Folks

Was chatting to my partner in crime, who is helping me with Daisys rustoration, and he asked weather i was going to have 4 bolt mains installed vs the standard 2 bolt mains.

Since the motor is expected to have some reasonable get up 'n' go, he suggested that I have that done.

Is it worth the cash to have that done?? Can an a series really put out enough power to make the strength in the mains bearings a concern??

not neeeded, v8s share two rods with each main and at high rpm they usually need them but the trusty a series is solid as a rock with the right balancing etc.

ahhh righto

since there is one rod per main bearing in the a series, its more than strong enough, even for a crazy as blown off its nana donk.

The V8 shares each main bearing with two rods, so they only have 4 main bearings in the block??

Is it the torque or the HP that means you need 4 bolt mains??

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Well some engines came stock with 4 bolt mains.

It stops the crank flexing.

Yeah I was aware of that. Some of the older chevy stuff comes with 4 bolt mains.

I guess the question is answered, an a series cannot put out enough hp/torque to warrent 4 bolt mains mods, so thats one less expense that i need to worry about.

Is it safe to assume then, that it is the torque that means a motor needs the extra support for the crank?

Neither ... a weak design needs more. The A-series block is beefy enough to start with. Crank flex is mostly with RPM, not torque, and the stock A block can handle 8,000 rpm.

if u think about it the force on the crank is on the compression stroke meaning its the conrod likely to give, the smaller force on the end caps main bolts is on the piston upstroke. Even the A10 can rev hard and it only has 3 main bearings.
Having saaid that i think engines with 4 bolt mains feel better than those with 2, nissan VG30 or Mitsubishi 6G72 V6's feel heaps smoother and stronger than commodores old buick 3.8

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i think that comparison would still be the same regardless of the 4 bolt mains

there would also be a fair bit of inersial (speling??) force on the crank due to it spinning at so many 1000 rpm

DD, you say that its neither the torque or the HP but weak/poor design that means the upgrade is a good idea.

If it is weak design, then surely its either HP/torque combined with RPM that is the culprit.

If the majority of the pressure on the crank is during the power stroke (down after compession) then surley more cyclinders and in a v configuration would mean less stress/flex since there is a power stroke with less degrees of the cycle. (4 cyc is 180, 8cyc is 90), and a v config puts it at an even better angle for even distrubition of stress.


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Comparing two engines across different generations of cars is probally not fair as to smoothness.

Ok, so I am not an expert on this. But my understanding - which may be right or wrong -

Fact: the Chevy V8 was designed in the early 1950s. First was the 4.4 liter version in 1955.

Guess: It was probably designed for about 5,000 rpm tops.

To spin faster with later versions, say as with the 5.0 liter of 1967 that revved to (supposedly) over 6,000 rpm, they added 4-bolt mains to keep the flex of the crankshaft under control.

In no way does this mean the original design was "bad", only that it was not designed for high-rpm usage. Previous to the 4-bolt mains Chevy smallblocks put out more torque but didn't use 4-bolt main.

So I think torque is not the factor, it is RPM.

Wikipedia also states it is related to flex at high RPM: wikipedia: 4-bolt main


Now the Nissan VG v6 was designed from the ground up as a "high" rpm engine (6,000 being high back in the 1980s). So it doesn't need "upgrades" to spin up. NO FAIR to compare 1950's designs to 1980's designs.