{"id":1595,"date":"2018-04-17T01:10:05","date_gmt":"2018-04-17T05:10:05","guid":{"rendered":"http:\/\/maxtorqueperformance.com\/?p=1595"},"modified":"2018-04-17T01:11:23","modified_gmt":"2018-04-17T05:11:23","slug":"bearing-clearances","status":"publish","type":"post","link":"https:\/\/maxtorqueperformance.com\/staging\/index.php\/2018\/04\/17\/bearing-clearances\/","title":{"rendered":"Bearing Clearances"},"content":{"rendered":"<div>\n<div class=\"post-content-wrapper\">\n<div class=\"row\">\n<div class=\"col-md-12\">\n<article class=\"dt_post_content clearfix post-3418 post              type-post status-publish format-standard hentry              category-features covercategory-march-2013\" id=\"post-3418\">\n<header class=\"post-header\">\n<div class=\"post-by-line\">                  <\/p>\n<div class=\"author-meta\">\n<p class=\"author-name\"><span class=\"author vcard\"><a                          class=\"url fn n\"                          href=\"http:\/\/www.enginebuildermag.com\/author\/carley\/\"><b>Larry                            Carley,<\/b><\/a><\/span><span                        class=\"author-role\">author<\/span><\/p>\n<\/p><\/div>\n<p>                <\/div>\n<\/header>\n<div class=\"post-content clearfix\">\n<p>Is it better to build an engine with tighter bearing                  clearances and run it on low viscosity synthetic motor                  oil, or is it better to build an engine with more                  traditional or even looser bearing clearances and use                  heavier viscosity oil? <\/p>\n<p>\u00a0<br \/>                  Tight bearing clearances and relatively thin synthetic                  multi-viscosity motor oils work well in many performance                  applications from NASCAR and circle track racing to drag                  racing.<\/p>\n<div class=\"content-ad first-ad\">\n<div id=\"id_370_84832900_1523935964\">                  <\/div>\n<p>                  <noscript>  <\/noscript><\/div>\n<p>\u00a0<br \/>                  Keep in mind, however, that most of these engines are                  purpose-built engines that are machined with exacting                  tolerances. Crankshaft journals are precision ground to                  be as round, flat and true as possible, the journals are                  micropolished to a mirror-like finish of a couple                  microinches Ra or less, the bearings are precision fit                  to exact tolerances using a bore gauge and micrometer                  (not deformable plastic gauge), and the engines are run                  on high quality synthetic racing oil, not ordinary motor                  oil. <\/p>\n<p>\u00a0<br \/>                  The oil clearance is the gap between the inside diameter                  of an installed bearing and the outside diameter of the                  crankshaft or camshaft journal. The clearance is                  measured 90 degrees to the bearing parting line, which                  is the thickest part of the bearing (bearing thickness                  tapers slightly toward the parting line). <\/p>\n<p>\u00a0<br \/>                  Reducing the oil clearance between the rod and main                  bearings and the crankshaft has a number of advantages.                  A smaller gap spreads the load over a wider area of the                  bearing surface and distributes pressure more uniformly                  across the bearing. That\u2019s good, provided the bearing is                  strong enough to handle it. A smaller gap also decreases                  the volume of oil that has to flow into the bearing to                  maintain the oil film between the bearing and shaft.<\/p>\n<div class=\"content-ad content-ad-leaderboard\">                  <noscript><\/noscript><\/div>\n<p>\u00a0<br \/>                  That\u2019s also good, provided the oil is thin enough (low                  viscosity) to flow well into the bearing. This also                  reduces the amount of oil pressure the engine needs, so                  some extra horsepower is gained by reducing the load on                  the oil pump. <\/p>\n<p>\u00a0<br \/>                  In a NASCAR engine, rules limit the minimum diameters of                  the rod and main journals on the crankshaft. The rods                  are 1.850? in diameter while the mains are 1.999?. Most                  of these engines are running rod and main bearing                  clearances of .001? or less, and they are doing it with                  low viscosity racing oils such as 0W5, 0W30 and 0W50.                  These racing oils are as thin as water and are highly                  friction modified. <\/p>\n<p>\u00a0<br \/>                  They also contain extra anti-wear additives such as ZDDP                  (phosphorus levels up to 1,850 ppm or higher) to protect                  the cam lobes and flat tappet lifters. These are                  race-only oils and are not recommended for street use                  because they do not contain the same detergents,                  dispersants and corrosion inhibitors as ordinary motor                  oils. Ordinary motors have to handle extended oil drain                  intervals while racing oils do not. Also, the level of                  ZDDP is too high for late model vehicles equipped with                  catalytic converters.