August 1, 2000
Steven W. Ells
Engine case halves are the Rodney Dangerfield of engine parts. Often hangar talk about engines concentrates on the cylinders, or top end, of the engine. The crankcase halves, crankshaft, camshaft, and connecting rods—or the bottom end of the engine—are often ignored or, if thought of at all, are referred to as "bulletproof." It's true that, compared to the top end, the bottom end is remarkably trouble-free, but that doesn't mean the crankcase halves can be ignored at overhaul.
Check with any number of well-regarded engine overhaul shops, and you'll find that they all agree on the need to refurbish at least 80 percent of the cases that are cycled through their facilities. The good news is that once a case gets repaired correctly, the incidence of cracking is greatly reduced and in a lot of cases, it never recurs.
According to David Leis, vice president of Sales and Marketing at Western Skyways Inc., an engine rebuilder in Montrose, Colorado, "Cases definitely crack, and they definitely get welded."
Roger Fuchs, technical director of EC Northwest in Troutdale, Oregon, says, "The case that won't crack hasn't been built yet." There have been improvements in case castings over the years, but the requirement to keep the cases light enough for the airplane to maintain a reasonable useful load, coupled with the forces generated during operation, always result in stress and metal fatigue.
RAM Aircraft in Waco, Texas, a shop that specializes in rebuilding turbocharged big-bore Continental engines for Cessna twins, claims that they started putting the seventh-stud modification on their 520 series overhauls in an effort to lessen case cracking. The seventh-stud modification is a cylinder base clamp that adds strength to the existing cylinder hold-down studs and through bolts. This modification is now incorporated by Teledyne Continental Motors (TCM) on new and remanufactured engines. In spite of this effort to reduce case cracking, RAM, a company that rebuilds about 350 engines a year, must provide warranty repairs for case cracks on 12 to 18 engines a year.
A spokesman for RAM said the only time a case isn't sent out for repair (and refurbishment) is when it's a new case that has been purchased outright. Even then, the case will be run through a thorough in-house inspection.
Reputable FAA-approved repair stations with procedures for performing case weld repairs actually do a lot more than just weld up cracks. Cleaning; inspections; weld repairs; machining services to restore fit, alignment, and tolerance; and anticorrosion treatments are the norm at these facilities. Since the cost for the complete treatment is usually about 10 percent of the cost of a new case, business is good; so good that TCM has its own case-refurbishment facility.
Simply put, cases crack because of stresses. A typical light aircraft reciprocating engine is a much more dynamic assemblage of parts than most people realize. Forces created by the cylinder power pulses cause torsional twisting of the crankshaft because of the crankshaft being loaded and unloaded throughout the engine rotation cycle. The crankshaft and propeller are loaded, or torqued, during each cylinder power stroke and unloaded (untorqued) during the interval between power strokes. This loading and unloading occurs more than a quarter of a million times per hour for a four-cylinder engine at cruise rpm.
The pulling, or thrust, created by the prop rotation is applied to the engine case through a special flanged main bearing. The continual loading and unloading of the crankcase journal that supports the flanged thrust bearing is one example of the dynamic and repetitive forces that cases are subjected to.
Other areas that undergo repetitive stresses are the cylinder mounting pads, cylinder base studs, and crankcase through bolts. During the compression and power strokes axial loads are generated that bear on the cylinder head. These loads are transmitted to the engine case via the cylinder base studs. During intake and exhaust strokes these loads are reversed, or at least lessened considerably. Cracking at the mounting pads and cylinder hold-down studs occurs. Continental's seventh-stud modification was introduced to spread the forces and lessen the number of cylinder hold-down stud boss cracks.
Repetitive loads from many different force vectors bearing on the case throughout each flight hour, metal expansion and contraction during start, flight and shutdown cycles, and incorrect torque values introduced during maintenance all contribute to case stresses.
