Is my numbers matching 71, 4 speed, base engine, code CJL a 2 or 4 bolt main?

 

What are the block casting numbers

 

Block 3970010, heads 3973487

 

Absolutely no way to tell without looking at the mains, that block was made in both configurations. Try a borescope next oil change.

 

If your not building over 500HP it doesnt matter unless your just looking for braging rights. My 327 is over 400hp at the crank and its fine

 

Base engines are generally 2 bolt. But if a 4 bolt got mixed in, or a batch of 4 bolts cuz the 2 bolts weren't available at the moment, they would be used in base engines. Uncommon but did happen. Odds are it is a 2 bolt though. OTOH, if low on 4 bolts, they would not build LT1 or other SHP small blocks.

 

The whole 4 bolt thing is one of the overblown concerns of the musclecar era...........it makes very little difference especially if the fasteners are replaced with 8740 ARP bolts or studs during a block prep.
Not sure how it all started....but it must of had something to do with somebody years ago pulling a pan on an LT-1 and realizing it had 4 bolts holding the cap on rather than two. Thing that cracked me up since I was a kid in the 80's was very few people knew how to make a small block run to the point where a 4 bolt would even be a consideration of strength due to the fact that a real cylinder head was not produced for a small block til about 1982 in the Bowtie Catalog........guys who tell you they made 500 horsepower at 8000rpm are full of it unless they did countless hours of oil passage work and a thousand hours of cylinder head prep. A stock small block is limited to a sustained 6,500 rpm without oil mods.
It is in fact how I know that a person knows next to nothing about small blocks when they start spouting off how important 4 bolt mains are.......it also will do next to nothing for the value of your Vette unless sold to one of these people.
In 07' I dyno'd a 383 with Brodix heads and a solid roller cam that made 560 horsepower at 6,600 rpm with a studded 2 bolt block......on a 125 shot the 3700 pound Cutlass it was in ran 10.90's......the car had 50 runs on it before I lost touch with the guy......zero issues.

Jebby

 

The base V8 engine is supposed to be a 2 bolt main. 4 bolt mains were used on the optional high performance engines and some truck applications where the loads were more demanding. Steel cranks, forged pistons, larger harmonic dampers and other such heavy duty parts were usually specified along with 4 bolt mains.

 

The 69 I got with a base 350 had a 4-bolt in it. T

 

Anything over 500HP use a 4 bolt...

 

Everything Jebby states is correct. I regularly build engines over 500 hp on 2-bolt mains with no issues (I use good ARP studs). I share his observations and experiences 100%.

Lars

520 hp small block being assembled with 2-bolt mains - this engine is shifted at 6500 rpm:

 

Block 3970010 was the high nickel good block. I did some research and odds are that it is a 4 bolt block. I don't know for sure if all vette motors after 1968 came with 4 bolt mains. I owned a 68 with the 4 bolt large journal 327. I did come across a statement that makes sense. "one sure sign of a chevy small block being a 4 bolt is if it came with the larger 8 inch dia. damper" anything with the smaller lighter 7 inch was just a 2 bolt.

 

The "holy grail" is the 4-bolt with nodular main caps, easily recognized by the numbers "2482" cast into the center 3 caps.

 

Somewhere I still have the drill bits that are about 3 feet long. And you are right about what we called the turbo or angle plug heads being light years better. But I don't think I had a thousand hours in the old heads, but I think I still can taste the steel grindings in my mouth🤪

 

The high nickel block said 010 with 020 beneath it by the timing cover. And they were usually 4 bolts.

 

So why did GM engineers use 4-bolt blocks when nearly all hipo engines back then were not making anywhere near 500 hp? They had a good reason or they wouldn't have spent the extra money for the extra bolts.

 

Yes! The 3 foot long bits! Even when GM started making the “BowTie” block, they left the shitty little oil feed passages everywhere.

Jebby

 

There have been a few GM foundry workers who claimed that the 010 020 under the timing cover is BS. Wives tale is that these blocks contained .1% more tin and .2% more nickel. Tin makes 'em easier to machine and nickel makes 'em harder.

There's not a factory block that will hold a candle to an aftermarket block, including Bowties, when it comes to hardness. Ask any machinist - particularly the ones that bore and hone blocks. It's a completely different cast iron recipe and the ingredients are a small part of why these blocks are more expensive.

 

There is no good reason.....you will run out of oil before you move the main webs around....even with a 4 bolt...
It IS stronger....but it doesn’t matter if you do not drill every oil passage and spin it past 7k regularly....
A better question is why did they design an intake (LT-1) that could support 475 horsepower but yet waited til 1982 to release a head that was worth beans? Even then you could only buy it over the counter....
There is a fella running Top Stock in a 67’ Chevelle 283 that sings to 8k and runs 11’s....2 bolt main!


Jebby

 

At the time, guessing the engineers saw it as an advantage. It was my understanding they believed the truck motors would be more stable under lugging conditions. Since then, it's been found that a 2-bolt is just as stable by the circle trackers and others, and 2-bolt drilled for splayed main caps arguably stronger.