Part Numbers vs Casting Numbers
I often see ebay listers confusing Harley casting numbers with Harley part numbers. The confusion arises because they are sometimes the same and sometimes not.
A complex part like a frame has several casting numbers. None of these are the part number; each is simply the number for that particular casting or piece of the frame (the neck casting or the seat casting, for instance).
As I write this, there are several Sportster swingarms listed on ebay as part number 47558-52, but if you check your catalog, you won't find it listed. That's because it's the casting number for the front section of the swingarm. Sportsters from 1957-1967 have the same swingarm--part number 47504-52B. That number represents the entire swingarm; the casting number represents only a particular piece of the swingarm.
In 1968, XLH models went to an inch longer swingarm. This swingarm has a different part number, but the front casting number remains the same because it's the same casting.
On parts of a single casting, the part number and casting number are usually (but not always) the same.
And finally, just because a Sportster part has a casting number that ends in "52," it doesn't mean it came from a K-model (This claim is made for triple trees all the time). In fact, the odds are better than even it didn't. The 52 simply indicates the year the mold was made.
I hope this clears up any confusion. If you feel I've missed something, or if you'd like to add information, please contact me via the ebay mail feature.
Guide created: 10/31/07 (updated 05/07/13)