Pistons
This article deals with measuring and understanding clearance issues relative to the piston and piston rings. Here I discuss clearances involving piston to bore, piston dome to head, valve to piston, and ring clearances, in addition to file-fitting rings.
Measuring Piston Skirts
Prior to boring and/or honing cylinder bores, the pistons that are intended for installation must first be measured for skirt diameter to determine the required bore diameter adequate for piston-to-wall oil clearance. Piston diameter dictates the final bore diameter.
Measuring piston diameter is not determined at the ring area; rather, the measurement must be taken at the skirt, but only in the specific height location specified by the piston maker. For example, the piston maker may specify that the diameter measurement be taken at a point exactly .500 inch from the bottom of the skirt. Measuring at the specific point recommended by the piston maker is necessary, since pistons are not the same diameter from top to bottom, by design. A very slight “cam ground” or barrel-shape profile at the skirt area is designed to reduce friction and to promote piston stability, which in turn optimizes ring seal. Both the cylinder block bores and pistons (hypereutectic or forged) must be measured at room temperature approximately 68 degrees F, because metals expand and contract with temperature variations. The measured piston skirt diameter dictates the required cylinder bore diameter to achieve the piston maker’s specification for piston-to-bore clearance. Always measure skirt diameter at the point indicated by the piston maker.
Skirt diameter is measured using an outside micrometer. Considering the precise manufacturing techniques for today’s aftermarket performance pistons, you should be able to measure only one piston from the set and assume that all remaining pistons have the same diameter. However, it never hurts to measure each piston just to be safe. Most of today’s performance piston makers hold their tolerance to about .0005 inch at the gauge point.
Before final-honing cylinder bores, the piston skirt must be measured at the exact skirt height location specified by the piston maker.
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If pistons were sourced from an established manufacturer, such as JE, Wiseco, Ross, CP, Mahle, etc., it would be extremely rare to encounter a problem. Manufacturers such as JE normally hold to a very consistent tolerance of +/- .0005 inch, and even offer a “critical process” for custom piston orders held to even tighter tolerances.
Piston-to-wall clearance is specified by the piston maker. Hypereutectic pistons tend not to expand as much under operating temperature as forged pistons, so hyper pistons usually require a tighter piston-to-wall clearance. Always follow the piston-to-wall clearance that is recommended by the piston maker. As a general rule, given the same piston skirt measurement, a hypereutectic piston may require approximately .0015- to .0025-inch clearance, while a forged piston of the same size may require .0035 to .0055 inch for a naturally aspirated street application. Adding nitrous injection or forced induction may require an increased bore diameter of .0020 to .0035 inch for hyper pistons or .0045 to .0065 inch for forged pistons. Adding nitrous or forced induction may require an additional .001 to .002 inch of added clearance as opposed to naturally aspirated applications, depending on the piston maker’s specs. Again, follow the piston maker’s specifications for piston-to-wall clearance when the cylinder bores are being final-honed.
Although on the subject of skirts, some piston applications are available in an asymmetric design. This means that the skirt size on each side of the piston is different; a larger skirt area is on the piston’s major thrust side (exhaust side of the right bank and intake side of the left bank) and a smaller skirt is on the minor thrust side, where friction and load are minimal. This saves weight and reduces bearing wear.
Asymmetrical pistons feature a larger skirt area for the major thrust side and a smaller skirt for the minor thrust side. The wider skirt accommodates the high stresses of the piston as it is pushed against the major thrust side of the cylinder. The exhaust side is located on the right bank and intake side on the left bank. Using a smaller skirt on the minor thrust side saves weight. These JE pistons (shown) for the LS engine platform feature a forged side relief design, and it provides added clearance to the reluctor wheel in stroker applications. Asymmetric pistons are labeled for bank location and front orientation.
Checking Ring-to-Piston Fit
Never assume that the rings you have fit your pistons correctly. Refer to the piston maker’s specs and measure for ringside clearance and radial clearance. Insert a top ring into the top ring groove and use a feeler gauge to measure side clearance. With the ring seated against the floor of the groove, side clearance is the distance from the top surface of the ring to the roof of the groove. This is likely in the .001- to .002-inch range. Radial clearance, or back clearance, is the clearance behind the inside of the ring to the inside wall of the ring groove. This is likely in the .005-inch range. Perform this check prior to file fitting the rings to avoid wasting a bunch of time working on rings that don’t fit the grooves.
