Horsepower is always paramount in the minds of customers who salivatelike a Pavlovian dog when the terms "high performance"and "engine" are used in the same sentence. Unquestionably,horsepower is heady stuff, capable of not only moving a vehicleto obscene speeds, but also of propelling its owner to a positionof prominence in the eyes of performance enthusiasts and the localconstabulary.
Large quantities of it are what a customer deserves if he or shehas ventured into the world of ported heads, high lift cams and"trick" machine work. But if the end product doesn’toffer reasonable durability, a once happy customer will quicklydevelop the menacing facial expression and snarly tone of an attorneywho has to abide by a code of ethics.
As it pertains to high performance engines, durability doesn’tmean 100,000 miles with nothing more than an occasional oil change.Even emotionally impaired customers (of which there seems to bean inordinately high number) will usually acknowledge that a highperformance engine isn’t expected to enjoy the longevity of ahum-drum standard powerplant. Aside from the fact that stresslevels rise with horsepower, vehicles with high performance enginesare typically driven aggressively, which also takes its toll onengine life.
But expectations are that power levels and oil consumption willremain relatively stable until an engine, having served its masterwell, accumulates sufficient mileage to warrant another rebuild.Depending on customer and usage, that could mean anything fromtens of thousands of road miles to a few hundred laps on a racetrack. In either case, proper cylinder wall preparation and judiciousring selection are the arbiters that ultimately establish an enginebuilder as either a hero or a goat.
Rings and surface finish
Before a set of rings comes face-to-face with the cylinder wallsit will be up against for a lifetime, the block that’s to serveas an internal combustion condominium is typically mounted ona boring bar, with a deck plate attached, and each cylinder isusually machined to within .004"-.005" of the desiredfinished dimension. After the boring bar has carved its last bitsof metal from the cylinders, a honing operation is required tomassage the cylinder walls until a ring-friendly finish is achieved.
For maximum friendliness, cylinder wall finish specificationsare selected according to ring material. While opinions vary somewhatas to optimum finishes, a straightforward procedure invariablyprovides the best results. As Garry Grimes of Grimes AutomotiveMachine in Alpharetta, GA, notes, "The biggest concern whenhoning should be cylinder wall movement. Finish doesn’t mattera whole lot if the bores are out-of-round, so the use of torqueplates during both boring and honing is an absolute necessity.A lot of shops that dabble in performance work think torque platesare a waste of time, but eventually they learn that if you intendto stay in this business, you have to do things right."
One aspect of block machining that is often overlooked is heatinduced distortion. As a hone goes about its business removingcylinder wall material, it can generate a considerable amountof heat. To minimize heat-induced bore distortion, Fritz Kaylof Katech, Inc., Clinton Township, MI, notes that his company’sprocedures call for staggered honing so that two adjacent cylindersare never honed one right after the other.
Since the purpose of honing is to provide a suitable finish forthe rings, specific stones are required. With the almost universaluse of moly top rings in high performance engines, doing thingsright usually means a multi-step honing process for those leaving.010” for final honing, wherein the cylinder walls are firsthoned with 70- to 100-grit, then with 280-grit, and finally with400-grit stones.
All honing is done in a manner that leaves the walls with an appropriatecross-hatch pattern. Katech has been building race engines forGeneral Motors for more years than Kayl cares to admit, and dependabilityhas always been a top priority. Consequently, Katech has establishedspecific requirements for its multi-step honing process.
Blocks are initially bored on a CNC vertical mill to a dimensionthat’s .010" less than the finished diameter. Each cylinderis then rough honed at a reading of 40 on the hone load meter,to within .005" of final bore size using 70-grit stones.The next pass, also made with a load meter reading of 40, is with280-grit stones. Finally, four light passes (with a reading of20 on the load meter) are made with 400-grit stones (For all steps,the hone is set for 45 strokes per minute and 155 rpm). Surfacefinish is then checked with a Hommel T500 tester and a printoutis filed in the engine build book.
Many machinists feel as though the cylinder walls in a block destinedfor high performance use should be honed only with conventionalstones. In fact, one engine builder went so far as to say, "Youcan’t print what I have to say about diamond stones." Othershave an entirely different opinion.
