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Crankshaft bearings

All information about the Main bearing shells
by
Bartek Bartoszewicz
Tuning Professional
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Crankshaft bearings - inconspicuous but indispensable

Crankshaft bearings may be small and inconspicuous, but they are very important for the stability of the crankshaft and therefore also an important element of performance enhancement.

The bearings should always be adapted to the performance of your engine to avoid damage. Here you can learn everything about the main bearings, as they are also called: from function to installation.

What are crankshaft bearings?

Crankshaft bearings, also called main bearings, support the crankshaft in the crankcase. Essentially, bearings are there to guide the movement of two components against each other. As the crankshaft rotates and translates the up and down motion of the piston and conrod into a circular motion, it is held in the block by the main bearings. They consist of two semi-circular elements, each containing a bearing shell. Depending on whether they are plain bearings or roller bearings, they are lubricated either by pressure oil lubrication or by an oil mist. Due to the extreme performance we want to achieve through tuning, extreme forces also tug at the main bearings and the bearing shells, which is why we should always adjust them to our requirements.

What is the function of crankshaft bearings?

The crankshaft bearings hold the crankshaft in the block with almost no wear. Optimal lubrication prevents the crankshaft and bearing shells from touching and causing unwanted friction. Instead, the crankshaft simply slides over the oil film provided by the oil pump. The bearing shells are important for transferring the existing axial and radial forces to the bearing housings. They also embed the wear, which consists of small metal particles. This would cause high wear without the embedding as it does not get caught in the oil filter. Improved crankshaft bearings also have the function of withstanding the higher demands caused by the increase in performance. This is achieved, for example, by special coatings and improved oil bores.

Damage to the main bearings

How does damage to the main bearings occur?

Bearing damage occurs quickly when foreign bodies do make it through the lubrication gap and are so large that they cannot be fully embedded in the sliding or running layer. When this happens, the shaft journal wears and scoring occurs. These encourage bearing seizure and dramatically reduce the life of your main bearings. The cause of contamination can be anywhere: dirt particles can become embedded in the motor during manufacture, as well as during repair. Often the oil can also be contaminated, for example by a clogged filter. If you notice bearing damage, the first step is to know where the damage is coming from. Before you install new bearings, you should therefore clean everything thoroughly: flush out all oil bores and oil ducts of the turbocharger and the like and also clean the bearing seating surfaces. Check all seals and filters for dirt particles and replace them if necessary.

Instructions: Replacing the crankshaft bearings

You can only reach the crankshaft bearings if you disassemble the engine block and the crankshaft. You usually do this to find out if there is bearing damage. If this is the case, you should change your crankshaft bearings. This is how you proceed:

Step 1: First, disassemble the block. Pay attention to the markings on the bearings before you install them.

Step 2: It is especially important that it is absolutely clean when you install the new bearings. Dirt can directly destroy new bearings. Therefore clean all parts. Pay attention to cracked bearing caps: Here you have to be especially careful to avoid dirt and abrasion so that everything fits afterwards.

Step 3: Now measure the diameter of the base bore. You do this at three different places in order to find out if there are any ovalities.

Step 4: Now the actual installation of the new Main bearing shells begins. Now clean the crankshaft's bearing race and journals again and compare the new shells with the old ones. By means of the numbering you can determine whether you have selected the correct bearings. Also, the fitting bearing and the oil hole must be in the right place. Also, upper and lower bearings may be different, or sputter layers may be used on engines that are particularly heavy-duty (main bearings usually have sputtered lower bearings).

Step 5: After inserting the bearings, oil the running surfaces.

Step 6: Now insert the crankshaft. Be careful not to jam it and get help if you don't have a crane or similar.

Step 7: Now put the bearing caps on and tighten them. Pay attention to the markings and tighten the expansion bolts according to the manufacturer's instructions. Also check if the bolts can be reused.

Step 8: After installation, check whether the crankshaft can be turned or whether it still sticks somewhere. The axial play must also be checked now. If everything is within the tolerances, you can reinstall the block.

Assistance with bearing replacement

Do you have questions about the bearing change or do you need support? Then use our FAQ, our contact form or our chat. Our tuning professionals are looking forward to meeting you!

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Bartek Bartoszewicz
Tuning Professional
His first car was a Polo Mk1 with a 40 Weber twin carburetor and 129 PS (95 kW). His second was an Audi 50. Today Bartek tunes Lamborghinis to 1000 PS (735 kW). Even as a young boy, Bartek disassembled vehicles and put everything back together better. He wrote his high school diploma with oil on his fingers. The trained automotive mechanic with a focus on engines and gearboxes was determined to go into motorsports. In his 10 years in Formal 1, he supervised 73 races, including as engine mechanic for Ralf Schumacher at Toyota. Since 2010, he has dedicated himself fully to his company BAR-TEK® and helps his customers to bring VW and Audi engines to peak performance.
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