Modifying Domes
If domes are to be modified, a dedicated, adjustable piston vise is a necessity. This fixture allows mounting the piston securely and is adjustable for valve angle. Before attempting to mill piston domes, pay particular attention to existing dome thickness, specifically the thinnest section of the dome. Depending on the piston material, the thinnest allowable area might be .150 or .200 inch, as examples. If in doubt, check with the piston maker for advice on this. If an area of the dome is too thin, the dome can sag under heat and pressure and potentially break through.
Checking piston ring side clearance with a feeler gauge. Follow the specification provided by the piston maker. Side clearance is usually about .001 to .002 inch.
Ring back clearance is checked with the ring installed and pushed back to the inner groove wall. The distance from the face of the ring and the outer diameter of the piston surface is measured to determine back clearance. This is usually about .005 inch.
Before milling a piston dome, consider the thinnest section first to determine how much, if any, can safely be removed without compromising dome strength. The black mark on the cross-section indicates the thinnest section of this piston dome.
Depending on the combination of cylinder head design, piston design, piston compression distance, crank stroke, rod length, head gasket thickness, block deck height, and valve-to-piston clearance possibly due to camshaft profile and rocker arm ratio, it’s possible to run into clearance issues. First, consider clearance between the piston dome and head, and between piston dome and the spark plug. The suggested absolute minimum clearance (with the head gasket in place and fully torqued) between the piston and head and/or spark plug is .040 to .050 inch for steel connecting rods and .055 to .060 inch for aluminum connecting rods (due to potentially greater expansion rate of alloy rods).
If the piston domes require modification by fly-cutting or making valve pockets deeper or larger, a dedicated piston pin vise allows the piston to be secured in the proper location and angle for the milling machine. This piston is being cut to obtain more valve radial clearance at Ross Racing Engines in Niles, Ohio.
A common method of checking piston dome clearance relative to the head and spark plug is to apply modeling clay to a dry, clean piston dome; coat the chamber lightly with a thin oil to prevent the clay from sticking to the chamber. With a head gasket and head installed and fully torqued, but without pushrods, slowly rotate the crank to allow the piston to reach TDC. If you feel a dead stop, the piston may be in contact with the head, so don’t try to force crank rotation. Remove the head and inspect and measure the clay for contact. If you did not experience a positive stop and rotation to TDC was achieved, examine and measure the clay areas that were compressed for thickness by carefully slicing a cross-section of the clay with a razor and carefully measuring thickness with a small machinist’s ruler or depth caliper. If you find tight spots that feature less than the recommended clearance, remove the clay and mark those areas on the dome.
In preparation for piston dome machining, the combustion chambers are profiled digitally to obtain the required piston dimensions.
Reapply clay, reinstall the head, this time with a spark plug installed, and repeat the process to determine spark plug clearance.
Prior to dome modifications, the initial dome heights are measured with a digital height gauge.
The piston dome is receiving a custom dome profile on a CNC lathe.
Examine the point between valve pockets. If this is very sharp, use a light abrasive to slightly soften this to minimize the potential for a hot spot that could contribute to detonation/ pre-ignition.
The sharp corner between valve pockets was relieved on this piston, eliminating a potential hotspot.
Dome Shapes
Piston domes are available in three basic types: positive dome, reverse dome (dished or bowl), and fl at top. A positive dome features a rise above deck. A dished piston features a depression in the top below deck; a fl at top has a smooth fl at piston deck, with valve relief pockets where needed.
The additional piston volume provided by a raised dome increases compression ratio by reducing the volume of the piston-to-chamber area. Depending on the design of the cylinder head combustion chamber and the specific design of the piston dome, a high dome might help or detract from the efficiency of the ignited and burning air/fuel charge. One potential drawback with a high-dome piston is the possibility of increased piston rock as combustion pressure ramps over the dome projection. However, this can be impacted negatively or positively due to the variables involved in terms of dome shape and height, and chamber profile.
