Does 1978 CB550K OEM exhaust have drain holes just behind the header pipe weld? Can't really tell on this project, but they might be closed from neglect by PO. If not I may put some in there. Would find it hard to believe they did it in the early 750's and not the 550's?
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Just checked my '77 550K pipes Johnny (believe their the same as '78)..no drain holes in headpipes anywhere between header pipes and muffler. However as Nick said..there is a small hole directly below the exhaust hole/port at the end of the muffler. Can't see where that would function much as a drain tho..as the pipes are slanted upwards towards the back?.
I also remember the PO saying to be careful when riding cold..to make sure the pipes and mufflers warmed-up well..so that any condensation inside pipes/mufflers has chance to evaporate..before putting bike away for the day. Guess there have been cases of pipes and mufflers rusting-out prematurely on these late 550's..due to that fact. I've always tried to heed that advice..and thus far my pipes and mufflers on this bike look fine after many years of riding.
Ichi