I went to the website that Packardv8 posted above and found this statement.
Compare this with the FAA/PMA "equilivant" cylinder that is not countersunk (shown below).
Attention to details is important on troublesome exhaust ports. No to put too fine a point on it, BUT there is no such word known to Merriam-Websters as "equilivant". Attention to detail is important in English also
Seriously, I have learned a little more with this THREAD.
(pun intended). I just thought rolled threads were cheaper to make. That obviously is only part of the reason for using them.
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