Are 4 bolt main blocks stronger than 2 bolt main blocks?

Lets start with what a 2 bolt and a 4 bolt main block is anyway. What holds your crankshaft in place are the main caps. These are what the crank bearings sit in. Most engines have 5 main caps.

2 bolt main caps are fastened to the block with 2 bolts (one on each side), and a 4 bolt cap is held in with 4 bolts (2 on each side) so there is "about" double the holding force keeping that 4 bolt main cap fastened to the block and keeping the main bearings securely in place as compared to a 2 bolt, which in turn makes it stronger and more suited for performance and race use. This is especially true for boosted or nitrous engines which put a tremendous amount of downward pressure and force on the crank... and what's holding it in place? The main caps and bolts keeping them tightly fastened to the block. The block in this picture is a stock big block Chevy 4 bolt main block.

And the block in this picture is a 2 bolt main as you can see by only 2 bolts holding the main caps on. 

Main caps are the only thing keeping your crank from falling out the bottom of your engine. When you increase the load on the crank, you inherently increase the load on the main caps. Any time you increase horsepower, torque and cylinder pressure, you increase crankshaft load. It would take quite a bit more stress to rip-out 4 bolts than it would just 2, so that's why the 4 bolt mains are stronger than the 2 bolt mains. This is the theory behind it anyway. I have to say though... I have never seen a crank get "blown-out" of an engine, (other than something like a nitro engine), be it a 2 bolt or a 4 bolt main engine. I have seen engines "blow-up", leaving pieces of crank and engine block all over the track, but that's usually due to metal fatigue and failure somewhere else and not the main caps themselves blowing-off.

I have also seen 2 bolt main engines in low budget race cars running 9 and 10 second ET's that last season after season. The more compression & cylinder pressure an engine has, the more the crank is trying to be pushed out the bottom of the block, especially when a blower or nitrous is used, so you want to make sure it is held in good and strong, and 4 bolts hold stronger than 2 bolts (in most cases).

Sometimes the outer bolts on race blocks are splayed, which means they don't go straight in. Instead they angle outward so the main cap has 2 different angles of clamping force with the 2 big ones in the center and the two smaller ones on the outer edges going out at angles. It's a bit hard to tell but if you look close at this picture of a Dart Sportsman race block, you can see the outer bolts on the center 3 caps don't go straight into the block and are a bit angled outward. This adds to the "pulling" strength of the bolts, or at least that's the theory anyway. It's debatable whether splayed bolt caps are actually stronger or more reliable under harsh conditions than straight bolt caps are. I made the image enlargeable so if you click on it you can get a little better look at the outer splayed main cap bolts. 

Sometimes you can't get a block with 4 bolt mains though, so a way to make a 2 bolt main block even stronger than a typical 4 bolt block is to use a "main support". This is a brace that bolts across all 5 main caps, thus tying them all together and sort of creating a 10 bolt main engine. The theory behind this is simple, if one main cap tries to walk, move, flex or come-off, it has to take all of the others with it at the same time because they are all inherently attached to each other by that main support. It disburses the load across all of the main caps evenly. Again, one can't move or come out without taking the other 4 with it because the brace ties them all together. It's the same theory behind a stud girdle that hard core race engines use on rocker studs to prevent individual studs from flexing, breaking or pulling out. You can see what a typical billet main support looks like in this image. These are common with Ford engines, especially when building big strokers so that extra weight and leverage of that big stroker crankshaft doesn't rip and pull at all of the main caps causing main cap walk, bearing failure, cracked main webs... or worse.

If you are going to be building a street engine that has 11:1 compression or less, then a 2 bolt main block would be just fine. Anything higher than 11:1 or so, or if you are going to run nitrous or a blower, then I would definitely recommend a 4 bolt block, or a 2 bolt block with a main support on it just to be safe.

So for a basic performance engine making 400 HP or so, do you really need to build it on a 4 bolt main block? The answer is; not really. Think of it like this, ALL of the early Corvette 327's with 365 and 375 HP were all 2 bolt main blocks. There was no such thing as a 4 bolt main 327 back then. How about the infamous 68 "MO" 302 Z/28's which were famous for 8,000 RPM shifts in stock trim! They came stock with 11.25:1 compression and were indeed high winders that saw race track use quite often, and they were also 2 bolt main blocks!

How about the famous Shelby GT-350's? Well ALL of those were 2 bolt main "K code" 289's that were worked-over by Shelby Motors to make 306 HP for the S model cars and 360 HP for the R models who routinely saw 6,000 - 7,000 RPM use which were all 2 bolt main blocks. Now ask yourself, how many Vettes, early Z/28's or Shelby's have you ever seen on the side of the road with the crank blown out? How many vintage races or SCCA events have you seen any of those cars blow a crankshaft out onto the raceway? I'm not saying you should go out and build a race engine using a 2 bolt main block, but I want you to understand "reality". If you're building an engine that is going to be making 400 HP or so in a small block, or upwards of 450 or so in a big block, then a 2 bolt main block will be just fine if it's set-up well, especially if it's a short stroke engine. Big strokers are another story... if you are wanting a Chevy 383 stroker, I would suggest using a 4 bolt main block simply because of the longer stroke.

Now, the 351W Ford's on the other hand are only 2 bolt mains BUT I would venture to say that they are stronger than a 4 bolt main small block Chevy. Why? Because the main caps are about 50% ticker than a Chevy's, and the 2 bolts they have are huge 1/2" bolts, not little 7/16" bolts like the Chevy's use. The engine in the image with the main support is one of the common 408 cubic inch strokers I used to build for people for their Mustangs, Cobras, etc based on a 351W block and fitted with a huge 4.000" stroke crank. In MILD trim with decent heads and a small hydraulic roller cam they'll typically make in the 520 - 530 HP area and a whopping 540 - 565 Ft Lbs of torque, which is why you really need to run a main support on something like that if a 4 bolt block isn't available.

So as always, when dealing with subjects like this, you have to compare apples to apples and take everything into consideration, not just what's on the surface. 

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