at some point around 1980 the 617 engine received a modification to the inside of the valve cover in the way of an internal baffle which may help. Removing the valve cover to adjust the valves would allow you to find this out, you may also be able to tell by removing the oil filler cap and using an inspection mirror.

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'82 300SD - 361K mi - "Blue"

"Good judgement comes from experience. Experience comes from bad judgement."

listen, look, .........and duck.