The MGA With An Attitude
SPIN-ON ADAPTER - Oil Filter Will Jam - FT-026

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.

Spin on adapter and oil filter 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.

Spin on adapter with mounting bolt 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.
Spin on adapter bolt height
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).

Filter drawing with gauge Spin on filter bolt height
I measured a new NAPA Gold 1516 filter. Distance from the seal surface to the center thread face is .094 inch. The thread is 3/4-16-UNF (16 threads per inch). Instruction for installation is to hand tighten 3/4 turn after the seal contacts the surface. This will compress the seal 0.047, reducing distance from seal surface to thread face to .047 (after compression of the seal). With a protrusion on the adapter of .092 this makes mechanical interference of .045 inch. The filter then cannot be tightened 3/4 turn, as the seal barely touches the surface and is getting very little compression before the filter center hits the hex fitting on the adapter.

For this adapter to work (without modification) the filter must have sufficient recess to clear the hex fitting when installed. Make that dimension 0.092 protrusion +0.047 compression +0.031 additional clearance for various manufacturing tolerances or 0.170 inch minimum required depth in the filter. To assure that you can buy a filter that will fit, you would need an offset gauge in hand for measuring when you go to buy the filter. See drawing. I have no idea which brand filters might meet this minimal dimension requirement.
Filter gauge drawing
One possible solution to salvage this part might be to chuck the center bolt in a lathe and reduce the thickness of the hex fitting by at least 1/16 inch so it will not protrude more than 1/32 inch beyond the face of the adapter.

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. I also do not know the consistency (or inconsistency) of machining of these spin-on adapters.

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 still doesn't work. The adapter configuration has not been changed, only the advisory note added. The NAPA Gold oil filters in my possession (made by Wix) measure 0.094" 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 0.113" (as measured by Moss Motors) is definitely not enough space. The rubber seal will compress 0.047" when hand tightened 3/4 turn leaving only 0.066" final recess, and that the filter should have a minimum of 5/32" depth from seal to thread. Be advised that you should check every filter you buy to verify at least 5/32" 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 over tightens the filter against the hex flange while trying to stop the leak. The car had to be towed when the filter fell off and lost all the oil.

Addendum, June 11, 2013:
Terry Sheppard in Hampton, Victoria, Australia offered this suggestion:
"I had the same problem with the Moss spin on adapter, both the first one I fitted, and then with the replacement I fitted because I though the first one must be defective, even with Moss supplied filters. I have attached the sketches I gave to a local machine shop, who machined a counterbore in the aluminum spin on adapter housing enabling the hex to sit deeper in the housing. This allows about an extra turn of compression on the filter seal. Haven't had a leak from this area since doing this. Seems to be plenty of remaining material so it is not significantly weakened".

Addendum, June 15, 2013:
Keith Turner in Jackson, Mississippi, USA wrote:
"Took your advice and milled out the casting to lower the bolt/nut arrangement. Milled out 1/16 and it has solved the problem".

Now going on 17 years (or more), and Moss still refuses to acknowledge that this is a problem. In all this time they must have procured multiple production lots of this part, but still refuse to change the design. Apparently the parts are still selling well with minimal returns, so the customers must like this problem.

Addendum, August 2, 2016: Today I was pleasantly surprised to find the Moss Motors has corrected the long standing problem with their spin-on oil filter adapter, having reduced thickness of the hex fitting so it will not jam on the filter to prevent removal. Installation then was quick and easy. But it still has one remaining issue. When you bend up the lock tab washer, the vertical edge of the tab will stand higher than the surface of the hex fitting, still jamming against the new oil filter screw on fitting. Solution there is to grind back the exposed edge of the lock tab to be flush with the hex fitting.

Beware that there are many of the earlier faulty adapters still out there in the real world, having been sold for many years prior. There will still be some of these in sales stock with resellers for some time to come (never officially recalled). There will be some that were previously purchased and laid in wait for years before installation. There will be many of these parts installed on cars never having been fixed, still having the same interference problem.

Thank you for your comments -- Send e-mail to <Barney Gaylord>
© 2005-2013 Barney Gaylord -- Copyright and reprint information