If you go to ebay you can get them for pretty cheap.. I had a single tube overflow box and wanted to increase the flow to a double tube so while looking on ebay I found one that I think was 40.00 plus shipping to B.C was 12.00 so about 65-70 candaian for a double siphon overflow which is pretty good.. Or if you only want a single one then King eds has one used for 40.00.
For mine I just took out the side slot (dremelled) and drilled another bulkhead into it and put in 2 stockman pipes to make her nice and quiet.
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ZX900F