I have heard multiple reports of oil filters jamming onto spin-on adapters, and I have the same problem myself. This problem pertains specifically to Moss Motors part number 235-940. Moss has been notified, and they are reviewing the situation. I have seen a preliminary re-write of the installation instructions, but so far nothing which would actually fix the problem.

The large hex on the mounting bolt protrudes below the bottom sealing surface of the adapter when installed, and the recess in the mounting end of the filter is not sufficient to clear the bolt. This allows the central inner face of the filter to jam against the face of the hex on the bolt. Depending on tolerances and dimensional buildup, the interference may occasionally be sufficient to prevent complete sealing, and the assembly may leak oil under pressure at the sealing face.

With a little less interference the filter may seal okay but still jam against the hex. The next time it has to be removed a lot of torque may be required to unscrew the filter, and the center bolt may come out with the filter, in spite of proper set on the locktab washer. The picture shows a locktab which was bent up firmly against two sides of the hex, but the filter was jammed so tightly on the bolt that both sides of the locktab were spread open and the bolt came out with the filter upon removal.

For one part which I removed from my car and measured, the recess in the bottom of the adapter is 0.230" deep, the locktab is 0.045" thick, and the hex is .267 thick. This leaves the hex protruding 0.082" below the sealing face. Another part which I have new, still in the box, measures .0.218" deep, 0.046" locktab, and 0.264" thick hex, leaving a protrusion of 0.092" (.010" more than the one which was on my car).

The NAPA Gold 1516 filters I use (shown at top and at right) have a recess of 0.110" from the seal to the center thread face when new, and 0.095" after use when the rubber ring has taken a set (0.015" permanent compression set). The difference of 0.013" (compared to the 0.082" protrusion noted above) is apparently accounted for by additional compression of the rubber ring when installed (spring back when removed), so it must compress at least.028" when hand tightened (as I always do). Compression might have been a little more if not for the center interference. To be sure there is no interference I suppose we should allow for a minimum of 1/32 inch seal compression when hand tightening, and there should be some additional clearance (perhaps 1/32 inch more) to be sure to avoid the jam with various filters.

One possible solution to salvage this part might be to chuck the center bolt in a lathe and reduce the thickness of the hex part by about 1/16 inch. Even then there is no guarantee the problem might not occur with a filter from a different source. The cross reference filters from various manufacturers are not all the same. One Fram PH3600 filter has a measured recess of close to 3/16 inch and will probably work on this adapter without jamming. I do not have a complete list of dimensions of every filter available, nor do I intend to make such a list (unless some folks might volunteer to submit some measurements). I also do not know the consistency (or inconsistency) of machining of these spin-on adapters. If the bottom recess was 0.250" minimum it might be (marginally) acceptable as functional for the NAPA (WIX) filters I have in hand. I would prefer to see the bottom recess of 0.281". I do not know the original target design dimension.

The two adapters I have in my possession were purchased in November 1996 and October 2000, so this problem has been around for quite some time. There may be thousands of these spin-on adapters in use. I have no way of knowing how many may have this dimensional problem, or if there are some with a deeper bottom recess and no problem. If you own one of these parts, it merits checking.

Addendum, May 15. 2006:
Thanks to Tysen McCarthy for a copy of the latest installation instruction sheet included with the Moss spin-on adapter. It now includes this paragraph:

Please note that the relationship between the threaded fitting in the oil filter and the rubber seal on the filter is not the same for all filters. In the filters we offer the lip of the threaded hole is between 0.113" and 0.125" (about 1/8") below the surface of the rubber seal. If the lip of the threaded hole sits too high, it will bottom out against the shoulder of the adapter bolt (2B) before the rubber seal contacts the surface of the adapter itself (2A).

This is a nice note to make the customer aware of the problem (after he has already bought the part), but it is still too close for comfort. The adapter configuration has not been changed, only the advisory note added. The NAPA Gold oil filters in my possession (made by Wix) measure 0.110" deep from seal surface to the center thread, and they do definitely jam on the adapter. A few thousandths of an inch difference would not avoid the problem. That 0.013" is still too close. My educated engineering calculation tells me that the rubber seal can compress up to 1/32" when hand tightened, and that the filter should have a minimum of 1/8" depth from seal to thread. Be advised that you should check every filter you buy to verify at least 1/8" of depth, and no hedging allowed. Sorry Moss, but 0.113 doesn't cut it. If you have a lathe handy you might shave a little off the thickness of the hex fitting. Moss should really be doing this before selling the parts rather than passing the problem along to each individual customer.

Addendum, November 25, 2006:
At 04:09 PM 11/25/2006 -0700, Lee Najman wrote:
"On the moss spin on oil adapter .... that i purchased just three months ago .... the problem .... still exists. .... I just ground off both sides of the hex bolt with a hand grinder, pretty simple to do, and this has solved the problem rather nicely. Removing about a 1/16" is easy to do, particularly if the crown of the hex bolt is taken off. that allows the bolt to sit flush and flat with the adapter."

Addendum, October 29, 2007:

An internet friend sent these pictures showing what can happen when a filter hits the faulty adapter, and it leaks, and someone overtightens the filter against the hex flange while trying to stop the leak.