Cailler: v. tr. Make clots (rennet curdles milk).
Be cold. Curdle blood / grapes / bile: freeze.
To exasperate, push to the limit.
Curdle the grapes, the blood: worry.
To curdle: to lie down.
Curdle the slack: worry.
The expression "Curdle the loaves / the wheels": To be (very) cold.
And to begin with, why do we say curdle? The image would have appeared at Céline in the 1930s. We know that curdle denotes coagulation under the effect of cooling or fermentation. Imagine then that it is so cold that your blood curdles inside your veins. Curdle would then mean "to cool to the point that the blood can no longer circulate, so it freezes".
Now let's move on to loaves. Take a nice loaf of bread, a large ball split lengthwise. Wouldn't it remind you of a nice pair of buttocks separated by a nice vertical smile?
It is in any case the image which, in the slang of the end of the XIXth century, called loaves buttocks. And to reinforce the metaphor, a little before, in the middle of the same century, the miche also referred to the moon, to which the buttocks are often compared.
Finally why the grindstones ? There, the explanations are much less affirmative. This name for buttocks, which appeared in the middle of the XNUMXth century, could be due to the analogy of shape with the rounded top of a haystack. The expression is strictly equivalent to "freezing your buttocks" or "freezing your ass".