The 4 bolt main LIE !!! Or is it???

[Strick]

Everyone gets excited to pull the pan on a 3970010 & see the four-bolt caps in the center.
Having said, the teenage version of me was helping my bud build a 350 with 041 heads & Melling cam. This was Christmas of 1990 & I knew about as much about engines than I did p$ssy...I did like both though!
My buddy's dad was a fella that had worked in garages & salvage yards his entire life & he really knew what made an engine "do it" & knew how to make it "do it better".
We worked on the bottom end as Earl came over with a tall boy Clydesdale & made certain we got the bearing tabs & ring gaps right. He made certain that we were going to deliver a healthy SBC.
As we got the bottom end finished with all the amenities & started to rotate the assembly over to install the heads, Earl Jr remarked how he wished he had a 4-bolt main block to build.
Earl, who had more than a few tall boys retorted "dammit boy, I built a 302 at the track one weekend & nobody knew it would bust off...they didn't know & really I didn't know but that 2-bolt engine busted even Smokey Yunick's ass that weekend, so don't think your 300 horse engine is gonna snatch a 2-bolt journal turning 6 Grand"! From that day forward I've never been afraid to build an engine regardless if it was 2 or 4 bolt mains.
Wharn the older folk leave us maybe they do so with great recounts just like this.

Have Stories

 

[idahovette]

@Strick yeah, I gotta few.........if I could only remember them

 

[Vbb199]

My 2 cents, a 4/6 bolt only is justified if you can produce the cylinder pressure for it. If its <300hp n/a.... or high compression n/a (couldn't be much more than 5-600 n/a in a bbc) or nitrous, I wouldnt worry about sheit. It cant much hurt itself if balanced



Supercharged or anything else forced air type, definitely 4 or 6 bolts. Youre packing those cylinders more dense ever.

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But
Assuming your rods, head studs, pistons, crank, and 69 bolt mains are good

Then youre standing on the blocks strength lol

 

[Bennyt]

It was never about the number of bolts in SBC/ BBC, it was main cap surface area that was important to GM. When the stroke increased as the 350 was introduced the bigger and heavier rotating mass increased the instance of cap walk. So yes, a 4 bolt is technically weaker than a 2 bolt as it has less metal in the block, but the 4 bolt has more cap surface area. On factory long arm motors like the 400 and 454, both available as 2 or 4 bolt, they have larger registers.

Take a skinny guy and a fat guy and have them run in a circle around a pole holding on to it. The fat guy runs a lot slower but the pole moves around a lot more. RPM or HP wasn't the factor, mass is.

 

[Blue Ox]

One thing I can say with certainty is the four-bolt mains were the undoing of the 6.2 and 6.5 Liter diesel blocks.

Everyone thinks the cranks were weak, but the cranks failed because the mains in the block would crack from the outer main bolts, right up to the cam bore. When mine failed all three bulkheads were broken. Some of them you could move by hand. No crank can survive that.

GM later lengthened the bolts, and ultimately reduced the diameter of the outer bolts to mitigate the problem. While the bolts revealed the weakness, ultimately if they had made the block out of better material and/or made it an inch or so longer there would have been enough material to support it. In the end, by using smaller bolts they kind of acknowledged that a two-bolt probably would have been adequate and the extra perforations that weakened the block could have been avoided.

 

[HotRodPC]

Interesting take. I've heard all my life that what ya want is a 4-bolt main block but never looked into it more than that. I never even knew if the bolts were splayed or not. Splaying a 2-bolt block definitely makes more sense.

Just FYI, all LS engines are 6-bolt mains cuz of the deep skirted block. 4 straight down and 2 cross bolted through the skirt

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Yes, I personally feel they got it right with LS blocks. Not only 6 bolt mains but the way the main caps just complete the block and go all the way across and flush with the oil pan surface. Which is why they hold up so well. You can take a stock junkyard LS engine and start makeing your changes to the top end to make tons of power and not even have to worry about the bottom end. Only things missing is provision for a distributor and fuel pump. Even that later SBC and BBC didn't have provisions for fuel pumps anticipating TBI and SFI.

 

[HotRodPC]

It was never about the number of bolts in SBC/ BBC, it was main cap surface area that was important to GM. When the stroke increased as the 350 was introduced the bigger and heavier rotating mass increased the instance of cap walk. So yes, a 4 bolt is technically weaker than a 2 bolt as it has less metal in the block, but the 4 bolt has more cap surface area. On factory long arm motors like the 400 and 454, both available as 2 or 4 bolt, they have larger registers.

Take a skinny guy and a fat guy and have them run in a circle around a pole holding on to it. The fat guy runs a lot slower but the pole moves around a lot more. RPM or HP wasn't the factor, mass is.

