Very Big Snip
But I do like return line filters as they are better than relying on a bit of gauze.
I guess what we really need is a window in the timing cover so we can still look at that rather expensive blue device pumping the engines lifeblood rather than it being hidden away...
I had expected substantially better knowledge of the BSA filtering system from you.
The gauze in the sump is there to protect the oil pump.
If it can get through the gauze it will pass through the return side of the pump without damage to the pump.
The gauze in the oil tank outlet is there to protect the pressure side of the oil pump.
If it fits through the gauze it will pass through the oil pump without damaging the oil pump.
The gap between the main shaft & main shaft bush is in the order of 0.0001" to 0.0005", so any particle bigger than that can not get into the bush to do any damage and will pass through the hole in the timing bush into the crank.
The same thing happens at the big end.
Only particles bigger than the clearence between the slippers and the journals can pass into the bearing, the remainder will pass into the FULL FLOW CENTRIFUGAL OIL FILTER , commonly incorrectly termed the sludge trap.
here anything that has a heavier specific gravity than the oil will be flung out of the oil and FORM A SLUDGE in the sludge trap.
Or to put it another way, the sludge trap creates a sludge it does not filter sludge out .
The oil then gets pumped back up to the SETTELING TANK where the output on the top allows volatile contaminats ( like water & fuel ) to exit via the breather. Particles with a lighter Specific Gravity than oil will form a scum on the top of the setteling tank and particles with a heavier specific gravity will drop down below the outlet level.
Eventually the lighter particles will agglomerate ( if you are not using a detergent oil ) into heavier lumps and sink to the bottom of the settleing tank.
Thus each and every part of the 4 level filtering system has a specific job and does that job relatively well when the motor is being used within the design parrameters.
Note, a paper filter mearly filter everything out in one hit, by particle size alone . typically 50 to 120 microns.
However unlike the BSA filters, it does restrict the oil flow which is why it is best put in the return line as the gear pump used by BSA in not a particularly high pressure pump and fairly low volume and never designed to be hooked up to a cartridge filter.
If we have a good think, we will remember many people on this list stating that after fitting the spin on filter, their sludge trap was empty so all the external filter is actually doing s replacing the internal one.
No matter what external filter is fitted, you still need the 2 gauzes to protect the oil pump.