Had problems with a customers filter not sealing/leaking and here's what I found:
I checked on the leaking problem you experienced with the large can spin-on filters . This is a German filter and may be described as "over engineered" but the O-ring seal on a shaft is common to numerous very high efficiency filter types. The O-ring that is around the extended portion of the spin-on filters threaded section, is believe it or not simply placed there for shipping (it provides no sealing function at all where it is). Installation calls for removel of the existing O-ring from inside the machines filter mounting head assembly and installing the O-ring that's shipped on the replacement spin-on filter, up inside the mounting head and then screwing on the spin-on filter. Hard to believe they don't include a direction sheet in the box.
Page #2 shows a cut-away drawing of the entire assembly
http://mpfiltri.com/attachs/articolotradotto/2459.pdf
