Results
New displacement
5,820 cc
New displacement
5.82 L
New displacement
355.15 CID
Displacement gain
86 cc
Formula / model
New bore = original bore + overbore x 25.4, displacement = (pi / 4) x bore^2 x stroke x cylinders / 1000
The overbore displacement calculator shows how much cubic inches or liters you really gain from a bore change before machine work starts.
Enter your current numbers or target values below, then use the live results to review new displacement, new displacement, new displacement, and displacement gain before you commit to the next parts or setup change.
An overbore increases cylinder bore diameter by machining material from the cylinder wall. Because displacement scales with the square of the bore (area = π/4 × bore²), even a small overbore produces a measurable gain. A 0.030" overbore on a 4.000" bore V8 adds 5.4 CID — from 350 to 355.4.
Overboring is primarily a machining correction for worn or damaged cylinders, but it also serves as a low-cost displacement increase. The practical limit depends on cylinder wall thickness, verified by sonic testing. Most production cast-iron blocks tolerate 0.030–0.060" overbore. Thin-wall aluminum blocks may be limited to 0.010–0.020".
The calculator computes new bore diameter, then recalculates total displacement:
The displacement gain scales with overbore amount squared. A 0.060" overbore gains approximately twice the displacement of a 0.040" overbore — not 1.5× as a linear approximation would suggest. This non-linear scaling means each additional thou of overbore is worth slightly more displacement than the one before it.
Sonic testing uses ultrasound to measure actual cylinder wall thickness at multiple points around each bore. Factory casting variation means one cylinder may have 0.200" walls while another has only 0.140". The thinnest point sets the safe overbore limit. Without sonic testing, an aggressive overbore risks breaking through the wall — destroying the block. Minimum safe wall thickness is typically 0.100" for cast iron and 0.080" for aluminum with iron sleeves. Learn more about accurate bore measurement.
Interactive — linked to form inputs above
The table below shows displacement gain for common overbore amounts on a 4.000" bore, 3.48" stroke, 8-cylinder engine (Chevy 350 baseline). Gains scale with bore size — larger bores gain more per thou of overbore.
| Overbore | New Bore | CID | Gain (CID) | Common Name |
|---|---|---|---|---|
| Standard | 4.000" | 350.0 | — | 350 |
| +0.030" | 4.030" | 355.4 | +5.4 | "355" (rebuild standard) |
| +0.040" | 4.040" | 357.2 | +7.2 | "357" |
| +0.060" | 4.060" | 360.9 | +10.9 | "361" (maximum safe for most blocks) |
Sonic testing costs $50–$100 and takes 30 minutes. It measures actual wall thickness at 4–8 points per cylinder. Factory casting variation can put one cylinder 0.040" thinner than adjacent bores. Never assume uniform wall thickness — especially on used blocks that may have been overbored previously.
Each overbore amount requires matching pistons. A 0.030" overbore on a 4.000" bore needs 4.030" pistons — which are the most common aftermarket size for the Chevy 350. A 0.060" overbore (4.060") has fewer options and may require ordering from specialty manufacturers with longer lead times.
Overboring increases cylinder volume but does not change combustion chamber volume, gasket bore, or piston dome/dish volume equally. The net effect is a slight compression ratio change. A 0.030" overbore on a 9.0:1 engine typically raises compression by 0.05–0.10 points — small enough to ignore for most builds.
These are the next calculator pages most likely to be useful once you have this result in hand.
Engine Geometry
01Static compression ratio from swept and clearance volume inputs.
Engine Geometry
02Required stroke and compression height for a target displacement.
Conversions
03Convert CID, cc, and liters without re-entering bore and stroke.
Displacement gains, piston selection changes, and compression ratio effects — the complete overbore reference.
How ultrasonic testing measures wall thickness and determines the safe overbore limit for any block.
The 6-point bore measurement protocol that feeds accurate data into this calculator.
It estimates new displacement, new displacement, new displacement, and displacement gain from values such as original bore (mm), overbore amount (in), and stroke (mm).
Start with original bore (mm), overbore amount (in), and stroke (mm) because those are the core values that move new displacement the most. Then refine the secondary inputs to match the exact combination.
It is a solid planning tool built around the stated formula and assumptions, but final results still depend on real measurements, hardware tolerances, tuning, and operating conditions.
Yes. Change the inputs to reflect your exact parts, operating target, or comparison scenario, then review how the outputs respond before you make the next decision.
A useful next step is to compare the result with Compression Ratio Calculator, Stroker Engine Combinations Planner, and Metric to Imperial Displacement Converter so the rest of the combination stays aligned.
