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Old 12-19-2005, 04:56 AM
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SBC how much is too much when you bore them out

I have a used motor, it has had a performance rebuild and has been bored 40 over. It was done before I got the motor and I didn't have any say in it. I have heard of going 30 over on a rebuild but what is too much on a SBC, mine is a 4 bolt main from some 72 something

All of my experience has been with bolt on stuff, intakes, headers and other external power adders. This is the first motor that I have had that is not "stock" inside.
It also has a hot cam but the person that sold it to me doesn't have the spec sheet on it anymore. Glad I trust him

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Old 12-19-2005, 05:54 AM
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Nothing wrong with .040 over. Usually considered the max for hi-performance. Up to .080 overs are available for stock rebuilds.
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Old 12-19-2005, 04:54 PM
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Thanks, this is going to be a street car. The last owner had problems keeping it cool. His biggest problem was getting all of the air out of his system. I wont have that problem. I hope.
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Old 12-19-2005, 05:30 PM
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40 over will run warmer than a std. bore or 20 - 30 over cause the cylinder walls are now thinner, but no it's not too much and it shouldn't run "hot". just a little warmer than usaul. it should still be dependable though

J
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Old 12-19-2005, 05:38 PM
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Quote:
Originally Posted by jesse01
40 over will run warmer than a std. bore or 20 - 30 over cause the cylinder walls are now thinner, but no it's not too much and it shouldn't run "hot". just a little warmer than usaul. it should still be dependable though

J
It is a myth that an overbored engine will run warmer. My rule of thumb is the 1975 and older SBCs are usually safe up to .060". The 1976 up to 1985 are usually safe to .040". The 1987 and newer are usually safe to .030", I've not had trouble with .040" but saw one at .020" crack a cylinder with using a small centrifugal blower (that one was a 93 block).

tom
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Old 12-19-2005, 05:46 PM
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Quote:
Originally Posted by jesse01
40 over will run warmer than a std. bore or 20 - 30 over cause the cylinder walls are now thinner, but no it's not too much and it shouldn't run "hot". just a little warmer than usaul. it should still be dependable though

J
I heard the same thing

Quote:
Originally Posted by machine shop tom
It is a myth that an overbored engine will run warmer. My rule of thumb is the 1975 and older SBCs are usually safe up to .060". The 1976 up to 1985 are usually safe to .040". The 1987 and newer are usually safe to .030", I've not had trouble with .040" but saw one at .020" crack a cylinder with using a small centrifugal blower (that one was a 93 block).

tom
I looked up the casting numbers and it is an older 4 bolt main block. I was told it came out of a 1974 but dont remember what one.
I have a very large aluminum radiator that I will install if I need it.
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Old 12-19-2005, 05:55 PM
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I bored one to .80 for my sons monte carlo with flat tops and a good sized cam.We drilled 400 steam holes in the block and heads and used 400 head gaskets. Not sure if that helped that much but he never had a heating problem the time he owned it.
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