in the stock setup the filter housing fills with dirty oil and is sucked through the filter to clean it. If you just put a spinon bolt in the block the filter will not match up to the passages that are outside the bolt bringing the dirty oil. You need the adapter to 1) prevent oil leaks, and 2) make sure the filtering process is working correctly.
The stock filter housing BTW is also a crude cooler. Changing to a spinon can have an effect on this, but I doubt it is significant. On the 77/78 cb750Fs there is an extension to this to help the cooling process that most people throw away when fitting aftermarket pipes. They don't realize that their oil is running a little hotter than the K bikes before it, and that honda installed these to help a hotrodded cb750 engine cope with oiling issues. It is for this reason that I think running an oil cooler when doing a spinon filter is a good idea, and espically a good idea on the 77-78 F engines. As long as you are getting the adapter from terry, why not spring for the extra bucks and get the cooler too, espically if you live in warmer climates or urban areas.....