<\/p>\n<div class=\"content-ad first-ad\">\n<div id=\"id_610_84845700_1523935964\">                  <\/div>\n<p>                  <noscript><\/noscript>                <\/div>\n<p> With fuel injection, many NASCAR engines are now                  making close to 900 horsepower without a restrictor                  plate, and are turning 9,500 rpms for 500 miles. The                  bearings take quite a pounding but hold up extremely                  well (when was the last time you heard of a NASCAR                  engine blowing because of a bearing failure?). But what                  works great for NASCAR may not work in other forms or                  racing or on the street. <\/p>\n<p>\u00a0<br \/>                  One of the disadvantages of closer bearing clearances is                  that it can increase both bearing and oil temperatures.                  That\u2019s no problem as long as the bearings and oil can                  handle the heat, but if they can\u2019t it increases the risk                  of lubrication breakdown and bearing failure. That\u2019s why                  high quality synthetic motor oil is absolutely essential                  if you are building an engine with tighter than normal                  clearances. <\/p>\n<p>\u00a0<br \/>                  The old rule of thumb is to provide .0007? to .001? of                  bearing clearance for every inch of shaft diameter in a                  stock engine. Consequently, if the crankshaft has                  two-inch diameter journals, the rod and main bearings                  should be assembled with about .0015? to .002? of                  clearance. <\/p>\n<p>\u00a0 <\/p>\n<p>For performance applications, some bearing                  manufacturers recommend adding an extra half a                  thousandth of clearance. Why? Because the rod bores                  don\u2019t stay round in a performance engine at high rpm.                  When the piston reaches top dead center on the exhaust                  stroke, inertia stretches the rod and elongates the bore                  on the big end of the rod. This, in turn, deforms the                  bearings and reduces bearing clearances on the lower rod                  bearing while increasing it on the upper rod bearing. <\/p>\n<p>\u00a0<br \/>                  For high revving performance engines, some bearing                  manufacturers recommend rod bearing clearances of .002?                  to .003?, with an absolute minimum clearance of no less                  than .0015?. The tighter the clearances, the tighter the                  geometry requirements are for the crank journals (as                  round, straight and smooth as possible with little or no                  taper). <\/p>\n<p>\u00a0<br \/>                  Street engines can benefit from tighter tolerances and                  thinner oils for everyday driving. But when power adders                  such as nitrous oxide, turbocharging or supercharging                  are used, or the engine\u2019s power output gets up in the                  450 to 500 plus horsepower range, looser bearing                  clearances are probably safer to accommodate crankshaft                  flexing, main bore and rod bore distortion. <\/p>\n<p>\u00a0<br \/>                  The same reasoning applies to drag motors, truck pull                  engines and other performance engines that produce                  serious horsepower. Many of these engines are built with                  rod and main bearing clearances in the .0025? to .003?                  range. <\/p>\n<p>\u00a0<br \/>                  For the Saturday night dirt track racer, clearance is                  your friend because of the contaminants that often get                  into the crankcase. Looser is usually safer. <\/p>\n<p>\u00a0<br \/>                  Rod and main bores should be as round as possible with                  no more than plus or minus .0005? of variation for a                  performance engine (.001? is close enough for stock).                  You also have to take into account the fact that the                  bearings themselves may not be perfect. Manufacturing                  tolerances of up to plus or minus .00025? are not                  unusual in some bearings, while others may vary only                  .00015? or less. <\/p>\n<p>\u00a0<br \/>                  Main bore alignment is also critical. Some bearing                  manufacturers say adjacent main bores should have no                  more than .0005 inch of misalignment (.001? overall) if                  you are using tri-metal bearings, and no more than .002?                  