 Techline-June09-H-D 110" CVO -Part VI: Ending thoughts and the wrap-up
This article continues in the next thread.
This month we finish an excerpt from Chapter 3, H-D 110" CVO from Donny’s Unauthorized Technical Guide to Harley-Davidson 1936 to Present, Volume II: Performancing The Twin Cam. (Some content has been altered to fit AIM’s style and format.)
In this last article on the CVO 110, I want to continue with my installation recommendations concerning Axtell cylinders, starting with the piston rings.
Compression Rings
Do not overextend the rings when you are installing them, as this may cause stress points that can lead to eventual breakage during engine operation. Molybdenum-faced or simply moly top rings are best used in freshly bored and honed cylinders. Moly rings have a lighter color on the cylinder interface of the ring than the side faces. Moly is harder than cast iron, which is a common ring material. Look for markings on the two top single rings. Cast top rings are best used with deglazed cylinders. Deglazing is a honing procedure used to remove glaze from cylinder walls and create the correct crosshatch pattern on them.
A pip mark, indentation, or any other marking on one face indicates that side should be installed facing up, toward the top of the piston, unless otherwise noted by the instructions. A beveled inside edge on the top compression ring always faces up, unless otherwise noted. If there are no markings or bevel, the ring may be installed either way. If there is a bevel on the bottom compression ring, which is the middle ring, it must face downward, unless otherwise noted.
All modern quality pistons utilize a three-piece oil ring design with a center expansion ring and two outside rails. Older pistons use a thick cast-iron scraper ring, while not-so-old oil rings use a single cast-iron scraper ring with a relief in the center portions for oil removal. Grinding the rails to attain correct endplay is acceptable and necessary, but do not grind the center expansion ring or allow it to overlap itself.
Ring gap is a combination of the ring expansion itself and the expansion of the cylinder bore. Therefore, the metallurgy of the cylinder is important in determining correct end gap. Axtell took the time to measure stock Evo and Twin Cam aluminum cylinder growth, and found expansion rates up to a full 0.009" and slightly beyond at operating temperatures. Cast-iron or ductile cylinders have smaller growth issues.
The formula to calculate extra piston ring end gap is Cylinder Expansion Rate times Pi equals Extra Ring Gap. Pi is a mathematical constant with the value 3.14159. It represents the ratio of a circle’s area to the square of its radius. If we plug in the cylinder expansion rate for an aluminum cylinder as determined by Axtell we get 0.009" x 3.14159 = .0283".
Calculation of extra ring gap is not necessary in most cases, as the manufacturer will give correct specifications for the intended use. However, another way to calculate ring gap, called the Aluminum Cylinder Top Ring Gap Calculation, uses the cylinder expansion rate and cylinder bore estimate. This formula is Bore Diameter times 0.004" equals Ring Gap. If we plug in the bore spec for a H-D CVO 110 aluminum cylinder, which is 4.000", we get 4.00" x 0.004" = 0.016" Ring Gap. When using
aluminum cylinders the minimum ring end gap is the way to go since the
cylinders expand so much.
Cast-iron or ductile cylinders have a lower expansion rate, so ring gaps should be beyond the middle of the recommended figures, toward the maximum. The formula used to calculate ring gap, called the Cast Iron or Ductile Top Ring Calculation, is Bore Diameter times 0.006" equals Ring Gap. If we plug in a 4.00" bore spec we get 4.00" x 0.006" = 0.024" Ring Gap.
Middle rings are somewhat protected from excessive heat by the top ring, which bears the brunt of the combustion event. Therefore, the middle ring does not usually need the maximum ring gap settings since there will not be as much expansion as the top piston ring. The Middle Ring Gap Calculation formula is Bore Diameter times 0.004" equals Minimum Ring Gap. If we plug in a 4.00" bore spec we get 4.00" x 0.004" = 0.016" Minimum Ring Gap.
Many mechanics and riders want their engine tolerances set to minimum specifications, but this is not always wise, even though the engine will have proper break-in. Larger ring gaps should not concern the mechanic because gaps tighten up considerably when the engine reaches operating temperatures. Therefore, don’t be too conservative with gapping rings. Also, a thinking mechanic will look at aggravating heat factors such as too-high rear cylinder air/fuel ratios with closed loop EFI, or restrictive stock or stylishly long exhaust systems that add substantially to engine heat. When there is a high heat issue, always use the higher end of the recommended ring gaps.
