all I can imagine is that you have the cap from a different rod & the bore is offset enough that it's binding up
each rod/cap assy is machined to each other, they can't be mixed or flipped in any way, they must be re-assembled exactly the same as they were originally
worst case you may need to get them 'Big Sized' or 'Minor Rebuilt' at the local shop, this can be done with the pistons on the rods
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