not a stupid question at all.
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the girdle links the main journals to the outside of the block and to each other thus reducing movement and block flex.
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it is a form of strengthening the bottom end in that way.
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high revving engine with vibration etc.
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v engines suffer a lot more than straight fours and block girdles are more common on them as the crank gets pushed in different directions in relation to the mains.
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