An example of a flat-top piston with 5-cc valve reliefs to be installed in 4.125-inch bore with a 4.000-inch stroke. In combination with 64-cc cylinder head chambers, this piston provides a 12.2:1 compression ratio. With 68-cc chambers, this piston provides 11.6:1. With 70-cc chambers, you have an 11.4:1 compression ratio.
A flat-top piston with valve reliefs and an inverted dome. Note the double-cut intake valve relief. This piston fits either a standard small-block Ford (using the deeper pocket) or a “twisted wedge” application where the head chamber is tilted.
A domed “pop-up” piston featuring a healthy 44-cc dome. This 44-cc dome piston accommodates a big-block 124-cc open chamber head. The high dome is needed to maintain the high compression desired in combination with the large head chamber.
These pistons are sized for a 4.185-inch bore, feature a dome volume of 14.6 cc and are mated with 69-cc cylinder head chambers. The reverse dome increased volume allowed lowering compression ratio to a very streetable 10.54:1.
Reverse dish or bowl pistons reduce the dome height at the center area, providing an increase in piston-to-chamber volume, decreasing compression ratio depending on the volume of the chamber. Dished pistons are commonly used in forced induction applications to maintain combustion pressure under boost conditions to avoid detonation and excessive chamber pressure. The size of the dome volume (positive or negative) is matched to the volume of the combustion chamber to obtain the desired compression ratio. A flattop piston that features valve relief pockets is considered by many to be a preferred choice, again depending on the shape of the combustion chamber. Dome shape often boils down to a builder’s preferences, past experience, and trial-and-error testing on the dyno or track.
Checking Valve-to-Piston Clearance
Check valve-to-piston clearance in both the vertical and radial planes. In the vertical plane, you’re checking for the clearance between the valve face and the valve pocket. In the radial plane, you’re checking for the radial clearance of the valve head to the radius of the valve pocket in the piston dome.
Although clay checking, as described earlier, is a common approach, this method poses variables such as the compression and possible spring-back of the clay, a potential separation of the clay from the piston, and clay sticking to the valve.
Using a degree wheel on the crank snout to check intake valve to piston vertical clearance, rotate the crank at least twice and then bring that piston to 10 degrees ATDC on the intake stroke, generally a position where the valve is closest to the piston. Be aware that two crank rotations are required to rotate the camshaft once.
Install a light checking valve- spring onto the valve being checked. When checking valve-to-piston clearance, a solid lifter is required, because a hydraulic lifter may bleed down and provide a false reading. If the engine will be fitted with hydraulic lifters, locate a solid lifter of the same pushrod cup-to-tip length (a spare hydraulic lifter with the plunger tack welded to prevent plunging works).
Clay was applied to both intake and exhaust valve reliefs. The piston dome must be dry prior to applying the clay to ensure that it sticks to the dome. A light coat of thin oil is applied to the combustion chamber and valve faces to prevent the clay from sticking to those surfaces.
With clay applied to the piston valve pocket, the crank was rotated twice to rotate the cam one full turn. With the head removed, you see the valve impression in the clay. After cutting the clay with a razor, you see a cross-section of the clay thickness, which is then measured. This intake valve shows about .200-inch valve-to-piston clearance. Minimum suggested intake valve clearance is .080 inch, while minimum exhaust valve clearance is .100 inch.
A dial caliper’s depth tip can measure the clay thickness, or the clay section that was removed can be carefully measured with the caliper jaws. Due to the compressibility and potential spring-back of the clay, the clay method offers a rough estimate of clearance.
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With a lifter, pushrod, rocker, and light checking valvespring installed, adjust valve lash and set up a dial indicator onto the retainer and preload the gauge dial with enough travel to accommodate valve movement, and zero the gauge. With your finger or a small screwdriver, push the valve down until it touches the piston. The difference between the zero reference and piston contact indicates how much clearance the valve has to the piston. Generally speaking, for a naturally aspirated application, a minimum of about .080- to .090-inch intake clearance should be adequate. Follow the same procedure to check exhaust valve clearance, but with the piston located 10 degrees BTDC on the exhaust stroke. A generally accepted exhaust clearance is .100 to .110 inch.
With a light checking valvespring installed in this intake valve, the crank is rotated to bring the piston 10 degrees ATDC. A dial indicator is set up, contacting the spring retainer. The dial is set to zero.
