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389 Block
Pontiac V8 389 Block
Keep the 389, Lose the Guesswork — 389 Block for Pontiac V8s
An assembled 389 short block is a finished bottom end: a machined 389 block with the crank, rods, pistons, rings and bearings installed and the rotating assembly internally balanced. You supply the cam, heads and induction.
Butler builds these as Budget Short Blocks in three strokes, rated for up to 500 hp. Each starts with a 389 block through the Butler Process, then gets a cast or forged crank, Ross pistons in flat top, dish or dome, Total Seal file-fit rings and a Viton rear main seal, assembled by a Butler engine specialist.
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1. Three Strokes, Three Displacements
The 389 block responds well to stroke, and Butler builds the assembled short block in three of them. Pick by how far you want to go from stock.
| 3.750" stroke, 395-400 cu. in. | essentially a stock-stroke rebuild with modern parts. Runs 6.625" I-beam rods. This is the one for a numbers-conscious car that still needs a fresh, balanced bottom end. |
| 4.000" stroke, 421-427 cu. in. | a forged crank and a real jump in torque without changing the outside of the engine. The middle choice for a street GTO. |
| 4.250" stroke, 447-454 cu. in. | the long-stroke 389. Cast or forged crank, and the most cubic inches available out of the small-main block. |
2. What the Butler Process Does to the Block
A 60-year-old 389 casting is the variable in this equation, so Butler machines every one to the same standard before anything goes in it.
| Inspection first | cleaned, magnafluxed and sonic tested, so a cracked or thin-walled core is found before you have money in parts. |
| Machine work | completely deburred, bored and honed with a torque plate, line honed with ARP main studs, and deck squared. |
| Ready to run | cam bearings and brass freeze plugs installed, then assembled by a Butler engine specialist and shipped on a reusable engine cradle. |
3. Choosing Pistons and Setting Compression
Piston choice is where the short block gets matched to your heads, not the other way around. Butler offers flat top, dish and dome, with performance rod and main bearings under them.
| Give us the head information | add your head casting or chamber cc at checkout and Butler confirms the compression ratio before the pistons are ordered. |
| Internally balanced | these assemblies are balanced internally, which changes what flexplate or flywheel you need on the back. Plan for a neutral balance part rather than a factory offset-weighted one. |
| Rated for up to 500 hp | these are Budget Short Blocks. If the build sheet is headed past that, talk to Butler about a custom short block instead of pushing this one. |
Have your cam profile, head choice and transmission picked before you call. Those three decide the piston, the compression and the balance spec, and it is far cheaper to settle them now than after the bottom end is together.
Not sure what your Pontiac needs?
Call our tech team at 866-762-7527. Tell us your engine, heads, car, and power goal, and we will put the parts together as one combination that works.