... The cable idea is really aimed at cleaning the water jacket.
Correct me if I'm wrong, but aren't there chanels in the head, and these match up with those in the block? If you can clean the coolant channels in the block with frayed wire, can you not do the same with the head?
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I cannot advise you on where to buy cable ... The cables I am talking about are MUCH larger than what a bicycle would use. Think 3/16" or 5mm diameter.
I assumed something thciker than bicycle cables were required. Though I'd guess if you took everal short lenghts, platted them together, it would have the same effect?