flatsfisher - there should be a duckbill valve to prevent fuel from coming out, but allowing atmospheric air pressure in to make up for the fuel as it is consumed.
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There are other insert styles, hopefully your cap has the brass one as they are the easiest to service.
Ed, regardless of what figures have been published, there were only two cylinder bores for the 600 Series saws, 1.837 and 1.813 and only one crankshaft/stroke. McCulloch was quite confused themselves.
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As far as I know, those specifications were never corrected anywhere...
When I was working for a living, I used to get very frustrated with our Engineers for turning out drawings and part lists with inaccuracies for the machines we sold. Now that I spend a lot of time looking at old McCulloch documentation, I think our guys did a fairly respectable job.
Mark