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For those of us with replaceable element, cartridge-style, bypass-type AC or Fram oil filters, this is some important information that I recently discovered.

As you know, bypass-type oil filters do not filter all of the oil before the oil is pumped up into the engine. Rather, they only filter a small portion of the oil and return it to the oil pan. This has some benefit. Since the oil filter is not required to handle the full volume of oil delivered to the engine, it is able to filter the oil very finely and remove very small particles from the oil that a higher flowing full-flow oil filter cannot.

Bypass filters are usually rated at around 5 microns nominal and 25 microns absolute. Full-flow filters are usually rated at around 18 microns nominal and 36 microns absolute. Full-flow filters are coarse rock-catchers compared to bypass filters. They have to be, in order to handle the full flow.

The problem is that every bypass filter made by Wix (owned by Mann+Hummel) that I checked on, including the #51006 used by most postwar Packards, is rated by Wix at 32 microns nominal, which is awful! I thought this might be a typo error, but looking closely at the filter media, it appears to be the same type of coarse media used in a regular spin-on, full-flow oil filter rather than the finer media used on a proper bypass filter.

That is not what we want. A bypass filter that doesn't filter any finer than a full-flow filter might as well be no oil filter at all.

I checked with Baldwin. Their equivalent to the Wix 51006 is the P41. It is rated 5 microns nominal and 25 absolute. This is fine enough to filter out soot, sludge, and even water. That is what we want.

So just be on the lookout.

That's excellent information. One additional thing to consider: The bypass type filter canister itself will trap larger particles of carbon and other debris and water. The flow throw the canister is slow enough that debris can settle to the bottom. It's important to clean out the canister to the very bottom when changing the element. Some canisters have a drain near the bottom, and the bottom is usually conical so as to trap more debris. I used to have a 1952 Cadillac with a very dirty engine. The bypass filter canister would trap a remarkable amount of sludge and small carbon flakes. It would be worthwhile to check the bottom of the canister for debris from time to time even if you don't replace the cartridge every time. In the case of my Cadillac, the debris that settled to the bottom of the canister tended to stick together and remain there. In my opinion, the correct Baldwin filter element is the best choice. Removing fine soot and metal particles from the oil will help prevent the formation of deposits. But even using a less efficient filter element will be beneficial by reason of the ability of the canister itself to trap debris and help cool the oil. Sticking a small, powerful magnet to the lower part of the canister might be a good idea, too.

That is a very good point. On mine there was a lot in the filter bowl after 500 miles on the rebuild, to be expected and why you have a few quick changes after one. Mine has no drain so I stick a clear piece of vinyl tubing in and simply put my finger over the end drawing out the remains. I like the clear tube as it flexes and I can see the oil being drawn up through the tube's wall. (I guess I could add a drain but this works and is original to the engine.)
Once the liquid is gone, a thorough cleaning with brake cleaner and shop towels preps the bowl for the next round. Their is no chance of introducing the cleaner to the oiling system as the entry line is up near the top and the return line is inside the core tube isolated from the bowl.
Bob J.