piston removal on a 1946 d4?

I would say no. Someone else that knows for sure, please correct me if I'm wrong. I think the piston skirt is going to hit the main bearing saddles of the block.

The D2 with either 3.75 or 4" bore depending on the series requires that the connecting rod/piston assembly (or at least the ROD) come out the bottom, but only because the smaller bore does not allow the big end of the connecting rod to come up through the cylinder sleeve. The smaller engines have quite a bit of space between main bearing saddles/cylinder bores compared to the larger 4.25/4.5" bore engines used in the D4's

The answer to your question is no, your D4 has the D4400 diesel engine and the head has to be removed and the piston and rod assemblies taken out the top.

As ccjersey said, the D2's are the only models that have to have them removed out the bottom since the big ends of the rods are wider than the cylinder bores, only those engines (D3400 - 3 3/4" bore "J" series, and D311 - 4" bore "U" series) were designed to allow the pistons to clear the crankshaft on their way out.

[quote="Sasquatch"]
As ccjersey said, the D2's are the only models that have to have them removed out the bottom since the big ends of the rods are wider than the cylinder bores, only those engines (D3400 - 3 3/4" bore "J" series, and D311 - 4" bore "U" series) were designed to allow the pistons to clear the crankshaft on their way out.[/quote]

Not quite the only model, the R2 (4J, 6J series) also have to have the pistons removed from the bottom.