The upper portion of the filter is the by-pass portion of the filter that has a much finer filter media. The under pressure the a small portion of the unclean oil is forced through the cannister holes, through the filter media, into the small hole on the filter cover stem, down the stem, past a check valve, and into the oil pan. It would seem that the size of the hole in the stem is the factor that normally determines how much oil moves through the by-pass filter element at any given pressure. If the stem hole, stem, or the check valve in the stem where blocked, the oil under presure would have no effect on the filter cannister.

It would seem that if the small cannister holes became restricted, perhaps a simple over-loading or clogging of the filter media just inside the cannister holes, maybe from dirtier oil from driving harder than usual, a situation would occur where the perforated metal canister in combination with restricted holes/media would be the barrier directly acted upon by the dirty oil under presure and as long as the stem hole/stem/check vale remained functioning, the upper cannister metal portion could be collapsed inward towards the stem.


It might also be that if the oil cover stem check valve has failed there could be a condition that would exist where the normal check valve resistance is absent and the lessening of the designed oil flow restriction ( from the functioning check valve spring pressure) would contribute to a greater force than normal being applied to the cannister/filter media barrier from it's outside collapsing it inward towards the stem. At the same time if the oil flow through the by-pass filter increased in volume due to a decreased restriction (again, from a failed check valve ) it would stand to reason that the filter media just inside of the cannister holes might get clogged more quickly.

So the failure of the check valve to provide the resistance as designed would be allowing a greater flow of dirty oil initially that could be clogging the filter media/opening prematurely creating a pressure barrier that is acted upon by the unfiltered oil under pressure collapsing the upper portion of the cannister.

When I change the oil on my 603 engine I usually use air applied to the stem hole to blow the stem clean and then I put a squirt of WD-40 or similar product into the stem hole, let it sit for a couple of minutes and then blow that out, usually two or maybe three times. I don't know what the design spec is for the stem check valve but I'm sure it at least a couple of PSI the way it releases (yeah the garage wall will forever show evidence of the first time I did this without having the stem end INSIDE of a suitable rag!). I can't imagine that you should be able to create a flow in reverse if the check vale was operateing as designed. So perhaps if the check valve is absolutely not fuctioning that could be the root cause of the filter collapse.

There also appears to be a couple of pressure releif bypass valves in the oil system, a 7 Bar at the oil pump and a 3.5 Bar in the filter housing I suppose if one of these failed to operate as designed an over pressure/flow condition could exist that might overtax the cannister/filter media and lead to a collapse due to an over pressurization state?