4 bolt or 2 bolt mains

[G Winter]

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Generally, a four-bolt main bearing block is used in high-performance applications. For example, After 1967, typically, when power got above 340hp gross or 290hp net, GM used four-bolt mains in small blocks.

First , I highly respect your knowlage of all things related to cars. But I have to question you on the hp in which GM used the 4 bolt mains. Unless I missunderstand the hp ratings. In the late 60s 300 hp rated 350s had 4 bolt mains. Am I ncorrect in assuming this rating was gross hp ? This has nothing to do with cars but I belive most 350s in the pickups were 4 bolt mains.

 

[Ken]

They say pictures are worth a lot of words ...

2-bolt mains: (I'm using longer studs for pan accessories.)

4-bolt mains:

12-bolt third member:

 

[Achilles]

I don't know what the horsepower ratings were for passenger cars in the 70's, but I had a stock 1970 Monte Carlo w/350 cid that had for bolt mains. I replaced that motor with a GM crate motor (I believe it was called a Target Master). A friend of mine had a Camaro (early 70's), and he wanted the old 4 bolt motor I pulled to replace his 305 cid w/ 2 bolt mains.

 

[geekinavette]

2/4-bolt mains refers to the number of bolts that hold down the crankshaft main caps.

This is a 2-bolt late-model ('92) truck block. If the crankshaft weren't in place you'd see a series of "U-shaped" cuts in the block, the main caps are also "U-shaped"...well half-circle really, when the main caps are bolted on the two "U's" come together to make a circle, the center of that circle is the center of rotation of the crankshaft. The first, second, and third (from the front, or right) main caps are clearly visible in this picture, and have only one bolt on either side of the main cap...a 4-bolt block would have two bolts on each side (except for the first and last caps, they're always two).

This doesn't apply to the LS-series engines ('97-up)...that's a whole different animal.

Bill

 

[schmaltzr]

I'm no expert, but I'll let you know what I know. When refering to 2 or 4 bolt mains, they are refering to the caps that hold the crank shaft in place on the engine block. As horsepower is increased, the thrust on the crank shaft can be tremendous and more bolts are needed to keep the crank from comming loose from the block.

The rear end bolt counts are a little more confusing, but a typical GM rear end for a 1/2 ton truck usually is a 10 bolt. It is the number of bolts on the housing holding the cover in place. The 3/4 ton truck get a 12 bolt differential. Usually becuase the rear end is larger to be more durable there is a need for more bolts to hold the cover in place. In this case the number of bolts coincidentally mean more strength, but the bolts have nothing directly related to that fact. Ford has a 9 bolt rear end that is supposidly very strong and it matters on what years, etc you are talking about as far as durability. Being a GM guy, I don't keep up on the Ford stats all that much. If anybody else has some corrections to my opininion, please educate me, I'm always willing to learn. - Ron

 

[Ken]

Ford's rear end is referred to as a 9-inch, in reference to size of the ring gear. GM's is 8.875".

 

[Hib Halverson]

Hib...I used to think that the 12 bolt rear end was a posi, or limited slip posi rear end and the 10 bolt was just a single drive unit. My reason for thinking this is when I blew out the stock rear in my 69 GTO, I replaced it with a 12 bolt out of a Chevelle that had a lower gear and a posi. Am I correct or misinformed?



John

The 12-bolt and 10-bolt axles could be ordered with a limited slip. Most 10-bolts probably didn't have them but most 12-bolts probably did. The 12-bolt axle out of a Chevelle would bolt into a 69 GTO. 12-bolt passenger car axles were available from the factory with ratios from 2.29 to 4.56. The aftermarket has ratios up to 5.13, I believe.

 

[Hib Halverson]

First , I highly respect your knowlage of all things related to cars. But I have to question you on the hp in which GM used the 4 bolt mains. Unless I missunderstand the hp ratings. In the late 60s 300 hp rated 350s had 4 bolt mains. Am I ncorrect in assuming this rating was gross hp ? This has nothing to do with cars but I belive most 350s in the pickups were 4 bolt mains.

The 350 was not introduced in a Corvette until 1969. I don't believe the 300hp version had four-bolt mains. As far as I know, the only 350s in Vettes of that vintage to have them were LT1s and they didn't come along until 1970. As far as I know the only SBV8 to have four-bolt mains prior to 1970 were the 68 and 69 302s.

 

[Ken]

Gear ratios for the 12-bolt are available from West Coast Differentials, ranging all the way up to 6.14:1, although all gear set from 5.14 on up require a notched cross pin shaft.

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Most rear end housings can be identified by carefully observing the bolt pattern on the rear end cover gasket. This chart should help you to identify some of the most common rear ends: Rear-End Identification.​

If Hib would come out of his shell and share with us us all his vast knowledge of differentials and axles, I wouldn't steer you to Marlan Davis' article from Car Craft magazine: 12-Bolt Chevy vs. 9-inch Ford: Which is Better? :L

 

[nmacdonald]

Thanks to everyone for the wealth of information. I have a much better understanding of the whys and wherefors of this whole subject.

And a big thank you to those who posted pictures to help understand. That was great. And the links to other sites.... wow!!!!

Thanks again and HAPPY HOLIDAYS to all.