Prop strikes, even minor ones, have the potential to cause case cracking. When the normal reciprocating motion of an aircraft engine is interrupted or disturbed, abnormal forces are generated that the crankcase has to absorb. The abnormal loads generated during even a minor prop strike generates loads the crankcase was never designed to handle. The result of any prop strike is almost always some cracking in the nose-bearing support structure of the case halves. This damage can only be detected after disassembly. Prop strikes also cause damage in the counterweights, accessory drive gears, accessories, and crankshaft. Engine manufacturers recommend a teardown inspection whenever the strike is serious enough to require propeller repair or rework by a prop shop.
Prior to Service Bulletin M90-17, which superseded bulletins M77-14 Revision 1 and M83-10 Revision 1, TCM repeatedly printed that weld repairs were frowned upon, saying, "Salvage welding of cracks in crankcase cylinder decks, main bearing journals, and adjacent surfaces is an unsatisfactory repair. Welding often induces abnormal material stress which can result in subsequent crack formations in areas adjacent to the welds."
In 1990, Service Bulletin M90-17, titled "Crankcase Inspection Criteria," announced that improvements had been made in crankcase castings for many TCM models. According to the bulletin, "New crankcases incorporate new casting methods which reduce material porosity and increase thickness in certain areas to improve structural integrity. These changes should significantly reduce the rate of crack incidence in all areas." In contrast to the earlier bulletins, a paragraph in this bulletin says, "TCM has established that welding of crankcases is an acceptable repair process. The weld procedure must conform to approved FAA repair procedures and dimensional integrity of the crankcase must be maintained."
Why the change? Simply because TCM wanted to develop its remanufactured and factory overhauled engine business. Welding and refurbishing cases was necessary to be competitive.
Prior to the mid-1980s, TCM and Lycoming were busy selling new engines to manufacturers. Before many general aviation manufacturers quit building airplanes in the mid-1980s, neither company was very interested in the rebuild market. At that time more than 80 percent of all GA engines were being rebuilt in the field—and there were a number of companies that had developed FAA-approved case repair techniques and procedures for repairing worn and cracked cases. Some of these companies were in business as far back as 1963.
When TCM and Lycoming realized that the rebuild and remanufactured engine markets were the only markets left, their attitude toward case weld repairs changed. TCM began contracting with DivCo for welding and machining services for the cases they were using in their reman and rebuilt businesses.
A company spokesman for Lycoming said that the practice of weld repairs on noncritical areas of factory overhauled engines was discontinued in 1997. The present policy is to replace any case that needs repairs with a new case on all overhauls or zero-time rebuilds.
TCM service bulletins allow airplanes to continue to fly if case cracks are found in noncritical areas. The noncritical area can be roughly described as the part of the case that is above a line formed by the upper cylinder base hold-down studs and extending up to the top parting surface, or backbone of the engine. If a crack is discovered in this area that is less than two inches long, there is an approved procedure to scribe mark the ends of the crack and inspect it for further cracking after 50 flight hours. If no further crack progression has occurred after 50 hours, the inspection interval can be extended to 100 hours. If the crack is leaking oil, rather than seeping oil, the case must be repaired or replaced.
According to Fuchs, one of the manufacturers suggested the FAA look into the weld repair business in the late 1980s. The FAA investigated and, realizing that they had approved this type of repair as far back as 1966, issued an advisory circular.
The purpose of AC 33-6, issued December 20, 1994, is to "provide guidelines for the development of repair procedures for weld repairs on crankcases and cylinders of piston engines." This circular put into print the procedures that the repair stations developed.
As noted earlier, Lycoming stopped welding cases in 1997. According to Charles Jarvis, CEO of DivCo in Tulsa, his company provided weld repair services from 1991 through 1998 for TCM. Since 1998, TCM has been doing case repairs at its own facility.
John Pava, vice president of Victor Aviation, an engine shop in Palo Alto, California, says that at least 75 percent of the cases that come through that shop are rebuilt. According to Pava, "Fretting is a big problem." Fretting is wear characterized by a gradual wearing away of material when two surfaces rub against one another. The wear occurs where the case halves touch when the two halves are bolted together.
