Auto Service World
Feature   December 1, 2000   by Jim Anderton

From the Bottom Up

From the harmonic balancer to rear main seal, reliable, dry engines start with a solid bottom end


There’s no doubt about it: engines today hold up longer than ever before. Advances in lubricants, materials and electronic engine controls are main factors, but Canadians drive more, and expect more miles out of their engines. For installers, the increasing numbers of service procedures that require removal of the oil pan present an opportunity to service the vital crankshaft and connecting rod bearings.

If there’s good news about bottom end service, it’s that bearing installation isn’t rocket science. It is; however, essential to engine longevity, so a little extra care on the bottom end is good insurance for installer and customer both.

Fit and forget?

Can you drop the caps and just snap in fresh shells and get away with it? The answer to that question depends on factors such as the condition of the journals, the number and severity of mileage on the engine, and the economic sensibility of bearings versus a reman engine. A typical candidate might be a well-maintained high-mileage engine with reasonable compression and a slight rod knock or telltale rumble when the clutch is depressed. A seal or pan gasket job could be an opportunity to address a rod noise issue, and let valvetrain or compression determine the ultimate life of the engine. That life can be shortened considerably, however, if the technician falls into the biggest single mistake in bearing service today: inadequate oil system priming. Assembly lube is a given in modern bearing service, but reliance on that heavy lubricant to protect bearings during a dry start is, at best, risky. The only sure way to protect new bearings is to pre-pressurize the oiling system before startup, a process that ranges from “easy” if you’re priming a small block Chevy, to nearly impossible on modern engines with the pump buried in the block. Spinning up an in-pan immersion-type pump with a drill motor is an old technique, but in many modern engines there’s little alternative to cranking the engine to generate initial pressure. The key is to turn it over, but not start it, allowing oil to circulate to the bearing surfaces before the tons of force exerted on rods and mains pounds out the assembly lube, a process which can begin in mere seconds. Remove the plugs (another service opportunity there) and consider a small amount of light engine oil into the cylinders for ring and wall lube during cranking. Disable the ignition by the book, and be careful when hooking up boost batteries or chargers during cranking. As the starter motor spins, voltage levels drop considerably, enough to set trouble codes and wipe vehicle presets if the process is taken to extremes. Spin it in bursts, and allow the starter plenty of time to cool in between. A better way is to use an accumulator type pre-oiler operating through the oil filter mount or pickup tube. However it’s accomplished, the name of the game is to prime until noticeable pressure is indicated. Failure to adequately prime is a problem that must be avoided, relates Bob Anderson, team leader, Clevite engine bearings: “It’s not uncommon on some of those (hard to prime engines) that you can do serious damage to the bearings by the time you get oil in the engine”. Anderson also notes that the pre-pressurization procedure is a sure way to determine that the oil pump is working properly. Oil pump to pan clearance isn’t generally an issue, but if technicians reuse the original pickup, they should at least inspect the assembly carefully, washing the tube with solvent in the direction of oil flow. If parts are readily available, simply replace, fastening the tube according to manufacturer’s instructions, including anaerobic sealants as required. Concerned about pan clearance? A tiny dab of grease on the lowest point of the screen can act as a quick check. Offer up the pan dry, and the compressed gasket thickness will give a little extra margin. This process only takes a minute, and helps if a damaged or bent pickup is suspected. At the other end of the pickup tube, a new oil pump won’t mean a thing if it sucks air during the vital initial startup procedure. Strange as it may seem in this age of specialty lubricants, Vaseline is still the universal choice for packing new oil pumps among experts contacted by SSGM.

Care with cold cracked rods

Those “cracked” rods such as the O.E. 4.6 L Ford V-8 present a special problem to technicians performing bottom-end service. The issue is connected to the way the caps meet the rods, in that the fracturing process leaves an irregular but intimate mating surface that allows the manufacturer to achieve unprecedented levels of concentricity in a mass production rod. If you’re a rebuilder, however, the traditional technique of shaving the caps and boring to accept standard shells won’t work because of the need to preserve the unique texture of the mating surfaces. As a result, rebuilders generally bore these rods out to accept oversize shells. If you’re replacing bearings in a reman rod, and you install a conventional bearing, it will spin due to inadequate crush. The only way to be sure is to measure the bore before ordering replacement bearings. And remember, the undersize specs stamped on the back of most bearings refer to reground journals, not housings. At the very least, Plastigage before reassembly.

