A displacement figure is only as good as the two measurements behind it, and getting them right is a matter of the correct tool used the correct way. Bore wants a bore gauge or inside micrometer, not a caliper laid across the hole; stroke wants a crank spec or a doubled crank throw, not a rod length. This guide walks the measurements in order, then hands them to the calculator above.
Step 1 — measure the bore properly
Set a bore gauge to the nominal size, zero it against a standard, then read the cylinder at several depths and at a couple of angles around the bore. Two things you are hunting for: taper, where wear leaves the bore wider at the top than the bottom, and out-of-round, where the bore has gone oval. A caliper across the bore misses both and is only ever a rough check. Because bore is squared in the formula, this is the measurement to get right — aim for 0.01 mm or 0.0005 inch on a build.
Step 2 — get the stroke
Stroke is the piston's full travel from top dead centre to bottom dead centre. If the engine is together, look up the crank specification, or measure the crank throw (the crank's radius of offset) and double it — stroke = 2 × crank throw. What stroke is not is the connecting-rod length; the rod is longer, and using it is the single most common measuring mistake, which is why the calculator warns when a "stroke" exceeds three times the bore.
Step 3 — decide: published or measured?
Use published bore and stroke for a stock engine you have not opened; the figures are reliable and no teardown is needed. Use a measured bore the moment an engine has been bored oversize or has worn, because the real diameter — and therefore the displacement — has changed. A 4.030-inch cleaned-up small-block is genuinely larger than its 4.00-inch nominal.
Step 4 — measure cold, and write it down
Measure at room temperature; that is the reference for published specs and machine work, so your figure matches everyone else's. Record the unit and where in the bore you measured, because taper means depth matters and a bore number without its unit is meaningless. Good notes let a machinist reproduce your result.
Step 5 — turn measurements into displacement
Put bore and stroke in the same unit, enter them with the cylinder count into the calculator on this page, and it applies π/4 × bore² × stroke × N and returns the result in cc, litres and cubic inches with a formula trace. If you rebored or honed between readings, remeasure the bore before recalculating both displacement and compression — even a light cut moves the numbers a machinist cares about.
References & standards
- NIST SP 811 — Guide for the use of the SI, including the exact inch and unit-conversion factors.
Frequently asked questions
What tool measures bore accurately?
A bore gauge or inside micrometer. A caliper across the bore is only a rough check and misses taper and out-of-round, which matter once an engine has worn.
How do I measure bore step by step?
Set a bore gauge to the nominal size, zero it on a standard, then read the cylinder at several depths and around the bore to catch taper and ovality. Record where you measured.
How do I find stroke without a teardown?
Look up the crank spec, or measure the crank throw and double it, because stroke equals twice the crank radius. A degree wheel and dial indicator on the piston also work.
Should I measure the bore hot or cold?
Cold, at room temperature — that is the standard reference for published specs and machine work, so your figure matches the numbers everyone else quotes.
What is cylinder taper?
Uneven wear that leaves the bore wider at the top than the bottom. Measuring at several depths and using a representative diameter keeps your displacement figure honest.
What if the bore is oval or out-of-round?
Note the out-of-round; a worn or oval bore usually needs boring to the next oversize, which then sets a new, larger displacement you should recalculate.
Should I use published specs or a measured bore?
Published specs for a stock estimate; a measured bore once the engine has been bored or worn, because the real diameter — and therefore the displacement — has changed.
How precise does the measurement need to be?
Bore is squared in the formula, so bore precision matters most — aim for 0.01 mm or 0.0005 in on a build. Stroke enters linearly and tolerates slightly less precision.
Can I estimate displacement without any tools?
Yes, from published bore and stroke for a known engine. You only need to measure when the engine has been modified or you cannot trust the badge.
How do I turn my measurements into displacement?
Put bore and stroke in the same unit, enter them with the cylinder count into the calculator on this page, and it applies the formula and shows the result in every unit.
Why record the unit and location of each reading?
Because a bore figure is only meaningful with its unit, and taper means the depth you measured matters. Good notes let you or a machinist reproduce the result.
Does deglazing or honing change my numbers?
Light honing barely moves the bore, but a full rebore does. After any machining, remeasure the bore before recalculating displacement and compression.
What is the most common measuring mistake?
Using connecting-rod length as stroke. The rod is longer than the stroke; stroke is only the piston's travel, which is twice the crank throw.
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