Bearings Are An Important Part Of Any Engine
In life, sometimes there are objects which need to be sacrificed for the greater good and henceforth the term “sacrificial lamb.” When it comes to an engine, the sacrificial lamb just might be a crankshaft or connecting rod bearing but they’re really much more than that.
In the case of just about any engine; small engines like lawn mowers or golf carts not withstanding; without the use of a soft piece of material between the crankshaft and its
mating surfaces, it wouldn’t last too long. However, the bearing is still there as a last defense as there should be a thin film of oil between the bearing surface and the crankshaft; thin in as little as the thickness of the cover paper of this magazine. Still though, there are instances during the rotation of the crankshaft where the bearing does come into contact with the crank journal and it’s these instances where the proper bearing for your application becomes necessary.
We spoke with Bill McKnight of MAHLE, the manufacturers of Clevite bearings who said, “First of all, if you’re going to run an engine hard, then by all means use a race bearing. A race bearing has a slightly different geometry because of the heavy loads it sees. Also, we alter the Babbitt overlay thickness to make it less prone to cracking under heavy loads. Coatings are also in wide use today mostly for their anti-friction properties which help under heavy loads to protect the bearing from damage.”
One other thing to bear in mind when it comes to race engines is the fact of most aftermarket forged crankshafts have a large fillet radius, which is a rounding of the exterior corner of the rod journal. A connecting rod bearing must therefore be chamfered enough to clear that fillet. The use of a narrow H-style bearing clears that fillet and should be the bearing to look for in those applications.
“One of the comments we often hear,” says McKnight, “is ‘My bearing delaminated, it must be defective.’ Clevite bearings are actually manufactured from three materials. A steel back with a cast alloy layer of lead-tin-copper material called bronze and finally an electroplated Babbitt overlay. At roughly 450-degrees F, the Babbitt melts. Then about 600-700 degrees F, the bronze melts, and finally the steel backing, if your bearing ever got to 2500 degrees F, it would completely melt. The first signs of damage are smeared or ‘wiped’ material and as the heat climbs, delamination is next. Since oil temps in most engines are 300-degrees F or less, lack of lubrication is almost always the cause of both wiping and delamination.”
With bearing clearance the vital part of any engine build, adjusting it can be a project all in itself, albeit a very important venture. Bearings are available in a wide range of sizes to match crankshaft dimensions with the ultimate clearance left entirely up to the engine builder. In addition to standard and undersize bearings for crankshafts which have been undercut, bearings are also available in what is termed as .001-under and over to allow builders the opportunity to properly adjust clearances. One myth regarding that is “You shouldn’t mix standard size and one under bearings on the same journal.”
“Bearings have a feature called a parting line relief machined into them,” says McKnight. “That allows you to mix these sizes as mentioned with no problem and gives an engine builder a means of dialing bearing clearance in much closer. Put the thicker shell on the loaded side, that being in the rod for rod bearings and in the main cap for main bearings.”
Obviously to accurately measure bearing clearance requires both inside and outside micrometers and/or bore gauges. But an old and little used measurement tool is Plastigauge®, a thin strip of “string,” a soft material, which is placed on the crank journal and the rod or main cap tightened on it. The string is then compressed after which the cap is removed and the thickness of compressed string is matched against a template standard to give you an idea as to bearing clearance. Definitely “old school” but still relevant today.
“Plastigauge is not accurate enough to check bearing clearances on performance engines,” McKnight said. “For everything but the really high-end engines, Plastigauge still works quite well. For anyone building their own engines at home, it’s the best tool we have. We used to sell it to the Navy to check the door fit on the water-tight bulkheads in submarines.”
It’s pretty obvious everyone understands the purpose of bearing clearance, but it stands to reason that the type and weight oil used should somewhat be matched to clearance. Looser clearances require heavier oil and vice-versa for tight clearances. Today we often see the use of “zero-weight,” or very thin oils which should only be used
in engines where bearing clearance is adjusted for that purpose.
“One thing to bear in mind,” adds McKnight, “is that as the bearing clearance gets looser, the oil film contact patch gets narrower and the pressure load on the oil film gets higher. When that happens, there’s more likelihood the crank and bearing will contact each other, not a good thing. For most applications, we recommend .001” of clearance per one-inch of shaft diameter. For example, a 2.200” rod journal would need about .0025” of vertical oil clearance. Tighter clearances need thinner oil, looser clearances require thicker oil is an important point to remember.”
Bearing tangs on the edge of the bearing are meant to locate the bearing in place. However, another myth perpetrated in the industry is that the tangs keep the bearing from spinning in the bore. “Definitely not true,” says McKnight. “Bearings have something called ‘crush’ built in to them. Basically this means each half shell is longer than half a circle and when the rod or main cap is tightened down, radial force is created and applied to the housing bore of the rod or the main and that’s what prevents spinning, all of which makes the diameter of the bore; be it block/cap or connecting rod; an important measurement.
“Having your shop hone the rod bores a bit big is not a good way to get extra clearance,” he adds. “This is actually a bad idea, not to mention has very little effect on clearance. We design race bearings with maximum crush because of the heavy loads seen in that type engine. Opening up the bore reduces the crush and increases the chances of spinning a bearing. If you need more clearance, we offer bearings in a variety of over- and under-sizes.”
Flipping the above statement around is another one McKnight hears frequently. “I like to make my bores a bit smaller to gain more crush.” The problem here is that bearings are designed with a high crush right from Clevite. Adding more can exceed the material strength of the bearing and cause it to buckle in and rub on the crankshaft. Often the problem is not severe because the soft Babbitt wears away quickly, but McKnight adds, “We’ve been making trimetal bearings for about 70 years. Many of us at Clevite have had 30, even 40 years in the business. When you decide on your own to start re-engineering things, you’re starting down a slippery slope.”
As mentioned earlier, coatings have been used for years mainly for their anti-friction properties. Using the bearings right out of the box is the best even though some people have used Scotch Brite® or the equivalent to clean the bearings before installation. “This is actually not a good idea,” says McKnight. “Babbitt on race bearings is thin and you can easily polish away 30-40-percent of that material with the pads, meaning you’ll be buying more bearings from Clevite. If you feel compelled to polish the bearing surface, use a piece of newspaper. The fiber in newsprint is just enough to do the job without wearing away the Babbitt.”
Both McKnight and a number of other Mahle representatives travel to numerous races and trade shows all over the country. Each one is very well versed in bearings from a design and engineering aspect. “We’re very accessible at the track and are always willing to work with anyone on your bearing issues,” adds McKnight.
Assuring the “sacrificial lamb” is one which lasts for a very long time and takes proper care of your crankshaft is the goal of any engine. The use and care of the correct components is the only way you can help that to happen.