What's the difference between a 2 bolt main and 4 bolt main?

 

Exactly like it sounds. 2 bolt main blocks have 2 bolts holding the main caps on, 4 bolt mains have 4. 4 bolts are of course a lot stronger, and take more abuse.

 

bolt are bolting up to the transmission or motor mounts or where?

 

The main bearing caps, inside the block.

 

If you still don't understand, jump on google images and look up 2 bolt mains and 4 bolt mains. The difference is pretty obvious. The main caps are what hold your crankshaft in place inside the block, and having 4 bolts on each cap makes it a lot stronger than having 2 bolts per cap. Like they say, a picture is worth a thousand words.

 

On an SBC, there are 5 main caps, front and rear, then 3 in the middle (correct me if i'm wrong on this guys, it has been a while).
The front and back are always 4 bolts. It's the middle 3 that can be either 2 bolt (straight), or 4 bolt (straight), or you can take a factory 2 bolt, and install aftermarket "splayed" 4 bolt caps. This requires a machine shop to machine. This of course is the "strongest".

Keep in mind, adding strength to the main caps is generally a waste of time for "normal" applications. Try to find someone who has failed a middle main cap due to lack of clamping load. I doubt you'll find it. The only application where the 2 additional bolts add some value is long term heavy loading. ie, truck pulling a 5th wheel up a hill, boating, etc. 3rd gen Camaro's wouldn't see much value there.

 

Sonix, you are spot-on. The front and rear are not always 4-bolt though. It is very common for all caps to have just 2 bolts.

 

Also, IIRC, on the 4 bolt mains the front and rear are still just a 2 bolt configuration, while the 3 center caps have 4 bolts.

 

I agree. My 70s truck 4 bolt 350 block is 2 front 2 back with 4 bolt centers. Like stated google has plenty of images.

 

Its been a couple of years since I've been into a chevy block, but I think everything I said was accurate information. Research would tell for sure, but I'm American and lazy and not going to do any

 

I've always heard 4 bolt main this and that and while looking up crate engines I found one priced (what seems to be) right but it said 2 bolt mains. Eventually I want the 6 speed to replace the 5 speed and it's been said on here to have a 4 bolt main for that or you'd have to get some kinda kit.

 

I think you may have either gotten your info confused or just got bad info.. If you want to go to a 6spd trans and have an older block you will need a special flywheel for that because the older blocks have a 2 piece rear main seal and the newer engines that you find 6spds on have a 1 piece rear main seal. This has nothing to do with the 2 bolt or 4 bolt main cap discussion. the trans doesnt care how many main bolts are on the caps. It only cares about the the flywheel mouting location on the back of the crank which is different on 1 piece rear main seal cranks as opposed to 2 piece units. I hope this helps and didnt confuse you to much.

 

That's right!!! I'm thinking (2) for the main.

 

So to put a lightly built 350 in our cars two or four bolts really doesn't matter then? Basically, if it was going to be built for major dragging or pulling then the 4 bolt is important because it'll hold the internals better?

 

Exactly...

 

4-bolt mains are stronger and less likely to experience cap-walk than 2-bolt mains.

4-bolt main blocks are weaker than 2-bolt main blocks.

While Sonix was wrong about the number of bolts for the front and rear main caps (there are only 2 in the front and rear main caps, even in a 4-bolt main block), he is right that a 2-bolt main engine will be just fine for your typical street performance build. If you're building an engine for towing in a truck, then you might want to look for a 4-bolt main.

If you're building an engine for more than street power, then get a 2-bolt block and get splayed 4-bolt caps for it. This will require getting a machine shop involved (but that's a good assumption, anyway), but the splayed bolts put the bolt holes in the part of the block webbing with more meat, avoiding the weakening that factory 4-bolt mains caused. Even better is an aftermarket performance block with splayed 4-bolt main caps.

But, that's more info than you can probably digest. Suffice it to say that a 2-bolt main will be fine for a street performance, or even street/strip, build.

 

Damn, I knew the front and rear caps were "different" then the other ones, I just couldn't remember why. The rear caps have the oil pump hanging off of it, so I guess I just remembered wrong