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318 crank shaft in a 340 ???

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Old 04-14-2012, 03:52 PM   #1
mmoose
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318 crank shaft in a 340 ???

How would this change the dis and compression of the engine??? its a 73 block.
I am unsure if the 340 rods were used or the 318??

Would this be an improvement over the stock 340 stuff??
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Old 04-14-2012, 03:56 PM   #2
1968FormulaS340
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The 318 and 340 cranks are essentially the same. Swapping from one to the other will not change the displacement or compression at all.
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Old 04-14-2012, 04:10 PM   #3
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The balance is different correct? the 340 is externally balanced and the 318 is a balanced neutral shaft correct?
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Old 04-14-2012, 04:21 PM   #4
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I believe all 318s were internal balance ( someone here will correct me if I'm wrong...). Your '73 340 external.
Either way there are multiple 318 cranks. Early forged ones, later on cast with balance for light duty rods, and later yet cast with balance for heavy duty rods. None of them balanced for the heavy 340 pistons...
Stroke and journals all the same, so, yes you could use a 318 crank...But needs balancing.
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Old 04-14-2012, 04:25 PM   #5
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and the right balancer and flex plate or torque converter...
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Old 04-14-2012, 04:26 PM   #6
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The point is that even with a 318 crank, a 340 will need to be balanced.
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Old 04-14-2012, 04:55 PM   #7
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Is the 318 crank forged or cast? 318 is internally balanced and 73 340 externally, as others have already stated. If you are swapping cranks you might as well put some higher comp. pistons in @ the same time. It will need to be balanced anyways. 10:1 would be much better than the 73 340 8.5:1 JMO. Otherwise your not going to gain a thing by just swapping cranks.
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Old 04-14-2012, 05:04 PM   #8
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All 273/318 poly/early and truck 318la's were steel light duty cranks. Earlier 340s had better steel cranks, later 340s were cast. Most passenger car 318s had cast cranks.
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