Barry Gowen, manager of the machine shop operations of SummitRacing Equipment, Tallmadge, OH, states, "We bore all ourblocks with a CNC mill to within .003" of finished size.Then we do a single honing operation to final dimension usingmetal-bonded diamond stones. Following that, we use a soft honeas a finishing step. Our process is idiot-proof and the diamondstones give us straight, round bores with no taper every time.And once you get the stones broken in, they’ll last (far longerthan conventional abrasives). I know there’s a lot of controversyabout diamond stones, but we have absolutely no ring problemsand cylinder leak-down is never over 5%."
Gowen, who used to sell machine shop equipment, also points outthat it took some time to perfect the diamond honing process.He pointed out that one of the keys is the use of coolant ratherthan honing oil. But, hone speed, abrasive and type of hone headare also important. As is the case with building engines, differentshops prefer different processes and procedures. However, whateverthe methodology employed, successful shops have carefully documentedand perfected the right combination of ingredients.
Employing abrasive stones of various grits results in aplateau-type finish where the tops are knocked off of the relativelydeep grooves left by the coarser stones. The effect is similarto that of filing the threads on a nut. While the removal of threadmaterial isn’t advisable if secure fastening is the objective,it does serve as an excellent means of illustration if you’re"visualizationally challenged."
If you had the time and inclination to file the thread’s tipsdown to within a few thousandths of an inch of the root diameter,and then applied motor oil to the surface, you’d find that theoil would collect in the small grooves that remained, therebyproviding surface lubrication. Voila! Having completed the experiment,you would have experienced first hand, the benefit of plateauhoning.
Having achieved a text-book plateau-honed surface, you might thinkthat you had reached machining nirvana. But according to proponentsof two processes known as "soft honing," and ball-brushhoning, there is yet work to be done. Whenever an iron surfaceis machined with a stone of any type, the surface is literallytorn. When an engine is fired for the first time, the rings scrapemuch of the torn material from the surface, resulting in minutevertical scratches in the cylinder walls.
"Soft honing," which is actually an abrasive cleaningprocess, was developed to produce a smooth surface, devoid ofeasily dislodged metal particles. The "soft hone" itselflooks very much like a bore-cleaning brush; some models may bemounted in an electric drill. But rather than bristles, thesehones are composed of silicone carbide filament strips which aretough enough to scrub metal from the surface. In essence, theprocess removes the jagged peaks, folded material and abrasivesleft by previous operations.
Having cleaned out the undesireable metal, "soft honing"leaves a more stable load-bearing surface on the cylinder wall,which hastens ring seating and eliminates cylinder wall scarring.Many performance shops swear by "soft honing," othersremain unconvinced.
In some instances, resistance to the process may stem more frompride than from logic. Experienced machinists often pride themselveson their cylinder wall preparation skills and are reluctant toadmit that their product can be improved. However, "softhoning" equipment is relatively inexpensive, and it servesas excellent insurance that cylinder wall surfaces have been optimized.If moly rings are to be installed, optimized means the relativelysmooth surface left by 400-grit stones.
Another means of abrasively cleaning cylinder bores and achievinga plateaued cylinder wall free of loose, torn and folded metalis with a ball-brush hone, also known as a Flex-Hone®. Like"soft honing," proponents of using a ball-brush typehone feel that it removes much of the folded metal that remainsfollowing normal honing. Ball-brush hones are also used to clean-upand deglaze cylinder walls when an engine is freshened.
The suppliers of ball-brush type hones and "soft hone"brushes continue to debate the benefits and final surface finishcharacteristics achieved when using one brush compared to theother. However, we have interviewed more than a few engine rebuilderswho use both types of brushes. In each case, they are more thansatisfied with the final surface finish achieved with their chosenprocedure. Again, success seems to boil down to documented processesand procedures that are consistently implemented from one enginerebuild to the next.
Chrome rings, on the other hand, require a coarser finish, usuallyachieved with 280-grit stones, before they agree to seal properly.Prior to the advent of moly, chrome-plated top rings enjoyed widespreaduse in high performance engines. According to Joe Moriarty ofTSE Racing Engines, Greenland, NH, "About the only applicationsin which chrome rings are still used are those in which abrasionis a major concern.