To check valve-to-piston valve relief radial clearance, instead of using clay, a spare valve can be cut and ground to a point to serve as a center-of-valve reference.
For a blown (forced induction) application, valve-to-piston clearances need to be a bit greater, approximately .125 inch at the intake valve and about .175 inch at the exhaust valve.
The spring is compressed until the valve contacts the piston. The gauge needle turns in a counterclockwise direction, showing this valve-to-piston clearance at .087 inch.
Measure the diameter of the valve head. After you measure the radius distance from the center mark, you factor one-half of the valve head diameter.
With the piston at 10 degrees ATDC to check the intake valve, the modified spare valve is inserted upside-down with the pointed end facing the piston. Using hand pressure, the pointed valvestem places a mark on the dome that indicates the centerline of the valve.
Instead of checking with clay, a more accurate method of checking radial clearance is to remove a valve and grab a spark/junk valve of the same stem diameter. On a lathe, cut the tip off and grind it to a point. Color the piston dome valve pocket center area with a marker. Install the head gasket and head. Rotate the crank to bring the piston to TDC or about 10 degrees BTDC. Insert the ground, pointed valve upside-down into the valveguide and gently contact the piston, applying enough hand pressure to place a mark on the piston that provides a valve centerline mark. Remove the head.
Measure the diameter of the head of the valve that is to be final installed. Set your caliper to half of the valve head diameter. For example, if the valve head measures 2.080 inches in diameter, set the caliper at 1.040 inches. Place one end of the caliper jaw at the marked center valve mark and sweep across the edge of the valve relief to determine how much radial clearance you have. A suggested minimum clearance is about .085 to .100 inch.
If clearance is an issue, consider using a thicker head gasket (which of course decreases compression ratio). An alternative is to mill the piston dome at the contact area(s). This is done on a dedicated piston vise on a milling machine.
As noted earlier, be aware of existing dome thickness to avoid weakening the dome. Depending on the clearance area, the valve pockets may be cut deeper and/or larger in diameter, and/or the flat area of the dome can be milled to increase clearance.
After removing the head, measure the distance from the center point to the edge of the valve relief. Using the radius of the already-measured valve head, you can determine how much radial clearance you have.
If piston-to-head clearance is an issue and the pistons feature a center-domed profile with no valve pockets (such as found on a vintage flathead Ford, for example), the domes may be reduced on a CNC lathe. The pistons are first measured for dome height and profile, and the cylinder head chambers are digitally profiled for shape. The data is then programmed and fed to a CNC lathe that can cut the dome to precisely the height and shape required for clearance.
If more than about 4 grams of material is removed from the piston domes, the crankshaft should be rebalanced, considering the change of piston weight. For high-RPM racing applications, any change in piston weight should require the crank to be rebalanced.
File Fitting Rings
When over-boring cylinders to repair or to increase bore displacement, it’s not uncommon for the supplied ring set to include top and second rings that are slightly larger in diameter, to require custom file fitting to achieve the desired end gaps. This is not a compromise on the part of the ring maker. Rather, this allows the builder to establish the precise gaps recommended by the piston maker, based on how the engine is to be built and used.
A rule of thumb for ring end gap is .004 inch per inch of bore diameter. However, it is absolutely essential to follow the piston maker’s specifications for ring end gap, as the recommendation may increase to as much as .006 inch per inch of bore diameter or more. The piston maker already knows how the specific piston material and construction is expected to grow in diameter during engine operation, so follow their specifications.
End gap is the clearance distance between the two ends of a ring when installed in the bore. Factors including forced induction (supercharging or turbocharging), the use of nitrous injection, and the operating environment (street, endurance racing, drag racing, etc.) all play a part in determining the optimum top and second ring end gaps.
File fitting top and second rings involves removing material from the ends with a fine file or, ideally, with a dedicated piston ring filing machine.
Rather than assuming that each cylinder requires the same final sizing, it’s recommended to file-fit each ring for the specific cylinder. For example, instead of using cylinder number-1 as the reference to file-fit all rings, you should file-fit the top and second ring for cylinder 1, then file fit rings for cylinder 2, etc., keeping all rings organized for cylinder location. Slight variances in cylinder wall dimensions are possible, so for achieving optimal end gaps, custom fit rings to each cylinder.