Oil Rings
A standard modern three-piece oil ring has a center expander and two outside rails, one on top of the expander and the other on the bottom. The corrugated expander ring ends will buttress together after the two rails are installed and the piston is fitted into the cylinder. The expander, as the name suggests, provides the needed tension on the rails, which scrape the oil off the cylinder wall. Do not modify the expander! The rail end gap should be between 0.015" and 0.050". This is a wide variance. Use the principles previously mentioned, such as cylinder metallurgy and excessive heat, to determine the best rail end gap for a particular engine.
When building an engine properly, it is necessary to think through every aspect of construction instead of daydreaming while blindly following specification tolerances. Modify tolerances to the specific needs of an engine, as well as the type of riding the engine will enjoy or suffer through. I build engines according to customer personality. It’s important for me to spend some time with a new customer to figure out his riding style. In simplistic terms, will the intended rider be overly conservative and lug the engine? Or will he operate it within the prescribed break-in procedures and, once broken in, stay within proper rpm ranges in all gears and change the oil regularly? Or will he simply beat it up? Tolerances are modified accordingly.
When adjusting ring end gap, grind or file the top, middle, or bottom oil ring rails from the outside in. Never file or grind from the inside out, or the ring face may chip, causing undesirable consequences in the running engine. Remove burrs after making your adjustment. And, as always, cleanliness is next to godliness. But remember to never adjust a three-piece oil ring expander’s end gap.
Axtell recommends testing the oil ring tension by installing only the oil rings onto the piston. After lightly oiling the bore of the cylinder, insert the piston into the cylinder. Use a fish scale to drag the piston through the bore at a constant speed and read the scale to see how much drag is generated by ring tension. Axtell states that its testing has shown that maintaining oil control is possible with a 9-pound pull for a maximum output engine and a 27-pound pull for a high-mileage touring application. The oil ring end gap is a determining factor in how much tension is applied to the cylinder bore, if the expander is doing its job.
As for proper ring orientation on the piston, follow the instruction sheet supplied with the piston rings concerning expander and rail gap orientation, as there are different methodologies used depending on design differences. In general, the top oil rail will be positioned about 1" to one side of the expander ring opening, while the bottom oil rail will be about 1" to the other side of the expander opening. Usually, the compression ring gaps are orientated equidistant from each other. This ensures that compression does not blow downward past the rings and into the bottom end, and oil from the lower end will not make its way into the
combustion chamber.
Boring & Honing
You should first face the head and base gasket surfaces of a cylinder before resizing its bore, so you have a flat surface on which to base your work. If the gasket surfaces are warped, every measurement you take will be off. Always use the proper torque plates when boring and honing a cylinder because the plates simulate actual installed-on-the-engine conditions. When a cylinder is untorqued during disassembly, its bore springs to a different shape that may not necessarily be round and concentric. In fact, the bore may go out of round and become tapered. Bore and hone a cylinder with torque plates installed, so that the cylinder will be round, perpendicular, and concentric top to bottom when installed on the engine.
The cylinder must be torqued to spec with torque plates installed when checking piston-to-cylinder bore clearances for this to be done correctly. Boring is superior to honing for piston fitment because the bore will be perpendicular to the gasket surfaces. Honing will create a round bore, but it will not necessarily be perpendicular.
Use the correct torque plates. Do not use larger cylinder bore torque plates on smaller bore cylinders, as the pressure of the plates will not be evenly distributed. Larger torque plates can distort the base and head gasket faces of the cylinder outward, thus pinching the formerly round bore inward at the center. If the cylinder is bored in this uneven situation, when the cylinders are torqued during engine assembly, the now even application of pressure will leave the cylinder bore too wide in the middle, tapering smaller to each cylinder face. The piston thrust surfaces and rings will suffer much trauma as they try to contend with these inconsistencies.
I mention this because torque plates are expensive, and there are currently so many different bore sizes, from 3.00" on earlier engines to present behemoth 4.50" big-inchers. I bet we have at least seven sets here at Heavy Duty, and our machinist “Marvelous” has even more.
