is parallel to the valve stem, and bring the piston to 10 degrees BTDC. Compress the spring on the exhaust valve and measure the movement required for the valve to contact the piston. (We used a tool similar to a valve spring tester with a solid bar instead of a flat spring to compress the valve spring.) Move the dial indicator setup to the intake valve, rotate the crankshaft to 10 degrees ATDC, and repeat the procedure.
    This procedure and a little patience will ensure that your engine’s piston-to-valve clearance is measured correctly. The ideal clearance dimension for your combination will depend on the weight of your engine’s valvetrain components (especially whether you use steel or titanium valves), the maximum rpm, the tension of the valve springs, the characteristics of the camshaft, and other factors.
    In most instances, off-the-shelf pistons have valve pockets that are too deep and provide much more valve clearance than is really necessary. This is perfectly understandable, because the piston manufacturers can’t anticipate every possible combination of cylinder head, camshaft, block height, valve height, gasket thickness, etc. They don’t want to hear from an angry customer who crashed all the valves in a new engine, so the piston makers typically machine the reliefs in shelf-stock pistons with clearance for the worst case scenario. Then to compensate for the oversize valve reliefs, the piston dome is made taller to produce the advertised compression ratio.
    The downside of this situation is that overly generous valve reliefs cost horsepower. For example, a 2.50-inch diameter valve pocket that is .100-inch deeper than it really needs to be has a volume of 8 cc’s. That much volume at TDC can significantly lower the compression ratio, reducing efficiency and power. It’s much better to have the proper piston-to-valve clearance and a shorter dome that doesn’t intrude as far into the combustion chamber.
    Imagine two engines with the identical compression ratio. One has pistons with valve reliefs that are too deep and domes that resemble Mt. Everest; the second has pistons with optimized valve reliefs and shorter, rounded domes. Both engines have the same volume above the piston at TDC, but the engine with the proper valve reliefs and shorter domes will have a substantial horsepower advantage.
    Measuring piston-to-valve clearance properly is one of the basic operations that every novice engine builder should master. It’s not as sexy as flow bench testing or as high-tech as running dyno simulations on your laptop, but it is an absolutely essential step in building a reliable and powerful racing engine.

  4. #4
    Member JUNIOR BUILDER
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    Method #3.....
    Checking Piston To Valve Clearance (PUTTY OR CLAY METHOD)
    Brand-X strongly urges you to check the piston to valve clearance on the larger street cams and all race cams. The easiest and most accurate way to check this is to place strips of modeling clay on top of one piston, and then rotate the engine over by hand with the cylinder head bolted in place and all of the valve train components adjusted. If there is any resistance during rotation of the crankshaft, STOP! The piston has probably hit the valve. Then you must decide whether to fly cut the piston, or exchange the cam for a profile that will fit into your engine. Note: Minimum piston to valve clearance is .080” on the intake and .100” on the exhaust valves. If aluminum connecting rods are being used, add a minimum of .030” to these suggested clearance figures. Aluminum rods will stretch and expand more than steel rods.
    Note: Be sure to check piston to valve clearance after the cam has been degreed. The positioning of the cam in the engine will greatly affect the piston to valve clearance.
    With the camshaft installed, remove the cylinder head from the block. Clean the combustion chamber and the top of the piston and valve reliefs. The cleaner the piston, the better the clay will stick to it.
    Apply a strip of model clay 3/8” to ½” wide approximately ¼” thick to the pistons. The clay strip should be long enough to run across both the intake and exhaust valve reliefs. Applying a small amount of oil to the clay will prevent it from sticking to the valves as they press into it.
    Reinstall the cylinder head with the head gasket that is going to be used. It will not be necessary to torque the head yet. All head gasket manufacturers can tell you what the compressed thickness of their gasket will be. Measure the gasket before you install it permanently and add the difference to the piston to valve clearance. Install a sufficient number of head bolts to secure the head in place while you are rotating the engine. Install the pushrods, lifters and rocker arms on the cylinder you have prepared for the clearance check.
    Adjust the rocker arms to their suggested clearance. If the camshaft you are checking uses hydraulic lifters, you must temporarily use solid lifters in their place. Hydraulic lifters bleed down and will provide a false measurement. Once the hydraulic lifters are replaced with solid lifters, adjust the lash to “zero.” Be sure not to pre-load the valve spring. Be sure to reinstall the hydraulic lifters before starting the engine.
    Now turn the engine over by hand in the normal direction of rotation. Be sure to rotate the engine over two times. This will be one complete revolution of the camshaft and assure you of an accurate reading on both the intake and exhaust. Remove the cylinder head from the block. Do this gently, so the clay is not disturbed. It may be stuck to the valves or combustion chamber, so be careful.
    With a razor or sharp knife, slice the clay cleanly -lengthwise through the depression, and peel half of it off the piston. The clay’s thickness in the thinnest area will represent the minimum piston to valve clearance.
    To accurately check the thickness, use a set of dial calipers. The clay can also be measured close enough with a thin steel rule.

  5. #5
    Senior Member MASTER BUILDER
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    The .080 on the int isn't correct.you only need the same clearance that you use for piston to head. the piston is never chasing the int valve,its moving away so .035-.040 is plenty of clearance.Bill
    Hendrens Racing Engines
    1310 U.S. Hwy 221 North
    Rutherfordton N.C. 28139
    828-286-0780 Bill, Steve, Mike
    www.hendrensracingengines.com


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