Reverse Engineering Internal Features: Scan, Section or CT

Nikita ShuvalovJul 17, 2026

A hydraulic manifold with galleries that cross inside the block, a pump housing with a cored water jacket, a valve body whose seat sits at the bottom of a bore: these are the parts where reverse engineering gets hard, because the geometry that matters is the geometry no surface scanner can see. This guide explains what a surface scan captures, what it misses, the four ways an internal feature can still be measured, and when a computed tomography scan from a CT lab is worth ordering.

What a surface scan captures

A handheld laser scanner records the surfaces its light reaches. A scanner of the HandySCAN SILVER class, by its brochure, projects fourteen blue laser lines over a scanning area of 310 by 350 mm and is rated at an accuracy of up to 0.040 mm, the brochure’s typical value for diameter measurements on a calibrated sphere. Moved around a part by hand, it builds a mesh of every face it can illuminate: the outside of a casting, the flange faces, the mouths of the bores, and the first part of a straight hole, as far as the lines reach the wall at a usable angle. The mesh is the measured record of the part’s exterior, and for a bracket, a blade or a worn gear that is the whole job: the CAD model is rebuilt from that mesh alone.

A gallery that turns a corner inside the block, a cooling passage cast into a cylinder head, the inside of a closed hollow casting and a thread at the bottom of a deep blind hole all appear on the scan as openings in the outer surface and nothing more. A CAD model rebuilt from the mesh alone would be a solid where the part is hollow.

Why internal features matter

For many parts the internal geometry is the function. A manifold routes fluid from one port to another through galleries whose diameters, lengths and intersections set the flow and the pressure drop, so a model with the wrong gallery is a different manifold. A pump housing’s water jacket decides how the pump cools, and a valve body’s seat angle and concentricity decide whether the valve seals. A reproduction that copies the outside and guesses the inside looks right and works wrong, and the error usually shows up after the part is made.

The question to settle before scanning, then, is which internal features matter for what the model is for. A housing being re-made as a casting needs the cores modeled; a housing being fitted with a new bracket needs only the outside. The scope of a reverse engineering job names the internal features that have to be measured and the method for each.

Four ways to measure an internal feature

The first is to measure from the openings. A straight bore is measured at its mouth with a bore gauge and its depth with a depth gauge, and a straight through-passage can be scanned from both ends and its two mouths joined in the model. For a straight gallery between two known ports this is often enough.

The second is to use what already exists: a drawing, a catalogue section or a casting pattern. Internal features taken from such a source are modeled from it and marked in the delivery as taken from a document rather than measured.

The third is sectioning. A sacrificial sample, a scrapped part of the same model or the worn part itself when the client agrees it will not be used again, is cut on the planes where the passages run, and the cut faces are scanned or measured like any other surface. Sectioning gives direct measurements at the cost of the part, and it is the answer we propose when a spare is available and the part is going to be re-made anyway.

The fourth is computed tomography. ASTM E1441, the Standard Guide for Computed Tomography (CT), describes CT as “a radiographic examination technique that generates digital images in three dimensions of an object, including the interior structure”, and notes that “because of the relatively good penetrability of X-rays, CT permits the nondestructive physical and, to a limited extent, chemical characterization of the internal structure of materials”. A CT volume shows the galleries, the cores and the wall thicknesses without cutting anything. As a rule of practice, the lab extracts a surface mesh from the volume, and that mesh can be modeled from in the same way as a scan mesh, at the resolution and accuracy the lab states.

What CT can and cannot do

The X-rays have to pass through the part, so what a CT system resolves depends on the part’s material and thickness and on the system’s X-ray energy; a thick steel part may be beyond a given system, and that is a question for the lab. ASTM E1695 is the test method for a CT system’s overall spatial resolution and contrast sensitivity, measured “based on examination of the CT volume of a uniform cylinder of material”; the standard itself says it is not meant to predict whether a specific feature will show in a specific part. So the question to put to a lab is whether it has scanned a part of this material and wall thickness at the resolution the feature needs. As a rule of thumb, the voxel size of a cone-beam CT volume grows with the field of view, so a region of a manifold scanned on its own resolves finer detail than the whole manifold scanned at once.

WeAre Capture does not operate a CT system and does not order CT scans. When a part’s internal features cannot be reached by the first three methods and the model needs them, the client orders the CT scan from a CT lab, and the mesh the lab supplies can be modeled from together with our surface scan: the exterior from the laser scan within the scanner’s rated accuracy, the interior from the CT mesh at the resolution and accuracy the lab states. The delivery says which features came from which measurement.

How the work is scoped

The part’s purpose decides the method. For a part that will be re-made as a casting with cores, the internal passages are measured, by sectioning a spare or from a CT mesh the client supplies, and the model is built as a solid with the cores subtracted. For a part being fitted to, or being redesigned on the outside, the internal features are left as the drawings or the openings describe them and marked as unverified. For a worn part being restored to its design shape, the internal features that wear, such as a bore or a seat, are measured from their openings and snapped to their nominal values in the model, and the delivery lists what was measured and what was restored.

The quote names the method per feature, because the methods differ by a great deal in effort: measuring a bore at its mouth takes minutes, sectioning a spare takes about a day, and a CT scan is a separate purchase from a lab. Sending photographs of the part, its approximate size, its material and a sentence on what the model is for is enough for a quote within one business day.

FAQ

Can a 3D scanner see inside a part?

No. A laser line scanner records the surfaces its light reaches: the exterior and the first part of any opening. Internal passages, cores and the inside of closed castings are not in a surface scan and have to be measured another way.

What is CT scan reverse engineering?

Reverse engineering from a computed tomography volume: a three-dimensional image of the part, including its interior, built from X-rays passed through it, as ASTM E1441 describes. The surface mesh a lab extracts from the volume can be rebuilt into CAD. Whether a part can be CT scanned depends on its material and thickness and on the system.

Does WeAre Capture do CT scanning?

No. We scan surfaces with handheld and terrestrial laser scanners and rebuild CAD from the scans. When a part needs a CT scan, the client orders it from a CT lab, and the lab’s mesh can be modeled from together with our scan.

How are internal features measured without CT?

From the openings with bore and depth gauges, from existing drawings and patterns, or by sectioning a spare part on the planes where the passages run and scanning the cut faces.

How accurate is the exterior scan?

A handheld scanner of the HandySCAN SILVER class is rated by its brochure at up to 0.040 mm, as a typical value for diameter measurements on a calibrated sphere.

What does the quote need?

Photographs of the part, its approximate size and material, and what the model is for. The quote names the measurement method for each internal feature and arrives within one business day.