I did checked the inner by-pass round tube of the t-stat housing. It is in good shape, no corrosion. I think the old t-stat just a bit worn out, and cannot maintain perfect blocking of the by-pass.

The small hole is 7/64", mainly for positively getting rid of the air pocket at the expense of "slightly" longer warm up time. I think some older models have the small hole. As I have/had many Bimmers, I'm paranoid about the air pocket and potential head gasket problems.

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Shane83SD