400 Small Block Chevy: Specs, History and 400 SBC Crate Engines
The 400 small block Chevy is the largest factory Gen I small block: 400 cubic inches from a 4.125″ bore and 3.750″ stroke, built from 1970 to 1980. Its siamese cylinder bores, larger main journals and external balance set it apart from the 350, and its long stroke is the reason it was a torque engine for trucks and full-size cars. New 400 SBC crate engines from BluePrint are rated at 500 HP and 510 lb-ft.
400 small block Chevy specs
| Specification | Factory 400 small block (Gen I) |
|---|---|
| Displacement | 400 cubic inches (6.6L) |
| Bore x stroke | 4.125″ x 3.750″ |
| Bore spacing | 4.400″, like every Gen I small block Chevy |
| Cylinders | Siamese (no water passage between neighboring bores) |
| Main journals | 2.650″, larger than the 350’s 2.450″ |
| Connecting rods | 5.565″ from the factory, shorter than the 350’s 5.700″ |
| Balance | External (balancer and flywheel or flexplate carry weight) |
| Main caps | Two-bolt or four-bolt depending on the block |
| Years | 1970 to 1980, mainly trucks and full-size cars |
Factory horsepower ratings were modest because the 400 was tuned for low-speed torque and, from the early 1970s, for emissions. Its value today is the block: the big bore and long stroke make it a strong base for torque-focused street engines and stroker builds.
What makes the 400 different from a 350
- Siamese bores and steam holes: to fit a 4.125″ bore within the small block’s 4.400″ bore spacing, the cylinders are cast touching each other. Cylinder heads for a 400 need small steam holes so trapped steam can escape from the block; heads made for a 350 without them should be drilled to match the block and head gasket.
- Bigger mains: the 2.650″ main journals need a 400 crankshaft or a crank made for the large-journal block. The Dart “Little M” block we list is described as a 400 main block with a 2.650″ main bearing diameter.
- External balance: a 400 needs its own balancer and flywheel or flexplate. Parts from an internally balanced 350 will not balance it.
- Cooling: with less coolant around each cylinder, a healthy radiator and cooling system matter more on a 400.
400 vs 350, 383 and 406
| Engine | Bore x stroke | Displacement | Notes |
|---|---|---|---|
| 350 | 4.000″ x 3.480″ | about 350 ci | The common small block; see our Chevy 350 guide |
| 383 stroker | 4.030″ x 3.750″ | about 383 ci | A 350 block with the 400’s stroke; see the 383 stroker guide |
| 400 | 4.125″ x 3.750″ | about 400 ci | Factory siamese-bore block |
| 406 | 4.155″ x 3.750″ | about 406 ci | A 400 bored 0.030″ over |
A 383 borrows the 400’s 3.750″ stroke but keeps the 350’s thinner bores and water jacket between cylinders. A 400 gives a little more displacement and a larger bore, which helps the cylinder heads breathe. Both are small blocks: for how they compare with a 454, see big block vs small block.
400 SBC crate engines we list
| Engine | Version | Rating | Price |
|---|---|---|---|
| BluePrint SBC 400 (BP4002CT1) | Base engine with aluminum heads | 500 HP, 510 lb-ft | $7,349.00 |
| BluePrint SBC 400 (BP4002CTC1) | Base dressed | 500 HP | $7,949.00 |
| BluePrint SBC 400 EFI (BP4002CTF) | Base dressed with Holley Sniper EFI | 500 HP | $8,999.00 |
BluePrint builds these on a new four-bolt main cast-iron block. The BP4002CT1 description lists a forged 3.750″ stroke crankshaft, 5.700″ connecting rods, forged 10.3:1 pistons, a hydraulic roller camshaft and aluminum heads with 220cc intake runners; the EFI version adds a dual-plane intake, HEI distributor and a Holley Sniper EFI system. Note that BluePrint uses 5.700″ rods, not the factory 5.565″. Prices change; check the product page for the current price and included parts. Every order ships for a flat $75 to any US state. See all small block Chevy crate engines.
Frequently asked questions
What is the bore and stroke of a 400 small block Chevy?
4.125″ bore and 3.750″ stroke, for 400 cubic inches.
What years was the Chevy 400 small block made?
From 1970 to 1980, mostly in trucks and full-size cars.
Do 400 small block heads need steam holes?
Yes. The siamese bores trap steam, so heads used on a 400 need steam holes that line up with the block and head gasket.
Is a 400 small block internally or externally balanced?
The factory 400 is externally balanced, so it needs a 400-specific balancer and flywheel or flexplate.
Is a 383 the same as a 400?
No. A 383 is a 350 block with the 400’s 3.750″ stroke and a 4.030″ bore. A 400 uses its own siamese-bore block with a 4.125″ bore.
How much horsepower does a 400 SBC crate engine make?
The BluePrint 400 crate engines we list (long block, base dressed and EFI) are all rated at 500 HP and 510 lb-ft.
Related: Chevy 350 engine, 383 stroker, Chevy firing order, 2-bolt vs 4-bolt mains, 400 SBC crate engines, all guides.
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