Yeah, I am not seeing any closing sleeves anywhere. Mike that is great verbage to use in the document if you don't mind. But again, no closing sleeves. All I see is it's open via the many holes around the shell all the time. As far as I can tell from looking, the only thing the valve does is add even more flow.
My casting numbers are the same as Tims. There is a machined surface. And.. Don't the holes we drill in the thermostat allow water to pass the thermostat as well? I do not think anything from an engine design perspective changed much in 1958. There are differences, but I would think the reason for the change of thermostat housing was GMs experience with them overheating and it was 1958 when they finally fixed it? Speculation of course. Matt that is exactly what that bypass thermostat looks like. It came fresh out of an engine, and I took it out, so there wasn't any evidence of closing sleeves anywhere.