When boring a cylinder, leave up to about 0.008" clearance before attaining the proper piston fitment measurement. This is because correct fitment and cylinder wall finish take numerous steps to complete. Axtell uses five steps for honing with four increasing stone grits: 70, 180, 220, 280, and a cork wiper finish. Of course, the honing finishes should be at a 45-degree crosshatch to both allow break-in and contain oil on the cylinder sleeve surface.
Axtell cylinders have a preventative coating on the sleeves to protect them from oxidation, also known as rust. Wash the cylinder liners thoroughly in warm, soapy water using a firm plastic bristle brush. Rinse cylinders with clean water and dry completely. Then oil the bore with a white paper towel, as rust will begin to appear on the liner surface. In this case, rust is an indicator of a great cleaning job. If the paper towel shows evidence of dirt, repeat the washing procedure and do a more thorough job. When the towel comes out clean, the job is done right.
Keepers & Buttons
Axtell uses two types of wrist pin keepers: 0.073" wire locks and Spiro locks. The wire lock has a groove on the bottom and a notch cut in the piston to aid in removal. The Spiro lock uses a 0.044" seating groove in combination with a conventional square bottom groove without piston notching. Some pistons have a double Spiro lock system with a 0.088" groove to accommodate them. Spiro locks resemble a small, flat, coiled spring. Only pull the spiral coil up to about a 1/4". If you overstretch the lock, start over with a new one. Do not try to compress the natural diameter circle of the spiral lock for ease of installation. This will cause deformation of the lock, and disturb correct radial preload. Gingerly place one end into the piston pin boss groove, and spiral the complete lock into the groove until the lock seats fully in the groove. (This is not as easy as it sounds.) Proper installation requires practice, dexterity, and, above all, patience. The spiral lock should not move in its groove after installation. If it does, remove it and try another new one. Never reuse a spiral wrist pin lock!
Some pistons do not use locks, but have a Teflon button holding system where the wrist pin and buttons float in the piston with preset endplay. Teflon buttons add more stability, particularly as the piston changes axis at Top Dead Center (TDC) and Bottom Dead Center (BDC). However, they don’t add extra thrust surface area since the wrist pins and buttons are housed perpendicular to the sides between the piston’s thrust surfaces.
Axtell supplies different depths of buttons to achieve the correct wrist pin endplay of 0.020"-0.060". The proper endplay spec depends on metallurgy, generated engine heat, and piston-to-cylinder tolerance.
Obviously, an engine operating with a closed-loop EFI and stoichiometric 14.7:1 air/fuel ratio will generate more heat than the previous lower air/fuel ratio engines. Furthermore, the rear cylinder will run even hotter because of restricted cool air flow conditions. Therefore, the rear cylinder wrist pins and dual Teflon buttons need more endplay. Compare this situation with a front cylinder on a carbureted engine running at around 14.0:1 air/fuel ratio, where endplay will be a tighter fit with less tolerance. Once you’ve decided on what the proper endplay is, add the length of the wrist pin and two buttons together. Subtract the length of the wrist pin and desirable endplay from the cylinder bore diameter and divide the answer by two. This will equal the Teflon button depth. The formula looks like this: Wrist pin Length plus Desired Endplay minus Cylinder Bore divided by 2 equals Button Depth. Let’s plug in some numbers: 3.200" (wrist pin) + 0.040" (Desired Endplay) – 4.00" (Cylinder Bore) / 2 = Button Depth. The actual equation would be 3.200" + 0.040" — 4.00" 2 = 0.380". That means each button will be 0.380" deep.
Balancing is crucial when changing piston weight. If necessary, equalize piston weight by grinding or polishing underneath the heavier piston, usually near the piston pin bosses, which are the meatiest portions. Compare new piston weight with the former pistons. If the weight of the new piston is equal or a little more than the old, then everything is okay. Furthermore, if you’ve static-balanced the engine previously, the various figures will allow calculations to bring the new pistons within the set balance factor. Otherwise, new pistons require the static balancing to a balance factor of 60 percent.
This article continues in the next thread. Check back issue for pix and extra information. |
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