"Dirt track engines running without air filters are a primeexample. Chrome is much more resistant than moly to dirt and otherabrasives on the cylinder walls. That’s one of the reasons you’llfind chrome rings in diesel engines; the carbon deposits are veryabrasive. But chrome doesn’t have a lot of self-lubricity so itneeds a relatively coarse cylinder wall finish so some oil remainson the surface."
Ring width is another consideration that must be "tuned"to a particular application. Since the days when flathead Fordsreigned supreme, 5/64" thick top and second rings, and a3/16" oil ring have been the standard in production engines.Many high performance engines are also fitted with a 5/64",5/64", 3/16" ring package, although thinner 1/16"or 1.5 mm top and second rings and either a 3/16", 1/8"or 4 mm oil ring are far more common in true race engines.
Extremely thin top rings ó .043" ó have beenthe hot lick in many drag race engines for quite some time, andmay also be found in a surprising number of endurance engines,e.g., Winston Cup. Although "thin" rings are shroudedin the same type of mystery and intrigue that surrounds a governmentinvestigation of toilet seats, the parameters for their use arerelatively straightforward.
In relative terms, "thick rings" (5/64") have thehighest heat dissipating capability and are heaviest. "Thinrings," by virtue of their lesser width and mass, are lessable to dissipate heat, but also more stable at higher enginespeeds. In round numbers, a 5/64" ring is suitable for applicationsin which engine speed doesn’t exceed 6000 rpm.
The primary incentive for minimizing ring width is to reduce massand thereby eliminate a phenomenon known as ring flutter. Whena heavy ring is cycled at high speed, it tends to flutter around,beat up the ring lands and lose contact with the cylinder wall.When that occurs, ring seal joins suicide knobs and wide whitewalls as a thing of the past.
In drag racing, where engine speeds of 7500 to 10,000 rpm havebeen commonplace for years, use of .043" and 1/8" toprings has long been standard practice. But now that many ovaltrack engines are seeing life above 8000 rpm, rings are growingthinner. As an example, some Winston Cup engine builders are nowusing 1.2 mm (.047") and 1.0 mm (.039") top rings, 1.5mm second rings and 3.5 or 4 mm oil rings.
Far more common in the universe of metric rings is a 1.5 mm topring which is only .004" thinner than 1/16". The switchto metric specifications, although not universal, is certainlyheaded in that direction. Certainly, ring seal would be questionableif metric rings were fitted to pistons with ring lands machinedto a fraction of an inch, but the trend in newly designed pistonsmachined for domestic V8 engines is metrification of ring grooves.
Much of the impetus to switch to metric rings has been to takeadvantage of new ring materials. Returning for a moment to oldertechnology, one means of improving ring seal is to increase theloading of the top ring against the cylinder wall. In drag racing,this is commonly accomplished through the use of gas ports ñsmall holes drilled through the top of the piston into the topring groove.
Combustion pressures are thereby allowed to reach the back sideof the ring, forcing it into firm contact with the cylinder wall.The extremely rapid rate with which rings and cylinder walls ingas-ported pistons wear demonstrates the extreme effectivenessof this process. It also makes it impractical for endurance racing.
With recent improvements in ring materials and block preparation,gas porting, even in hard core drag race engines, isn’t as prevalentas it once was. Dykes-style rings are another "old technology"method of improving ring seal in race engines. Use of .031"and .017" Dykes rings, which are "L"-shaped (toallow gas pressure to load them against the cylinder walls) ispretty well confined to blown fuel and alcohol drag race engines.
Another approach to improving ring seal is to eliminate end gaps.At least two companies offer rings, designed to be installed inthe second ring groove, that have no open gaps. One design employstwo-piece construction with an oil ring-type rail combined witha conventional cast or ductile iron ring. The end gaps of therail and ring are positioned 180° apart, so there is no openend gap. Another design utilizes overlapping steps at each endof the ring, which again, leaves no open gap. Gapless rings arewidely used in a variety of high performance and racing applications,(including Indianapolis) especially in engines fueled by alcoholand nitromethane, where oil dilution is a significant problem.
However, some engine builders prefer to stick with standard typerings. As with most other aspects of high performance engine building,each successful engine builder has a somewhat different opinionabout cylinder wall preparation and ring selection. But the factremains that some engine builders are enjoying success with componentsand techniques that others flatly state will not work. Clearly,the formula for success involves a good bit of experimentationcoupled with an equal amount of common sense.