Plastic for cooling sucks.

pishta said

My 61 slant head has the original stat housing, cast iron.

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Nope. The original part (№ 2121 254) was aluminum, right from start to end of production. An iron one on a Slant-6 is an aftermarket piece (many-to-most of the aftermarket ones were/are iron). Yours is aftermarket piece number at-least-one.

pishta said

I've just never had one explode on me like this one. Found the piece on top of the rad shroud 10 inches away.

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That sucks (and it's also awesome). Also, someone replaced the aluminum one on your '61 head with an iron one sometime along the line because it failed. It didn't fail the way your plastic one did, but it failed just the same—almost certainly at much lower mileage/fewer years than the one on your SC400.

I do not see a real path or line of sight from your Lexus event to plastic for cooling sucks.