BBC 2 Bolt Mains Limits.
Discussion
Can any of you clued up drag racer give me an idea of what it is that makes a 2 bolt mains not very strong ??
Is it the BHP, Torque or the high revs ! Or is it a combo of all them. What will they take before you start getting into risky teratory.
Ive just bought one that has just had a full rebuild and i plan on using it as it is this year and getting splayed caps done with it next year. It will be getting a supercharger sat on it to.
Is it the BHP, Torque or the high revs ! Or is it a combo of all them. What will they take before you start getting into risky teratory.
Ive just bought one that has just had a full rebuild and i plan on using it as it is this year and getting splayed caps done with it next year. It will be getting a supercharger sat on it to.
The rotating mass of the engine is held in place by the crank girdle, the forces trying to throw the engine apart are phenominal,even for a stock engine, and the problem increases with revs, double the revs does not double the forces, but quadruple them, so four bolt mains help to hold the main caps better by spreading the stresses more evenly throughout the block, .. or something like that...
Chevy spent loads of $ developing the small and big block v8s,and ran 375hp on the 327 in 1966 with 2 bolt mains, people forget how tough these motors are so stick a blower on, keep the boost conservative, and hang on!! Lets face it ,the BBC has enough torque in std form and can easily handle a bit of boost.
Thanks for all the answers and advice.
To get it splayed, What does actually it involve ?? Ive neer had a chevy before so it is new to me.
Ive not yet looked but ive been told that my new bottom end has arp right through. So it might have arp cap studs, The splayed conversion do they add 2 extra bolts at a splayed angle making it a 4 bolt? And what sort of price would i expect to pay to get it done and can anyone recomend a good reliable place to do it for me. Thanks Adam.
To get it splayed, What does actually it involve ?? Ive neer had a chevy before so it is new to me.
Ive not yet looked but ive been told that my new bottom end has arp right through. So it might have arp cap studs, The splayed conversion do they add 2 extra bolts at a splayed angle making it a 4 bolt? And what sort of price would i expect to pay to get it done and can anyone recomend a good reliable place to do it for me. Thanks Adam.
Ive read on a yank forum that the 2 bolts type block is stronger as the 4 bolt has some weak points where as the 2 bolt is a stronger block but the only thing that lets it down is the 2 bolt mains. Why is the 4 bolt stronger than the 2 bolt splayed conversion ? If the 2 bolt conversion has 4 bolts and 2 of them are on an angle where they wont come loose.
Sorry for all the questions. Imjust trying to understand the difference between the pair and then try to get it to stay in my head (Which is the difficult part)
Sorry for all the questions. Imjust trying to understand the difference between the pair and then try to get it to stay in my head (Which is the difficult part)
'It's not simple to answer, but modifying a 2 bolt block will never give you the strength of a 4 bolt block. The number of bolts and studs maybe the same, but the block isn't the way nature, or GM intended.'
Hi; the above statement is not true.
BB Chevy's have the same main webs & bulkheads whether they are two or four bolt.
Fitting spayed steel caps to a two bolt block makes them stronger than a factory four bolt.
The biggest problem with all these blocks is that when you start making a lot of torque, they start to shuffle the caps on the block, and they become loose in the register. A spun main follows shortly after.
Blocks with two bolt mains have this happen a little bit earlier than 4 bolt blocks. I reckon a two bolt block is good for past 550 bhp as it is, if the mains are within spec (by the way, they NEVER are unless you line hone them.)
If you want to make a ton of power, we have found that two bolt blocks fitted with splayed 4 bolt caps DOWELLED to the block are the best solution. The size of the fastener makes little difference. It needs locating better.
Incidentally I have sonic tested a large range of BBC blocks over the years & early to mid seventies two bolters are generally the thickest. I have often seen in excess of .500" wall thickness on these. This is more than a Bow tie block, but sadly, they aren't siamesed bore blocks like the Bow ties. Bear in mind though, the sample is still only a fraction of a percentage of the total production run, so nothing is guaranteed.
Good luck
Hi; the above statement is not true.
BB Chevy's have the same main webs & bulkheads whether they are two or four bolt.
Fitting spayed steel caps to a two bolt block makes them stronger than a factory four bolt.
The biggest problem with all these blocks is that when you start making a lot of torque, they start to shuffle the caps on the block, and they become loose in the register. A spun main follows shortly after.
Blocks with two bolt mains have this happen a little bit earlier than 4 bolt blocks. I reckon a two bolt block is good for past 550 bhp as it is, if the mains are within spec (by the way, they NEVER are unless you line hone them.)
If you want to make a ton of power, we have found that two bolt blocks fitted with splayed 4 bolt caps DOWELLED to the block are the best solution. The size of the fastener makes little difference. It needs locating better.
Incidentally I have sonic tested a large range of BBC blocks over the years & early to mid seventies two bolters are generally the thickest. I have often seen in excess of .500" wall thickness on these. This is more than a Bow tie block, but sadly, they aren't siamesed bore blocks like the Bow ties. Bear in mind though, the sample is still only a fraction of a percentage of the total production run, so nothing is guaranteed.
Good luck
v8 jago said:
Thanks for all the answers and advice.
To get it splayed, What does actually it involve ?? Ive neer had a chevy before so it is new to me.
Ive not yet looked but ive been told that my new bottom end has arp right through. So it might have arp cap studs, The splayed conversion do they add 2 extra bolts at a splayed angle making it a 4 bolt? And what sort of price would i expect to pay to get it done and can anyone recomend a good reliable place to do it for me. Thanks Adam.
I have four bolted a two bolt block before, simply by obtaining some four bolt caps and getting the bloc drilled and tapped to suit, then line boring after to get it all, er, lined up again.To get it splayed, What does actually it involve ?? Ive neer had a chevy before so it is new to me.
Ive not yet looked but ive been told that my new bottom end has arp right through. So it might have arp cap studs, The splayed conversion do they add 2 extra bolts at a splayed angle making it a 4 bolt? And what sort of price would i expect to pay to get it done and can anyone recomend a good reliable place to do it for me. Thanks Adam.
A splayed four bolt is stronger, as the thread goes into a more meaty part of the block.
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