there are two main differnet spin on filters available. The standard(if you want to call it that) thread size for Merc, Yamaha, Aqua Power, Sierra, Moeller, and just about every one else out there. then theres the OMC/Crusader filter that uses a larger threaded hole in the filter. If you compare teh two, you will see the difference. Racor has a whole otehr set of thread sizes and filter designs, but I'm talking about hte generally used spin on fule filters here. There are several sizes available in lengts, from the shoft late model Merc to the extra long Yamaha. If the box says "high performance" on it, its generally the 10 micron filter. Figure out which one you are using, most of them are the Merc/Yamaha size. Then get the 10 micron filetr with teh maximum lenght you can fit on the filter head. Once you figure out which one you need, buy at least three of them to keep around, one to install, one to keep on board(in a dry space) as a spare, and one in the shiop tp replace the one you used on board when you need it(weekend saver). If you use Racor filters, then it gets a little more complicated and expensive, but don't get me wrong, racors are great filters, they just cost more and people don't replace them as often as they should because of the price
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