From what I can find the GM # is 8526398 and it's an o ring. Does it look like an o ring would be right? I don't think I've ever looked one of those up for a Powerglide. Over the years I sold lots of them for the TH350's though but they used a tiny common looking seal.
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1966 Strato Chief 2 door, 427 4 speed, 45,000 original miles
1966 Grande Parisienne, 396 1 of 23 factory air cars (now converted to a "factory" 4 speed)
