"Take it thirty over." It is one of the most common sentences in an engine shop, and this calculator tells you exactly what it buys. Thirty over — 0.030 inch on the bore — turns a 350's 349.85 cubic inches into roughly 355, a gain of about 5.3 CID or 1.5 percent. Enter your original bore and stroke plus the overbore amount, or a new stroker crank, and the panel shows the before, the after and the percentage in one view.
Why bore changes punch above their size
Displacement grows with the square of the bore, so a change that looks tiny on a ruler still adds real volume. Boring adds only a thin ring of cylinder area — but because that ring sits at a larger radius each time, the same 0.030 inch adds more cubic inches on a big-block's 4.25-inch bore than on a small-block's 4.00-inch bore. Stroke, by contrast, adds volume linearly.
The overbore ladder on a 350
Here is what successive standard overbores do to a 4.00-inch-bore, 3.48-inch-stroke small-block. The gains are real but modest — which is the honest headline of this page:
| Overbore | New bore in | New CID | Δ CID | Δ % |
|---|---|---|---|---|
| +0.010" | 4.010 | 351.60 | +1.75 | +0.50% |
| +0.020" | 4.020 | 353.35 | +3.51 | +1.00% |
| +0.030" | 4.030 | 355.12 | +5.27 | +1.51% |
| +0.040" | 4.040 | 356.88 | +7.03 | +2.01% |
| +0.060" | 4.060 | 360.42 | +10.57 | +3.02% |
Bore versus stroke: where the big gains live
If you want a lot more displacement, stroke is the lever. Boring a 350 the full 0.060 inch adds around 10 CID; swapping to a 3.75-inch stroker crank adds far more, and combining the two is how a 350 block becomes a 383. The calculator lets you enter both the larger bore and the longer stroke together so you see the combined result rather than guessing at the sum.
How far is safe?
Displacement maths sets no limit — the block does. Cylinder-wall thickness varies by casting, so the safe overbore for one 350 is not the safe overbore for another. Many iron small-blocks accept 0.030–0.060 inch, but the only honest way to know is to sonic-check the walls before cutting. The tool tells you the displacement; the machinist tells you the ceiling.
What an overbore drags along with it
A larger bore needs oversize pistons, and it nudges the compression ratio up because it adds swept volume. That is why the result panel offers a button to carry your new per-cylinder swept volume straight into the compression-ratio calculator — so you can plan the piston dish or dome and the head gasket around the bore you have chosen, in one pass, instead of discovering the compression change after the fact.
References & standards
- SAE J1349 — Engine power test code — the standard behind 'SAE net' crankshaft ratings.
Frequently asked questions
How much displacement does a 0.030 overbore add?
On a 350 small-block, going 0.030 in over adds roughly 5.3 CID — about a 1.5% gain — because volume grows with the square of the bore.
Why does the same overbore add more on a bigger bore?
Because the added ring of area depends on the starting diameter. The same 0.030 in adds more cubic inches on a 4.25 in big-block bore than on a 4.00 in small-block.
Is boring or stroking the bigger displacement gain?
Stroking, usually by far. Adding stroke lengthens every cylinder's swept height, while a safe overbore only nibbles at the diameter — which is why stroker cranks make the headline numbers.
What does '0.030 over' actually mean?
The cylinder diameter is machined 0.030 inch larger, so a 4.00 in bore becomes 4.030 in. Overbore figures always refer to the diameter, not the radius.
How far can I safely overbore an engine?
It depends on the block's cylinder-wall thickness, which varies by casting. Many iron small-blocks take 0.030–0.060 in; sonic-checking the walls first is the only honest way to know.
Does overbore change the compression ratio?
Slightly, because it adds swept volume. The panel can hand your new per-cylinder swept volume to the compression tool so you see the combined effect.
How do I calculate a stroker's new displacement?
Enter the larger bore and the new, longer stroke together. The tool shows original versus modified cc and CID and the percentage increase in one view.
What is a common stroker combination?
A 350 block bored 0.030 over with a 3.75 in stroke crank makes the classic 383 — about 396 CID with the overbore, versus the stock 350's 349.85.
Why express the gain as a percentage?
Because a raw '+5 CID' means little without context. A 1.5% gain tells you honestly how modest a mild overbore is next to a stroker's double-digit jump.
Does an overbore always need new pistons?
Yes. A larger bore requires oversize pistons to match, which is why the overbore and the piston choice (dish or dome) are planned together.
Can I preview an overbore on the main calculator?
Yes — the home and cc calculators have an overbore field too, but this page adds the full before/after table and the stroker comparison.
Will a bigger bore hurt reliability?
It can if it thins the cylinder walls too far, risking distortion or overheating. Staying within a sonic-checked limit is what keeps a rebore reliable.
How do bore and stroke together set the final size?
Displacement is proportional to bore² × stroke, so the bore change is squared while the stroke change is linear — a small stroke increase often outweighs a big-sounding overbore.
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