Yes that type of design will open up like an accordion under too much pressure if it is not contained. One way to contain the core is to have an old top and bottom with the motor removed and just put the clips back on. I guess you could clamp it somehow with a couple of C clamps. The first time I took one to a radiator shop, the guy gave the core to the village idiot to do the testing and cleaning. When I got it back, it was about an inch bigger. I was able to put the core in a vise and smash it back down, so I could get it into the container. But, I do not think I ever used it again. I clean them out at home now by shaking them up side down over a sink, using soap and water and perhaps heat them in kinda boiling water on the stove. The old ones are fragile and the new ones are generic (you have to modify) or as Moss puts it, a Matrix, and they are expensive.
Sp53