The 83 RX-7 has a crappy Oil-Water oil cooler, it is this big ass thing that the old filter sits on. It is very common for these to be replaced with coolers from a 84-85 RX-7. I beleive the thread pitches are the same for the oil line fittings, but some fabrication might/will be needed to get the cooler down in it's right spot.
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