Ok, so Iīve recently removed the head on my om602 1992 300D and I took it to a machine shop to be inspected. They couldnīt find anything wrong with it and want me to check the block for 1) cracks in the cylinders and 2)complete flatness. They gave me a "flatness checker" and it seems to be fine in that respect. We could not get any sort of measurement feeler gauge under the ruler.
Now my question is checking for cracks, they say that itīll be difficult to find the crack, but should I be able to feel it? Iīve gotten in there with a light and canīt find anything. Are there any techniques to finding cracks?

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1992 300D OM602

Ben