No critical parts are made of plastic in a Nikkormat!
I am a world renouned engineer, not an amateur! There are at least 5 plastic gears in the FM3a.
They may last the usable lifetime of the camera, but this camera is in no way a Nikkormat!
No critical parts are made of plastic in Zenit cameras too. I do not mean to insult any, but in the world of internet you should weigh your words more carefully.
Anyway, on my long way on the technical side of photography (much longer than you might think), I never came across an FM3a with a failed shutter. I've came across quite many Nikkormats with a jammed shutter. And guess what, the problem with all of them was a cog missing at least one tooth (the main governor at the base of their gear box). I was really disappointing as I was looking for a broken one to use for parts to repair my father's Nikkormat. Eventually I gave up and ordered a custom made cog from McMaster by sending them the broken one. At least I have now parts for my Nikkormat to make it outlive even me.
Try this. Go to ebay and search for broken Nikkormats:
https://www.ebay.com/sch/i.html?_fr....l1313&_nkw=nikkormat+(parts,+spare)&_sacat=0
Now search again for FM3A:
https://www.ebay.com/sch/i.html?_from=R40&_trksid=p2334524.m570.l1313&_nkw=nikon+fm3a+(parts,+spare)
As of now, there is no broken FM3A available, but plenty of Nikkormats. I know that Nikkormat is much older than the FM3A, but at least for now, statistically the FM3A scores better than the Nikkormat.