Among the tools and devices for measuring the internal diameter of pipes and other cylindrical bodies is probably the most simple. Made of plywood, plastic or metal, cut an isosceles triangle with a hole of handle. On its lateral sides at the same distance from the top of the apply risks by placing near each a number corresponding to the distance between the sides of the triangle at this point. Enough to push the device top to measure the pipe until it stops — figure at this point it will show inside diameter.
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