The through bolts that provide the clamping force for the main-bearing journals and load-bearing sections of the case are designed to be torqued tight enough to prevent fretting. However, overboosting of turbocharged engines, particularly intercooled engines, can exceed design limits, and improperly calibrated torque wrenches or improperly applied or misunderstood torque application during cylinder work can leave the through bolts loose or the case stressed.
Fretted cases can be refurbished by cutting a little bit of material off each half of the case to restore the parting surface finish. After removing small amounts of metal to restore the case half parting surfaces, the case halves must be machined. During machining, main-bearing journals will have to be line-bored to restore clearances and maintain crankshaft alignment within the case halves. Line boring is a precision metal-removal technique akin to drilling a close tolerance hole.
Applying the proper torque value to the through bolts is absolutely critical to prevent fretting, and TCM has recently issued a service bulletin clarifying the proper thread lubrication material (engine oil) and increasing the torque values for cylinder hold-down studs and through bolts to prevent fretting. Since the through bolts project through the cylinder hold-down flanges and assist the cylinder hold-down studs in securing the cylinders, any cylinder removal also slacks off the torque of the through bolts.
RAM feels that maintaining the torque on the through bolts is so important for case integrity that they send all of their engines out the door with new cylinders. The unofficial feeling around RAM is that anytime a cylinder is removed and replaced the odds of case cracking due to uneven through-bolt torquing increases dramatically.
The first step in the repair process is to catalog and mark the case so a company tracking record exists. This protects the repair stations and enables them to gather data for procedural changes and repair improvements. AC 33-6 recommends that all marking numbers include the letter W if a weld was done, the repair station certification number, and the date. This data can be stamped, etched, or vibration peened into the top mating surfaces of the case.
The next step is to check the metal for hardness and thickness at critical points of the case. These checks are done before the case is even cleaned for inspection. If the case halves have been machined to the point that the flanges don't have enough metal to go through another machining, the case is rejected. Often what appears to be overzealous metal removal through carelessness is more likely an attempt by one of the repair stations to align the crankshaft main-bearing journals with the case centerline or equalize the cylinder deck heights.
Repair stations have amended their procedures so that any case cracking or wear induced by machining or repairs is minimized. Most engine manufacturers agree that once a case has been sent out for a repair, the chances of cracking and fretting wear are lessened.
If the case passes the metal hardness and thickness test, a complete cleaning is done. All plugs, dowels, and other steel parts are removed. Then the case is inspected for damage, using Xyglo nondestructive testing (NDT) and visual and dimensional methods. Xyglo NDT requires immersion of the case in a bath of very viscous liquid designed to infiltrate any cracks, then cleaning and looking for cracks under a special light designed to see the liquid, which will indicate the presence of cracks.
Experienced welders grind out the cracked areas and weld in new metal. Shop managers are reluctant to talk about the details of the actual welding technique, but there are requirements for heating the cases before, during, and after the weld to lessen warpage.
Some of the repair shops have approval to build up or reinforce areas on the case that experience has shown to be weak. After all welding is done the cases are sent to the machine shop, where the case may be line-bored or honed and surfaced. All welds are blended and smoothed into the surrounding metal. After rework is completed there's a final inspection before the cases are dipped in a surface treatment bath. This treatment, called Alodine, is a corrosion-inhibiting surface treatment for the aluminum and imparts a goldish hue to the metal.
The best engine rebuild shops in the country send most cases for repair during an overhaul. There's more to an overhaul than just putting new cylinders on the top end and turning the start key. Sending your case to the experts for a little TLC during an overhaul will increase your chances for trouble-free flying.
For further information, contact DivCo, Inc., 2806 North Sheridan Road, Tulsa, Oklahoma 74115; telephone 800/874-1351; fax 918/835-4801; Nickson's Machine Shop Inc., 914 West Betteravia Road, Santa Maria, California 93455; telephone 805/925-2525; fax 805/928-9117; or Crankcase Services Inc., 14700 West Shellcreek Road, Sand Springs, Oklahoma 74063; telephone 918/245-6767; fax 918/241-6205.
Links to additional information about engine crankcase maintenance may be found on AOPA Online ( www.aopa.org/pilot/links/links0008.shtml). E-mail the author at [email protected].
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