Sonic Testing - What, Why, How

Sonic Testing - What, Why, How
By Don Terrill © -

In the racing world a sonic tester is used to measure the thickness of metal in areas that can’t be accessed for direct measurement, the most common being cylinder walls and roll bar tubing. For this article I’m going to focus on the engine block’s cylinder walls.

Relative to the induction system, there isn’t much power to be had in the short block, but one of the most important areas is ring seal. The interaction between the rings and the cylinder wall is critical. Rings can handle some irregularity, but there is no question, the better the cylinder wall, the better the power.

How a sonic tester works:

Just like a bat or a radar gun, the sonic tester sends out a sound wave and then calculates the thickness of the metal by measuring the time for the reflected wave to return.

Calibration is the key to accurate readings. For the best results you should find two pieces of like material (cast iron), one thinner than typically readings and on thicker, to use as standards. You need to be able to physically measure these pieces and then compare them with the tester to make adjustments. You can try to find areas on the block for calibration or take a sledge to a blown up block and make your own. I was “lucky” enough to have a customer forget to put antifreeze in his block over the winter, which supplied me with the perfect donor block.

Where to test:
  • Major thrust – Located opposite the rotation of the motor. Facing the front of the engine, if it turns clockwise, the major thrust is the left side of each bank (V8). This is the location of the largest loads and thus it would be nice to see the thickest cylinder walls.
  • Minor thrust – Located opposite the major thrust and on the same side as the rotation of the motor - Typically the right side of both banks.
  • Front and back of block – This would be the front of the front cylinders and rear of the rear cylinders. Most manufacturers put a good amount of material in these areas, probably because they had the room.
  • Between cylinders – This is typically the thinnest section of the cylinder because of closeness of adjacent cylinders and the need to allow room for coolant. I’ve actually seen blocks run with less than .100 on the pin sides.
What’s a good thrust side thickness? .300+
What’s ok? Over .250
What’s bad? Under .200 on a thrust or under .100 on a pin side – I’d call this a nightmare

What can go wrong? Egg shaped cylinder walls and worse, a cracked cylinder wall. A good way to know how good a block is without a sonic tester is to see how out of round the cylinders get after a season of racing, good blocks will take next to nothing to clean up, bad ones may take .002+ every season.

  • Grouting the block - Filling a block with grout is no replacement for having a good block. Grout can be helpful, but for other reasons that I will discuss in a future article.
  • Sleeving the block - There are so many things that can go wrong with sleeving a block that it’s not worth the risk in my opinion.
  • Offset boring – Offsetting the boring bar away from thin spots may help a fuzz, but unless you’re doing a large overbore you just can’t make enough difference.
  • A better block – In the end this is the only true fix, with a few more pounds on the nose of the car being the only negative.
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