Oil filters - what the heck is going on here?

[WoodenBirdOfPrey]

I attempted an oil change today.  I got as far as trying to install the new filter, which I got from Napa, part number 21626, which appears to be correct through the specs and lookups.  The problem is the thread size is wrong, it goes on half a turn and that's as far as it'll go without forcing it.  The specs of this filter say the threads are 20x1.5, which is also what the yellow sticker in the engine bay calls for.

The filter that was already on the car is a WIX 51258 which appears to be for the 81 and older AMC vehicles, with a thread of 13/16 - 16, and this fits good.

As far as I can tell, this is the original engine to the car.  Is it possible that somebody switched out the adapter / stud/ whatever you wanna call it that the filter threads on to, so that they could use a more readily available filter?  That's the only thing I can think is going on.

I already bought 2 of the wrong size filters with the above Napa part number, no big deal, just 10 bucks.  I'd just go out and buy that WIX filter, the problem is that nobody around here carries WIX filters.  My options seem to be limited to Purolator, FRAM (out of the question), and NAPA filters.  Anybody know a good cross reference so I can get the right size filter and put my car back in the garage?

[carnuck]

The NAPA filter is made by Wix. 1258 is the gold and 21258 is the silver.

[TheWraith]

The NAPA filter is made by Wix. 1258 is the gold and 21258 is the silver.

True.  True.  Just give your NAPA the WIX number and they'll hand you the correct NAPA filter.

[WoodenBirdOfPrey]

I called in to Advance Auto, they cross referenced it and came up with the Purolator 30005 which worked fine.  It would have been nice to know from the previous owner that it was switched over to the older filters at some point.

[WoodenBirdOfPrey]

It's not hard to believe that somebody switched out the adapter somewhere along the line, this car has changed hands at least 4 times before I bought it.  I just need to remember that the parts lookups for make/model/year will yield the wrong filter.

[eaglefreek]

You could always change out the adapter to the metric one.

[WoodenBirdOfPrey]

What is involved in doing that?  Does the adapter just thread into the block, or is there more involved?  If it's cheap and easy I'd consider it, then I can use the filters I just bought.  If it's going to be a project where I get my knuckles busted up and curse all the stuff in my engine bay, I'll just leave it as-is since it's really not that big of a deal now that I know what filter I need.

[mudkicker715]

It is just pipethreaded into the block. just as easy as changing a filter. In my experience. never had a problem swapping them.