Pehaps a picture will help. This is what the exhaust port looks like on the OS 90. Looking at the discontinued engine list, the only os90 that I can see is an OS91FX. Since this is the only OS90 listed, then I assume that it is the same as my engine which says os90max. IF this is the case, then the part number is OSMG2840 which is available from several stores. All I am looking for is confirmation that this muffler will work for my engine. Thanks