My thoughts are junk heads. Probably big box store bad quality rebullds...
Edit: As if that's all you changed, the filings had to be in the head from mfg or remfg.
No where else could it have come from?
The block is cast, and nothing else was touched.
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7 3rd gens listed in the build thread (2 are parts mobiles)
Build Thread:
https://www.toyota-4runner.org/3rd-g...os-builds.html
Brillo's Bucket Fluid Ex changer:
https://www.toyota-4runner.org/3rd-g...ml#post3358086
Sparks Plugs Wire and Coil Information:
https://www.toyota-4runner.org/3rd-g...on-5vz-fe.html