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