Miking new bearings

Naturally, bearings should be miked before installation, but a key consideration is where to take the measurement. Bearing shells, or “precision bearing inserts” if you’re in the industry, need to be measured at the centre line or apex of the shell, regardless of the bearing material. The measurement must consistently be made at the centre, because modern bearing designs include a tapering wall thickness to promote the formation of an oil “wedge” between journal and overlay material. The taper also compensates for rod stretch during normal operation. Plastigage must also be used at the centre line of the bearings to accurately measure oil clearances. The best way to measure is, of course, a micrometer, but most “zero to one” micrometers use flat anvils, and most techs don’t have special purpose bearing micrometers. Some mikes have interchangeable anvils allowing a ball anvil conversion, but in pinch, an estimate can be made with the blade end of vernier calipers. Don’t squeeze hard, and take several readings on each measurement and think of average dimensions to control measuring precision. A ball mike, however, may be worth the investment for shops with extensive engine bottom-end business.

The “spun” bearing

“Spun” bearings are a common outcome of oil starvation, where bearing material overheats, locally melts, and briefly welds itself to rod or main journals. Correcting this problem begins with troubleshooting the cause of the original oil starvation. Pump failure, over or under filling of the crankcase, serious fuel dilution of the oil, incorrect oil grade, and sludge or debris restriction of galleries are all possibilities, as is catastrophic oil loss through damaged pans or loose drain plugs. Diagnosis should begin with the vehicle’s owner. Was the oil changed recently? When was the oil level last checked? Has the vehicle run over something or bottomed out in serious way? Does the engine require frequent topping up due to leaks or blowby? All these questions are relevant, and are especially important if the technician suspects gallery blockage due to debris or sludge. The risk of destroying new bearings is very real, so in these cases, an oil system flush is a sensible first step. On all engines, oil pump replacement is a practical suggestion regardless of pressure readings, along with a new pickup and screen assembly.

Some techs have used anaerobic sealants as a precaution against spun bearings in the hope of sticking the shells to the big end and mains bores. While some sealant products are available which are designed for industrial use in restoring lost press fit tolerances, bearing shells should be installed dry. The main reason is heat, or the ability of the shells to reject it. Bearing alloys are soft for many reaso
ns, including imbedability of particulates, tolerance for slight variations in journal shape, as well as fatigue and wear resistance. They’re also almost universally low melting point alloys that need to reject heat through connecting rod big ends or main bearing webs to stay alive. The thin film of sealant acts as an insulator, trapping heat in the one place in an engine where it can do the most damage. Any foreign substance will dam heat behind shells, so keeping it clean is especially important here. Spun bearings WILL damage rods and blocks, usually leaving the owner with reman as the only realistic option.

Contrary to popular belief, the locating lug on bearing shells doesn’t keep the bearing from rotating, but is only there to locate the shell during installation. Crush is the force keeping a bearing from spinning, meaning that the housing bore dimensions are critical. Unfortunately, crush is difficult to accurately measure with mechanic’s measuring tools. Bearing spread won’t affect crush, so reputable brands are the only guarantee of a quality installation. Cap bolts or nuts should be torqued to spec in at least two stages, then backed off and the process repeated. And don’t forget the “pry and check” procedure for crankshaft thrust bearings. This procedure is especially important where heavy-duty clutches are installed.

There’s far more to bottom-end engine service than can be covered in any one article, but a sound working knowledge of the environment and materials around crankshafts, connecting rods and their bearings can drastically reduce the likelihood of comebacks or warranty claims. The investment in time is minimal, and the ability to keep a higher-mileage machine on the road increases the potential for future business. And best of all, it saves the customer money, a true financial “win-win” for both parties. SSGM


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