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what is the difference between the 350 2bolt and 4 bolt mains. which would one prefer?

Refers to the number of bolts in each main crankshaft bearing cap. 2 bolt blocks had 1 bolt on each side of each main bearing caps. 4 bolt blocks had 2 bolts on each side of each main cap.

4 bolt blocks were used in high perf and some truck applications for high RPM/high HP operation. The 4 bolts holding each crank bearing provides more support for the bearing shell under higher forces encountered above 6,000 RPM. The 4 bolts prevent 'cap walk' which can distort the bearings and cause crank failure.

You only need a 4 bolt block if you are going make over 400 HP, or turn over 6,000 RPM, or use a blower or nitrous or other power adder.

The vast majority of SBC family like the 327s, 350s, 305s, 307s, 283s, 265s, etc had 2 bolt mains, and they held up fine in most applications.

4 bolt blocks are getting hard to find, as most of them have been hot rodded and blown to bits cuz the owner thought the 4 bolt was indestructable.

Some 2 bolt blocks can be drilled, tapped and line-bored by a machine shop to accept Milodon or other aftermarket 4 bolt main caps. Very expensive and often unneccesary for a street vehicle.

good luck

chip

thanks alot chip
was a mystery to me
i can "see" the difference just didn't understand
it. you are a vast wealth of knowledge

Each of the main-caps are not held with 4 bolts, only the center 3 caps. the ends had only 2 bolts. A person could build all the power one can legally use on the street with a well built 2-bolt block

Good point, Mech. Most modern V8s have 5 main bearings. The center 3 bearings are under the most stress, so they got the 4 bolt option.

I think most stock 4 bolt blocks had all 4 bolts going directly down through the cap into the block. Some blocks had "splayed" 4 bolt mains, where the outer two bolts were angled outward into the block. I don't know which provides more strength and stability for the bearing.

There are other bottom-end reinforcement methods for engines, such as using ARP studs instead of bolts in the mains, or using a block girdle that links all the main bearing caps together. Of course the wilder you get, the more it costs.

But the short answer is that a 2 bolt block with new bolts, new rods and a balanced rotating assy will make a bunch of power and live a long time in a street/strip application. But the block is only the starting point. Using well matched, quality parts throughout the engine is the key to success.

good luck

chip

This is true you usually don't need 4Bolt Mains unless you are over 400 HP and even if you are I would invest in conection rods and stronger rod bults first and formost as they are usually first to go South.(How many people do you know that have thrown a rod or spun a rod bearing verses had the crank fall out?) As far as splaid 4bolt caps go they really arn't too expensive but do require being align board at a machine shop (Summitracing.com has them for under$200) they are however much stronger than stock since they are splayed (angled) and lets face it any 4Bolt blocks out there are pricey and has probably been stressed papper thin, which makes a 2bolt block with splayed caps all that much more desirable!

Good info from all. Let me add one more point that Chip touched on. The 4 bolt main is also useful in truck applications where the engine is subjected to severe lugging for prolonged periods of time. I doubt if that is the case here, since this is the Hi-Po forum.