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In a piston engine, the main bearings are the bearings on which the crankshaft rotates, usually plain or journal bearings. The bearings hold the crankshaft in place and prevent the forces created by the piston and transmitted to the crankshaft by the connecting rods from dislodging the crankshaft, instead forcing the crank to convert the reciprocating movement into rotation.

While some small single-cylinder engines have only one main bearing, designed to deal with the bending moment exerted by the crank as a result of the force from the connecting rod, most engines have at least two main bearings, one at each end of the crank shaft and may have as many as one more than the number of crank pins. Increasing the number of bearings in an engine will generally increase the size and cost of the engine, but also provides stability to the crankshaft, which would otherwise have to endure greater bending moments from having the crank pin further from a bearing. The combination of bearing number and crankshaft design can also have an effect on engine balance; a more stable crankshaft will better avoid unbalance from flexing, but increasing the crankshaft length or girth to accommodate an additional bearing increases the rotational inertia of the crankshaft, which can have the opposite effect.

Most modern automotive engines have one main bearing at each end of the crankshaft and another in between each adjacent pair of connecting rod journals, but not all engines conform to this generalization. Notable exceptions include the Ford flathead V-8, which often had only three main bearings, and the Chevrolet straight-6 engine, which had in various iterations, between 3 and 7 main bearings.

When describing a crankshaft design, the number of main bearings is generally quoted, as the number of crank pins is determined by the engine configuration. For example, a crankshaft for an inline six engine will be described as three bearing or four bearing depending on its number of main bearings; The crank pins are not counted in this description. Similarly, when speaking of a crankshaft, the journals are the main bearing journals only. The crank pins are not normally called journals although they form the centre shafts of the big end bearings and are therefore journals in the more general sense.

Original courtesy of Wikipedia: http://en.wikipedia.org/wiki/Main_bearing — Please support Wikipedia.
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39 news items

Lewiston Sun Journal

Lewiston Sun Journal
Mon, 21 Jul 2014 20:56:15 -0700

On March 22, during a daily inspection of the resort's machinery, employees discovered that the main bearing on the return bullwheel at the bottom of the Wayback Chair was making strange noises and showed signs of wear-and-tear. The bullwheel is the ...
Virtual-Strategy Magazine (press release)
Wed, 23 Jul 2014 04:37:30 -0700

As the engine tuner explains, BRM ball hones can be used with main bearing tunnels, connecting rods, OHC camshaft tunnels, valve guides, DOHL lifter bores, and in-block hydraulic lifter bores. “If there is a hole and I need to surface finish it,” the ...
Tue, 22 Jul 2014 08:56:15 -0700

Six-bolt main bearing caps, a fully counterweighted forged steel crankshaft and cast exhaust manifolds are designed for heavy-duty operation. Both the upper and lower intake manifolds of the 3.5-liter V6 are tuned for responsive power, and both the ...
MotorTrend Magazine
Tue, 22 Jul 2014 10:50:45 -0700

The naturally aspirated 3.5-liter also gets a die-cast aluminum engine block, six-bolt main bearing caps, a fully counterweighted forged crankshaft, and cast exhaust manifolds for heavy-duty use. The engine is rated to tow a maximum of 7600 pounds ...
AftermarketNews.com (AMN)
Mon, 21 Jul 2014 09:30:00 -0700

BANNOCKBURN, Ill. – Magnum Gaskets (an MSI brand) has expanded its product offering beyond applications for traditional passenger cars and light-duty trucks, making inroads into the pickup truck diesel, heavy-duty diesel and high-performance vehicle ...
Popular Hot Rodding Magazine
Fri, 18 Jul 2014 06:18:45 -0700

The main bearing bulkheads on a production block (bottom) get very thin near the cylinder bores. This scalloped shape can potentially compromise crankshaft support under extreme loads. In contrast, the Ford Racing block (top) is built like a tank.
Overdrive Magazine
Thu, 10 Jul 2014 08:27:41 -0700

These can also include throwing and slapping of belts, broken accessory brackets and accessory drive gear wear, broken and loosening bolts, loosening bolts, excessive main bearing wear, loss of fuel economy, loss of torque and horsepower and broken ...
Popular Hot Rodding Magazine
Mon, 07 Jul 2014 06:07:30 -0700

This detail reveals the massive structure of the Kauffman block's billet steel splayed main bearing caps. The block features the 3.00-inch Pontiac “small” mains, as opposed to the 3.25-inch mains used by Pontiac on their largest engines. ARP studs ...

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