of misalignment between adjacent bores (.002? overall)                  with aluminum bi-metal bearings. <\/p>\n<p>\u00a0<br \/>                  One of the advantages of looser bearing clearances is                  that it allows more room for \u201cslop,\u201d which is important                  if the crankshaft isn\u2019t machined to near perfection or                  there is some misalignment in the main bores. Wider                  bearing clearances do require a heavier viscosity oil                  (such as a 20W50 multi-viscosity oil or a straight 30,                  40 or 50 oil). The heavier viscosity oil is absolutely                  necessary with wider clearances to maintain the oil film                  between the bearing and shaft so the bearing isn\u2019t                  starved for lubrication. This also requires more oil                  pressure from the oil pump and\/or more oil volume. <\/p>\n<p>\u00a0<br \/>                  The amount of oil that is actually between the bearing                  and shaft surface at the point of highest load isn\u2019t                  much. Though the installed gap between the bearing and                  shaft may be .001? to .0015? or more, the oil is                  displaced when the bearing is loaded. At its thinnest                  point, the oil film may only be .00002? thick (1\/100th                  the diameter of a human hair!). That\u2019s not much oil                  between the metal surfaces, but it doesn\u2019t take much to                  maintain hydrostatic lubrication. When the shaft starts                  to turn, an oil wedge forms between the shaft and                  bearing that lifts the shaft up and away from the                  bearing surface. The shaft then glides on the oil with                  minimal friction.                <\/p>\n<p>\u00a0<br \/>                  If a crankshaft grinder wobbles while a crankshaft is                  being ground, it can leave lobes around the                  circumference of the journal. These may be invisible to                  the naked eye and very difficult to detect with a                  micrometer. But if there\u2019s any distortion on the                  surface, it may interfere with the formation of the oil                  wedge under the shaft if the bearing clearances in the                  engine are too tight. Polishing the crank can reduce                  surface roughness on the journal but it won\u2019t get rid of                  the lobes or ribbing. <\/p>\n<p>\u00a0<br \/>                  Another factor to consider is that the upper Babbitt                  layer on a tri-metal bearing is very thin, typically                  .0005? to .0008? thick. The top layer of Babbitt acts as                  a dry film lubricant when there is no oil between the                  shaft and bearing. That\u2019s fine for a dry start that may                  only last a couple revolutions of the crankshaft, but it                  is quickly wiped away if the engine starves for oil when                  it is running under heavy load or at high rpm. <\/p>\n<p>\u00a0<br \/>                  And once the protective upper layer of Babbitt has been                  destroyed, the intermediate layer of copper\/lead alloy                  will quickly seize if there\u2019s no oil film to keep it                  separated from the shaft. <\/p>\n<p>\u00a0<br \/>                  One of the reasons why many performance engine builders                  use tri-metal bearings is because they want bearings                  that have good seizure resistance in high rpm                  applications. Tri-metal bearings also handle high engine                  loads well and have good fatigue resistance. The Babbitt                  surface layer also provides embedability if dirt or                  debris gets past the oil filter. Tri-metal bearings are                  typically recommended for use with forged steel                  crankshafts. <\/p>\n<p>\u00a0<br \/>                  Aluminum bi-metal bearings, by comparison, have high                  wear and corrosion resistance. With harder                  aluminum\/silicon alloys, they can also handle higher                  loads while providing good anti-seize properties.                  Aluminum bearings are often recommended for cast iron                  cranks because they have a polishing effect on the                  journal surface. What\u2019s more, according to some bearing                  manufacturers, a high silicon alloy aluminum bi-metal                  bearing will actually resist seizure longer than a                  tri-metal bearing if the protective oil film goes away.                <\/p>\n<p>\u00a0<br \/>                  That brings us back to the oil and bearing clearances.                  