2-Bolt vs 4-Bolt Main Blocks: Differences and Which You Need
A 2-bolt main block holds each crankshaft main cap with two bolts; a 4-bolt main block adds a second pair of bolts on some or all of the caps, which clamps the crankshaft more rigidly for high power and high RPM. For most street engines a sound 2-bolt block is enough. A 4-bolt (or a 6-bolt cross-bolted LS) block earns its place in high-RPM, high-power, boosted or racing engines.
What main caps do
The crankshaft spins in main bearings held between the block’s main webs and removable main caps. The caps are machined with the block, and the bolts clamp them in place against the combustion forces trying to push the crankshaft down and the caps outward. The more rigidly the caps are held, the less they can move (often called cap walk) at high load and RPM, which helps protect the bearings.
2-bolt vs 4-bolt vs 6-bolt mains
| Type | How it is held | Typical use |
|---|---|---|
| 2-bolt main | Two vertical bolts per cap | Most factory V8s, street engines, many spec racing engines |
| 4-bolt main (straight) | Four parallel bolts on some or all caps | Factory performance and truck blocks, stronger street builds |
| 4-bolt main (splayed) | Outer bolts angled outward into thicker parts of the block | Aftermarket blocks and race engines |
| 6-bolt (cross-bolted) | Four vertical bolts plus two side bolts through the block skirt | GM LS and many modern V8s |
On factory small block Chevy four-bolt blocks, the extra bolts are on the center three main caps, while the front and rear caps keep two bolts. Aftermarket blocks often use stronger cap materials as well: of the blocks we list, the Dart SBC Little M uses billet steel caps and the Dart Little M 305 uses ductile iron caps.
Do you need a 4-bolt main block?
- Mild street engine: a healthy 2-bolt block, properly machined, is usually enough. Plenty of factory engines spent their lives on two-bolt mains.
- High RPM, high power, nitrous or boost: a 4-bolt or cross-bolted block adds a margin of strength where cap movement becomes a risk.
- Racing classes: some spec and sealed racing engines use 2-bolt mains by design, so the rules, not the bolt count, decide.
Bolt count is only one part of block strength. Main web thickness, cap material, cylinder wall thickness, the fasteners and the quality of the machining (align honing the main bores) matter just as much. Many builders upgrade 2-bolt or 4-bolt blocks with stronger main studs; use the fasteners and tightening procedure that the block or fastener maker specifies.
How to tell a 2-bolt from a 4-bolt main block
- Look at the caps: with the oil pan off, count the bolts on the main caps. This is the only certain check.
- Casting numbers: on older Chevy blocks, the casting number narrows down the possibilities, but many castings were made in both versions, so confirm by looking.
- Listings: new blocks and crate engines state the main type in the description or specification table.
A machine shop can sometimes convert a 2-bolt block to 4-bolt with aftermarket splayed caps, if the main webs have enough material. It is a machining job, including align honing, not a bolt-on change.
Bare engine blocks and their main caps
| Block | Mains | Cap material | Price |
|---|---|---|---|
| Dart SBC Little M (31182225), 4.125″ bore, 400 main | 4-bolt | Billet steel | $3,999.99 |
| Dart SBC Little M 305 (31161411), 350 main | 4-bolt | Ductile iron | $3,799.99 |
| GM Performance BBC 454 block (19170538) | 4-bolt | Iron | $2,672.07 |
| GM Performance BBC Gen VI block (19170540), 4.500″ bore | 4-bolt | See product page | $3,288.95 |
| Dart Iron Eagle 2JZ block (31011010) | 4-bolt | Steel | $4,499.99 |
| GM Performance 6.2L LSA aluminum block (12673476) | 6-bolt | Iron | $3,808.32 |
A bare block needs a rotating assembly, heads, valvetrain and machining checks before assembly. See all engine blocks and rotating assemblies, and how a bare block compares with a short block in long block vs short block.
Crate engines by main cap type
| Main caps | Crate engines we list (from their specification tables) |
|---|---|
| 2-bolt | Quicksilver QS 357 CT and QS 383 CT spec sealed engines, BluePrint Windsor 408, BluePrint SBM 408 |
| 4-bolt | BluePrint SBC 350, BluePrint SBC 400, BluePrint SBF 347 EFI, GM Vortec L31 350, BluePrint BBC 632, BluePrint 426 Gen III Hemi |
| 6-bolt (cross-bolted) | GM Performance 6.0L LS, BluePrint LS 376 and the other LS engines in GM LS and LT crate engines |
Prices change; check each product page for the current price and the exact specification. Every order ships for a flat $75 to any US state.
Frequently asked questions
What is a 4-bolt main?
An engine block whose main bearing caps are held by four bolts instead of two on some or all of the caps, which clamps the crankshaft more rigidly.
Is a 4-bolt main block better than a 2-bolt?
It is stronger at the main caps, which matters for high-RPM, high-power and boosted engines. For a mild street engine a sound 2-bolt block is usually sufficient.
Which main caps are 4-bolt on a small block Chevy?
On factory four-bolt small block Chevy blocks, the center three caps have four bolts and the front and rear caps have two.
What does 6-bolt main mean on an LS engine?
Each main cap has four vertical bolts plus two horizontal side bolts through the block skirt, which is called cross-bolting.
Can a 2-bolt block be converted to 4-bolt?
Often, with aftermarket splayed caps installed and align honed by a machine shop, if the block has enough material in the main webs.
Related: long block vs short block, 347 stroker, 400 small block Chevy, big block vs small block, engine blocks, all guides.
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