Application: Measuring of wall thickness of cast blocks for automobile engines and other internal combustion engines, and wall thickness of cylinder bores in particular.

Background: Engine blocks for automobiles and trucks are complex aluminum or iron castings with numerous dimensions that must be controlled during manufacturing. Because of geometry, most wall thickness dimensions cannot be measured mechanically due to lack of access to the inside surface. Ultrasonic thickness gaging provides a nondestructive method of measuring most of these dimensions.

An area of particular interest is the wall thickness of cylinder bores, especially in the case of the high performance engines used in race cars and custom modified vehicles. When cylinders are initially machined or rebored, it is important that wall thickness not fall below a specified minimum, and ultrasonic gaging is the only practical nondestructive technique for monitoring this thickness.

Equipment: More than 30 years ago, Panametrics was the first company to introduce an ultrasonic gage specifically adapted for measurement of cylinder bore thickness, the Model 5224CB. Today, engine block and cylinder bore measurements are normally performed with the handheld Model 35 or 35DL gages, or with the advanced Model 38DL PLUS in cases where challenging alignment requirements make a large waveform display helpful. The transducer selection will depend on the specific measurements being made. While cast aluminum and cast iron in the thickness ranges encountered in engine blocks can generally be measured with common contact transducers such as an M109 (5 MHz, 0.5" diameter) or M106 (2.25 MHz, 0.5" diameter), the complex shape and access requirements of engine blocks often requires other transducers. In the case of cylinder bore measurements, radiused delay line transducers (usually 5 MHz M206 or 2.25 MHz M207) are used. The replaceable plastic delay line is cut to conform to the inside radius of curvature of the cylinder bore to insure proper sound coupling. Measurement in limited access areas such as wall thickness of exhaust ports may require use of transducers on pencil-type holders to reach into confined spaces. Panametrics-NDT sales representatives will provide assistance with transducer selection for specific cases.

Procedure: In general, ultrasonic measurement of cylinder bore thickness and other engine block walls with a properly set up instrument is straightforward. Calibrated accuracy will typically be ±0.005 inch (±0.12mm) or better. As with any application of this type, some general principles apply:
1. The inside and outside surfaces at the point of measurement must be nearly concentric or parallel to obtain valid and accurate readings. Surfaces must be clean and free of debris.
2. The transducer must be securely coupled to the outside surface at normal incidence. In the case of cylinder bore measurements, this involves use of an appropriate radiused delay line held in proper orientation. In the case of some measurements involving limited access, such as exhaust ports, this may involve monitoring waveforms to determine optimum alignment.
3. Thicknesses below approximately 0.050" or 1.25 mm will require a higher frequency transducer (typically a 10 MHz M202) along with an appropriate thin material gage setup.

Note: That ultrasonic thickness gaging can also be applied to many other automotive measurements, including body sheet metal (including thickness reduction rate at bends), roll bars, molded plastic parts, air bag tear seams, and thickness of paint over plastic body parts.