The oil doesn\u2019t care what kind of bearing and shaft it                  is lubricating. It only needs to maintain enough oil                  film between the two surfaces to provide hydrodynamic                  lubrication and prevent metal-to-metal contact. There                  has to be enough oil pressure and flow to keep the                  bearings lubricated and cooled, and the oil itself has                  to have enough shear strength so it isn\u2019t pushed out of                  the gap between the bearing and shaft at the point where                  the load is greatest. <\/p>\n<p>\u00a0<br \/>                  Multi-viscosity synthetic motor oils flow more easily                  than conventional straight weight oils at both low and                  high temperatures. So they can handle cold starts as                  well as elevated operating temperatures (which is really                  important with turbochargers). To reduce friction and                  improve fuel economy, most late model stock engines are                  factory-filled with 5W20 or even 0W20 oil. Combined with                  tighter engine assembly tolerances, these oil and                  bearing combinations work relatively well for everyday                  driving and even mild performance use. For racing                  applications, though, the oil needs to be formulated                  specifically for racing \u2013 especially if the engine has a                  flat tappet cam that requires plenty of ZDDP in the                  additive package. <\/p>\n<p>\u00a0<br \/>                  You can get oil viscosities ranging from 0W5 to 120W60,                  with 15W40 being a popular viscosity for stock car                  racing, road racing and spring cars. For wider bearing                  clearances, some prefer to use a heavier 15W50 or 20W50                  oil. In drag racing Top Alcohol and Pro Mod classes,                  AHDRA Nitro Bikes and blown alcohol tractor pulling,                  20W60 may be the lubricant of choice. For NHRA Top Fuel                  dragsters and Funny Cars, a 70WT oil might be used. So                  the type of oil that\u2019s used will depend on the                  application and the bearing clearances inside the motor.                <\/p>\n<p>\u00a0<br \/>                  An additional layer of protection can be achieved by                  installing coated bearings. Various types of proprietary                  coatings are available that provide scuff resistance                  where there is no oil between the bearing and shaft.                  Such coatings cost extra, but are good insurance against                  dry starts and may save a crank if the engine loses oil                  pressure in a race. <\/p>\n<p>\u00a0<br \/>                  Finally, regardless of what type of bearings you put in                  an engine or how close you set the bearing clearances,                  always use plenty of assembly lube to coat the bearings.                  Also, use the proper break-in oil when the engine is run                  for the first time. Break-in oils are typically a                  straight 30W oil without friction modifiers for fast                  ring seating. But they also contain extra ZDDP anti-wear                  additives to protect the cam and lifters. The break-in                  oil can then be drained and replaced with the type of                  oil (conventional or synthetic) that will be used from                  that point on. Be sure to tell your engine customer how                  important it is to use a high quality oil and that it                  has the correct viscosity to match the bearing                  clearances and lubrication requirements of the engine                  and application. <\/p>\n<p><em><br \/>                  <\/em> <\/p>\n<p>\u00a0<br \/>                  <img decoding=\"async\" alt=\"\"  src=\"\/wp-content\/uploads\/Articles\/03_01_2013\/111591crankshaf_00000062068.jpg\"                    align=\"right\" border=\"0\"><img decoding=\"async\" alt=\"reducing the oil                    clearance between the rod and main bearings and the                    crankshaft has a number of advantages. a smaller gap                    spreads the load over a wider area of the bearing                    surface and distributes pressure more uniformly across                    the bearing.\"  src=\"\/wp-content\/uploads\/Articles\/03_01_2013\/111591typesofbe_00000062069.jpg\"                    align=\"right\" border=\"0\"><img decoding=\"async\" alt=\"tri-metal bearings                    handle high engine loads well and have good fatigue                    resistance. aluminum bi-metal bearings, by comparison,                    have high wear and corrosion resistance. some engine                    builders use coatings for extra protection in case of                    contact with the journal, but others say it is not                    necessary.\"  src=\"\/wp-content\/uploads\/Articles\/03_01_2013\/111591coatedmai_00000062070.jpg\"                    align=\"right\" border=\"0\"><img decoding=\"async\" alt=\"&quot;if\"  src=\"\/wp-content\/uploads\/Articles\/03_01_2013\/111591crankjour_00000062071.jpg\"                    align=\"right\" border=\"0\"><\/p>\n<\/p><\/div>\n<\/article><\/div>\n<\/p><\/div>\n<\/p><\/div>\n<p>    <\/div>\n","protected":false},"excerpt":{"rendered":"<p>Larry Carley,author Is it better to build an engine with tighter bearing clearances and run it on low viscosity synthetic 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