The following is the text portion of the oil filter adaptor critique
appearing in the June Stag Owners Club newsletter / magazine:
I REFER to the letter from Tony Hart in last month's magazine, re the spin
off oil filter adaptor. Tony mentions that, in the May magazine, I did not
explain how the assembly allowed oil to bypass the filter. I hope the
following will adequately explain the reasons why I made the statement in the
May magazine about the potential for these devices to recirculate unfiltered
oil.
In 1987 I replaced the engine in my Stag using a short motor from a parts
supplier. As there were problems at that time with oil filter failures (now
thankfully rectified by a design change by Unipart), I decided to purchase
the engine complete with a spin off filter and the necessary adaptor.
The adaptor consists of two parts, a machined aluminium casting and a
machined hexagon steel bar. Please refer to photograph 1. The purpose of
the aluminium component is to channel unfiltered oil into the canister
filter, whilst the steel item retains the aluminium casting against the
block, acts as a retainer for the spin off filter and channels the filtered
oil through the six holes back to the oil supply gallery. Reference to
photograph 2 shows the assembly fitted to a bare cylinder block. From the
nature of the components, it can be seen that it is impossible to fit the
adaptor incorrectly.
Closer inspection of the device reveals the deficiencies:
1. The inner 'O' ring seal which should 'push' against the cylinder block
does not sit in a groove, it sits in a hole. For an 'O' ring to work in this
type of application, it needs to be fitted to a carefully toleranced groove
so that when the component is fitted, the ring seals at the four points which
it contacts. Reference to photograph 3 shows the effect the unfiltered oil
would have on this particular 'O' ring, ie it would be pushed towards the
main supply gallery.
2. When the adaptor was installed onto the cylinder block (using a standard
supply 0.125in thick outer seal ring in the cylinder block groove), it was
evident that a gap existed between the inner 'O' ring mating face on the
adaptor and the mating face on the cylinder block, ie the area which must
have zero clearance if unfiltered oil is to be prevented from bypassing the
filter. I measured this gap by sticking a piece of uncured 'plastic metal'
onto the adaptor which was then affixed to the block. Removal of the adaptor
enabled the slightly squashed 'plastic metal' to be measured using a
micrometer. Reference to photograph 4 shows the squashed substance on the
'O' ring area of the adaptor after it was removed from the block. The result
of this exercise was that a 0.155in clearance existed in the position where
there should have been a good seal. As the inner 'O' ring has a diameter of
1.5in an area of approximately 0.73in2 is available for the oil to flow
through from the unfiltered to allegedly filtered area of the adaptor.
Hence, it can be seen that if this device was fitted to my engine, the spin
off filter would be operating permanently on bypass, thus causing premature
wear on main and big end bearings. If the car had regular oil changes and
didn't accrue large annual mileages, it would probably take quite a few years
for the bearing wear to manifest itself if this assembly was used. I
therefore decided against using this device on my engine.
It is apparent that the assembly could be improved by machining a proper
groove for the inner 'O' ring to sit in, also by machining the outer lip of
the aluminium piece thus reducing the clearance of the inner 'O' ring and
cylinder block to zero but how much metal do you remove? The recess in the
cylinder block for retaining the outer sealing groove must be machined to a
certain tolerance. How do you effectively seal the filtered and unfiltered
oil at both ends of this tolerance range? If you machine too much off, then
the device leaks externally; if you don't machine enough off, then you have
the problem outlined in photo 2. I suspect that this device was manufactured
specifically to ensure it didn't leak externally.
Now, I must stress that the above relates to the items I was sold. Tony
states that 'there are no problems with these adaptors' which HRS and Rimmer
Bros are selling, maybe these are different. If so, I would be interested to
know how the deficiencies outlined previously have been overcome. I would
also recommend to all members who have an adaptor fitted, to check it out to
ensure it is not deficient like mine. If it is, I recommend you remove it
and return it to the supplier as unfit for purpose and ask for a refund.
After all, the correct oil filter assembly may be a bit fiddly, but if
fitted correctly, it will work.
Some newer members may not be aware the SOC went to a lot of trouble to get
Unipart to redesign the standard oil filter element in the 1980s so that it
performed to an acceptable standard.
A RESPONSE is awaited from Think Automotive, the manufacturers of this
product
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