Tis True !!! That's why small blocks love love love RPM and Big Block not so much. Take Olds for example, one of the best engine for all low end torque. There's so many things you have to do to the bottom end, like girdle the mains, lighten throws on the crank, lighten the pistons etc, and even then they're safe for about 6000 rpm with all those mods. Yep, sure they can pushed a bit more but you're getting risky. That heavy mass will for sure get loose and it all comes apart. Making a BB Olds go fast is NOT a challenge. Making is stay together is the challenge.

 

[82SquareBurb]

Agreed. I am building a forced induction 355 with a 2-bolt. The only thing I would recommend if pushing the "limits" is to pin the caps to lessen the amount of cap walk it may see. In my experience this does help the bearings live a bit longer. Less expensive than redoing the block with billet caps and splayed bolts.

 

[Fat 454]

From the BBC world of just "havin' fun with my truck" - I have found it increasingly difficult to find meaningful wisdom around engine building these days. There is an oversupply of opinion, and its getting harder to draw out the useful facts. No shortage of YT gurus etc. selling one story or another.
Given that I'm not building a race car, and that I am clear in my goals ( longevity, " fixability", daily driveability whilst unlocking the potential of the motor - even tho' I wont use it every day ) - my recipe for success appears to be a GEN VI roller cam BBC, oval port heads and then playing with a mix of fuel systems ( TBI and spreadbore carbs - no multiport - snipers etc. - refer goals above ).
I want a truck that is fun to drive, but its not a race car - so torque from 1500 - 4000 is the target. My son is into LS motors - huge HP potential, but power band comes in above 3K and up. No fun on the public roads.
The more I research, the less I know ( "old guy" wisdom ?? ) - cam selection and operation is fascinating. At the end of the day the proof to all this is in the doing - liveable fuel economy, serviceability and fun in a daily driver. As the build progresses, and we see some real world results I may post that up for anyone interested, as there seems to be very little "real world - long term driving/ ownership" info out there for DD classics?
PS - and the original point for this post ( we never get sidetracked - right ) - If you want to splay bolt a BBC you need a GEN VI, as the Mark IV had the main oil galley in the skirt. The GEN V / VI moved the galley up adjacent to the cam bearings out of the way. GEN V blocks appear to be half assed from what I can find out about them, however the GEN IV ( or GEN "fix") sorted alot of that out.
Finally - although block cooling and deck passage castings differ between IV and VI, apparently you can use any head on the later blocks, meaning there are more choices for intakes that match etc. ( although some "tweaking" may be required ).
Bottom line is either do your research, or find someone that has ( and probably pay them a bucket load for guaranteed results ) - but where is the fun in that ??....

 

[Wumbo]

From the BBC world of just "havin' fun with my truck" - I have found it increasingly difficult to find meaningful wisdom around engine building these days. There is an oversupply of opinion, and its getting harder to draw out the useful facts. No shortage of YT gurus etc. selling one story or another.
Given that I'm not building a race car, and that I am clear in my goals ( longevity, " fixability", daily driveability whilst unlocking the potential of the motor - even tho' I wont use it every day ) - my recipe for success appears to be a GEN VI roller cam BBC, oval port heads and then playing with a mix of fuel systems ( TBI and spreadbore carbs - no multiport - snipers etc. - refer goals above ).
I want a truck that is fun to drive, but its not a race car - so torque from 1500 - 4000 is the target. My son is into LS motors - huge HP potential, but power band comes in above 3K and up. No fun on the public roads.
The more I research, the less I know ( "old guy" wisdom ?? ) - cam selection and operation is fascinating. At the end of the day the proof to all this is in the doing - liveable fuel economy, serviceability and fun in a daily driver. As the build progresses, and we see some real world results I may post that up for anyone interested, as there seems to be very little "real world - long term driving/ ownership" info out there for DD classics?
PS - and the original point for this post ( we never get sidetracked - right ) - If you want to splay bolt a BBC you need a GEN VI, as the Mark IV had the main oil galley in the skirt. The GEN V / VI moved the galley up adjacent to the cam bearings out of the way. GEN V blocks appear to be half assed from what I can find out about them, however the GEN IV ( or GEN "fix") sorted alot of that out.
Finally - although block cooling and deck passage castings differ between IV and VI, apparently you can use any head on the later blocks, meaning there are more choices for intakes that match etc. ( although some "tweaking" may be required ).
Bottom line is either do your research, or find someone that has ( and probably pay them a bucket load for guaranteed results ) - but where is the fun in that ??....

All Gen 5 and Gen 6 BBC blocks are factory 4 bolt, with parallel drilled bolts. The only way to build a splayed cap BBC is to use a MK4 2 bolt, or buy an aftermarket block.

You're absolutely correct on the oil galley. My machinist drilled right into them. His approach to this was to chuck up 3 studs in a lathe and cut them down so they would bottom in the galley, but let oil flow around them. I didn't care much for that idea, so I made plugs that bottomed in the galley, then drilled through them from the side. It's hard to explain.

Next time I'll just buy a Dart Big M block. The machine shop costs, and material cost for me to do this was only about $1K less than a new Big M.

