Here is the difference. They are often stamped for a designated number, like the most common '65'. The question is whether 4 rods stamped '65' really make a set. The answer is NO. The smaller the number, like a '45' is lighter and often shorter. The heaviest are marked '75', although we have seen one set '76'. That being said, we made a set that matched weight and length 45, 54, 65 and 67. They balanced out without much ado within .1gr.
The average imbalance that VW allows is 8 grams, but a balanced set of rods can make a huge difference in the smoothness of the engine.
The AHU and ALH share the same design and dimension connecting rod, but the difference between rod is their weight and their length between centers. The heaviest to the lightest rod is about 30 grams. The longest to the shortest center is .040", when .004" is the difference between head gasket thicknesses. .004" difference makes .5 CR difference, so getting the measurements close is what we are very good at. And... we balance rod sets to within .2grams from the stack of rods we have marked for weight and length. We keep an average of about 120 rods, so we can pick out rods to match.
So, it may be the same part number, but there's more to it to make them match.