Before file fitting rings, check each ring to its intended piston to verify proper ring groove side clearance and radial depth to make sure that the rings are compatible with the pistons.
When you insert a ring into the cylinder for end gap checking, the ring must be set at an equal depth along the entire cylinder bore diameter. Using a ring-squaring tool makes this easy and quick. Insert the ring near the top of the bore and insert the squaring tool, which pushes the ring evenly down into the bore. Remove the tool and you’re ready to check gap.
When checking ring fit to the cylinder, a feeler gauge is used to determine end gap. The ring must be squarely installed at the same depth around the entire perimeter of the ring relative to the deck. If the rings require file fitting, the end gap should be checked in stages between filing to avoid creating a too-large end gap.
Side clearance is the vertical clearance of the ring relative to the upper or lower base of the ring groove. With the ring placed into its groove, push the ring down until the bottom of the ring is flush with the bottom land of the ring groove, then measure the clearance between the top ring surface and the roof of the ring groove with a feeler gauge. Side clearance should be .001 to .002 inch. Radial clearance is the inward/ outward clearance of the ring within the groove. This is the difference between the outer diameter surface of the ring to the inner diameter surface of the ring, relative to the depth of the ring groove. With the ring pushed fully inward until the back of the ring seats against the wall of the groove, measure the distance from the face of the ring to the outer edge of the groove. Radial clearance should be approximately .005 inch.
With the cylinder bores wiped clean, compress the number-1 cylinder’s top ring carefully by hand and insert it into the bore. Make sure that the ring is properly oriented. If a dot appears on one side, the side with the dot should face upward. Insert the ring about 1/2 to 1 inch into the bore, with the ring squared so that it is at the same depth along the entire circumference, relative to the deck. You can use a caliper or depth gauge or a small ruler to adjust this, or an easier and quicker way is to use a ring-squaring tool that automatically pushes the ring squarely into the bore. Check the existing end gap. In the case of file-to-fit rings, you may find zero gap or even an overlapping gap.
Top and second rings that require file fitting may be filed with a small, flat, fine file, but a dedicated ring filer makes the job easier and more precise. Each end of the gap must be filed by the same amount. The filer features a diamond abrasive wheel that provides a nice square cut. By applying slight hand pressure to place the ring end against the wheel, count the number of handle rotations during each filing. After correctly filing a few rings, you get an idea of how many turns are needed to achieve your desired end gap. This makes filing the remaining rings a bit quicker. Always file by a few turns, clean the ring and check the gap in the cylinder. Filing in stages helps to prevent accidental overfiling.
Remove the ring and file the ends, removing an equal amount of material from each end. Then insert the ring again and measure end gap using a feeler gauge. Perform the filing in small increments to avoid ending up with a gap that is too large. When the proper gap is achieved, remove the ring and carefully remove any burrs from the filed ends, using a small flat fine file. Wash the ring and store it on a clean workbench next to your number-1 cylinder’s pistons and rod. Repeat this process for all top rings, and for all second rings, cylinder by cylinder, and keep all rings organized for cylinder location. Remember to orient all top and second rings properly, because, depending on the ring profile, they may have a dedicated top and bottom surface. If the top and/or second ring have a small dot mark, the mark must face upward when installed.
If the piston wrist pin bore is raised due to the required compression distance, the pin bore can intersect the oil ring groove. In these cases, the piston maker provides a set of support rings that are installed to the floor of the oil ring groove, providing oil ring support at the voids at the top of the pin bore. The support rail features a small bump dot, which must be oriented at the center of the pin bore. This small protrusion prevents the support rail from accidentally rotating, keeping its end gap from moving over one of the pin bore voids.
The supplied oil ring rails should provide a minimum of the specified end gap, so you should not need to file any of the oil ring rails. By the same token, if your pistons require support rails (due to a short piston compression distance where the wrist pin bore intersects with the oil ring groove), there is no need to perform any file fitting for the support rail. Its end gap is not critical; it only serves to provide oil ring package support over each end of the wrist pin bore and serves no sealing purpose.
Written by Mike Mavrigian and republished with permission of CarTech Inc
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