photos courtesy of Redline Auto Parts
Redline Auto Parts has recently published a very comprehensive guide covering the interchangeability of parts among our two roadster variants. By looking at the photo above from their blog article, it becomes obvious that they actually disassembled one of each vehicle, in order to verify the cross-installation of parts for compatibility, and they they do a nice job of describing any noted issues.
Since Redline has indicated that they intend to keep the document updated, in lieu of publishing a copy of their great ★ ★ ★ ★ ★ (5-star) article, I’m sharing a link to it instead:
https://www.redlineautoparts.com/blog/fiat-124-spider-to-nd-miata-compatibility-interchange-guide/
They’ve also published a video featuring a side-by-side comparison of the two vehicles:
click to watch
21ˢᵗ Century Fiat / Abarth 124 Spider 
Very good article. I’m sort of surprised that they didn’t include the tilt/telescopic steering column on the Miata as a direct replacement for the 124. Other than that it’s pretty darn comprehensive.
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They probably didn’t tear down 2019 vehicles 😉
They also mention that the ND doesn’t have heated seats, but they are offered in some versions. This project had to be quite an undertaking, as I sure wouldn’t want to disassemble my vehicle!
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I want to add an additive to the gear oil on my Spider. Where can I find the gearbox oil container? I look forward to your help.
Greetings from Germany
Roland
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If you are referring to the LSD, look for the Germany equivalent to Mopar 68334697AA, which is 4 oz. of FCA MS-10111 Limited Slip Additive (friction modifier) to be added to the Mopar 68333589AA 75W-90 SG1 (GL-5) Hypoid Gear Oil.
See https://21stcenturyfiat124spider.wordpress.com/specs/#diff for the other differential specs, including wrench socket sizes for the filler and drain plugs.
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Hello, is it easy to find a fiat part ? And where can i buy parts ? Fiat dealer or can i buy at Mazda ?
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For a majority of body parts that are common to both the 124 Spider & Mazda MX-5, a Mazda dealership is one option; an online Mazda parts seller is also an option. For a majority of powertrain parts, a Fiat dealer (or an online Mopar new/used parts seller) would probably be your option (Redline has satisfied the needs of many seeking parts removed from salvage 124 Spiders & Mazda MX-5s, here in the U.S.).
Yes, thanks to your preferred search engine, it is pretty easy to determine the part #s needed, but as to parts availability – that’s another story, as many are “on back-order”. Your question was in reply to finding gear oil additive, and I think you have many more options for procuring lubricant products, starting with your local auto parts store.
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If you need a new part, part numbers are a big help and some dealers include complete parts diagrams on their websites. One group of dealers that has complete parts info for both Fiat and Mazda can be found at tasca.com . There can be a huge difference in price for any Fiat part also shown in the Mazda section. Not just on their website. I was in a hurry for a trim part recently so I took the Fiat part’s number and the identical Mazda part’s number to my local dealers. Fiat dealer: $85 plus shipping charge, tax and several weeks to arrive. Mazda dealer: $25 including tax, no shipping and available in 2 days..
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