As for the 2 bolt vs 4 bolt argument, I'm in the splayed cap fanclub.

 

[Ricko1966]

Agreed. I am building a forced induction 355 with a 2-bolt. The only thing I would recommend if pushing the "limits" is to pin the caps to lessen the amount of cap walk it may see. In my experience this does help the bearings live a bit longer. Less expensive than redoing the block with billet caps and splayed bolts.

Use studs and shuffle pins.

 

[xm20k]

I've noticed a few trends in the car influencer space in recent years. Probably why I can't watch most of it. It seems anything old school is either not understood or isn't trendy so it's crap or doesn't work. Also, your nothing unless your making 4 digits on the street. Ever notice the amount of turbo builds especially turbo LS builds.

2 bolts aren't worth building.

Big cams only make power up top. As if they do nothing under the stated RPM range. They make way more power off idle than the same stock engine.

They will say stall converters are not drivable on the street. Funny my 4K converter will idle down the road at 1K+ it drives normal from just off idle to red line it doesn't just sit there and do nothing unless I force it to by foot braking or holding the trans brake. Ya it'll flash/slip if you step into it hard enough but normal cruising it drives fine.

I also run a tunnel ram on the street, and everyone now a days says that doesn't work either, but people used to run them on street cars all the time. Guess what they make power everywhere, but some will also say they only make power up top (people forget what long runners do, I guess) I think this is from people putting race carbs on with no chokes and mechanical secondaries and then wondering why they can't just cruse it around as a daily.

My favorite was the dude that told me my truck was too heavy for a big cam and stall converter, yet my truck has a curb weight of 3600, his Challengers curb weight is 4300 at the same power level.

I don't think I've posted this because this is a pair of 750s on 110 from 4k (my stall/2 step launch RPM), not my street setup with the pair of 500 AVS carbs with 1 manual choke on 93.

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[Bennyt]

Sidetracking a bit..

The machine shop i worked at probably shipped 10-20 complete performance motors a day, 5 days a week for 20 years. Probably another 20 reciprocating assemblies daily as well. It was amazing how few failures we actually experienced. Unless a customer upgraded to ARP or Milodon bolts, rod bolts were reused with new nuts. 100's of rods a day resized and failure rate was almost non-existent. No white room, lab coats or really any quality control to speak of. 40-50 guys that just did their job. 2 bolt, 4 bolt, really doesn't matter unless winning/ losing is your paycheck.

 

[Vbb199]

From the BBC world of just "havin' fun with my truck" - I have found it increasingly difficult to find meaningful wisdom around engine building these days. There is an oversupply of opinion, and its getting harder to draw out the useful facts. No shortage of YT gurus etc. selling one story or another.
Given that I'm not building a race car, and that I am clear in my goals ( longevity, " fixability", daily driveability whilst unlocking the potential of the motor - even tho' I wont use it every day ) - my recipe for success appears to be a GEN VI roller cam BBC, oval port heads and then playing with a mix of fuel systems ( TBI and spreadbore carbs - no multiport - snipers etc. - refer goals above ).
I want a truck that is fun to drive, but its not a race car - so torque from 1500 - 4000 is the target. My son is into LS motors - huge HP potential, but power band comes in above 3K and up. No fun on the public roads.
The more I research, the less I know ( "old guy" wisdom ?? ) - cam selection and operation is fascinating. At the end of the day the proof to all this is in the doing - liveable fuel economy, serviceability and fun in a daily driver. As the build progresses, and we see some real world results I may post that up for anyone interested, as there seems to be very little "real world - long term driving/ ownership" info out there for DD classics?
PS - and the original point for this post ( we never get sidetracked - right ) - If you want to splay bolt a BBC you need a GEN VI, as the Mark IV had the main oil galley in the skirt. The GEN V / VI moved the galley up adjacent to the cam bearings out of the way. GEN V blocks appear to be half assed from what I can find out about them, however the GEN IV ( or GEN "fix") sorted alot of that out.
Finally - although block cooling and deck passage castings differ between IV and VI, apparently you can use any head on the later blocks, meaning there are more choices for intakes that match etc. ( although some "tweaking" may be required ).
Bottom line is either do your research, or find someone that has ( and probably pay them a bucket load for guaranteed results ) - but where is the fun in that ??....

Was some of this directed at me? Hehe
Ive had big blocks, small blocks, and LS. Enough variety to know what i like haha

 

[Rustisbest]

From the BBC world of just "havin' fun with my truck" - I have found it increasingly difficult to find meaningful wisdom around engine building these days. There is an oversupply of opinion, and its getting harder to draw out the useful facts. No shortage of YT gurus etc. selling one story or another.

Yeah it's like that with everything, not just engine building or cars/trucks even.
Then there's also the "experts" that give you advice on what they Googled or the "I'm gonna do this or that one day" guys that spend their time telling you how you're doing everything wrong instead of working on that project they've been telling everybody about for years