# Crankshaft Main Bearing - what size ?



## adamrysz87 (Nov 14, 2021)

Hello, In my GTO the engine stopped, probably I will need a new crankshaft main bearing. What does that mean - undersize 0.01 ", 0.02", ... which choose ?


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## O52 (Jan 27, 2019)

The crankshaft has two sets of bearings. One set for the crankshaft to engine block, or MAIN BEARING, the other for the connecting rod to crankshaft, or ROD BEARING 
When originally cast, the crankshaft bearing surface diameters are machined to the required design diameter. Over it's lifetime the crankshaft bearing surface will wear down. The bearing surface is then machined to a smaller diameter. 
Machining is commonly performed in Ten Hundreths of an inch or .010 
The depth of the cut depends upon the amount of wear from original and bearing clearance required and will round off to .010 or .020 undersized.
This is common knowledge for a good automotive machine shop. 
Until you have the crankshaft machined, you will not know what size bearing to purchase.


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## adamrysz87 (Nov 14, 2021)

OK, as I understand correctly, if I don't have to cut the crankshaft, I will need the standard crankshaft bearings. If there will be machining, I have to adjust to the depth of cut. That's right ?

Thanks for your help.


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## PontiacJim (Dec 29, 2012)

adamrysz87 said:


> OK, as I understand correctly, if I don't have to cut the crankshaft, I will need the standard crankshaft bearings. If there will be machining, I have to adjust to the depth of cut. That's right ?
> 
> Thanks for your help.


That is right. Typically the crank journals, main or rod, will be ground .010", .020", or .030" undersized and you will see replacement bearings in these sizes. The journals are ground only enough to clean them up to the next size, so if .010" will clean up all the journals, the only go .010".

Your main journals and rod journals can be of different sizes. For example, the main journals may clean up at .010", but the rod journals may need more and clean up at .020". This is OK to have different sizes.

The .001, .002" undersizes are used if the engine builder wants a little more bearing clearances over the .010" or .020" that the journals were turned to. With some race engine, more bearing clearance is wanted because the engine will see very high RPM's and more bearing clearances can provide more oil to remove the heat from higher RPM's.

A machine shop will have to grind the crank, then you order your bearings for the size they crank journals were ground down. VERY IMPORTANT that the crank grinder use a RADIUS cut where the crank journal meets the crank weight. You do not want a sharp edge as this will create a stress point and can lead to cracks and crank failure. AND, when a radius is cut into the crank journal, put some black marker on the radius, then install your bearing over the radius and move it about on that radius to see if it leaves any black marking. You want the bearing to have a slight cut/angle in the bearing material to clear the radius and not ride on it. If the bearing material rides on the radius, it can burn up the bearing and even lock up the engine. So always check for this.

Make sure the rods are installed correctly as well.


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## geeteeohguy (Feb 2, 2008)

Did a locked- up crank cause your engine to 'stop'?


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