Just my 2 bobs worth for interests sake. I'm like Desco, always remove and clean the screens, never had any trouble. That's been on a mixture of pre units, a T150V and 81/82 T140E's. Though I suppose it must have happened occasionally, since that was the reasoning behind the 4 valve oil pumps on the later T140's.
Ones point of view can often be described as looking through a funnel. Some are looking from the wide end focused on an specific event. Others are looking from the narrow end looking at a many events. From which end of the funnel you are looking you can come up with a much different opinion of something like whether you should specifically remove the crankcase drain plug and screen.
If you are anal enough to make sure you carefully clean the area around the plug, and make sure it is spotless, and you don't fumble around trying to put the plug back in the hole, and lucky enough without picking up a speck of dirt, your view of this would be much different than a dealer who has been exposed to the experience of hundreds of owners over a lifetime. In my experience over some 50 years of doing this when we see a pump that has failed because of something under a check ball the answer to, "Did you recently change the oil and remove the sump plug to drain the few ounces of oil in the crankcase?" the answer was almost always, "Yes!". Is this a double-blind scientific test, no, but it certainly leads me to believe there is more than a casual relationship with the behavior of removing the sump plug... just my opinion from looking up the funnel.
...yes; it s all about cleanliness. It s just that and then change the oil in time. I have a 48 pre unit so the engine parts does not has a perfect fit, etc but when I rebuilt it I triple checked for some debris, etc. Is the best that I can do; because I cannot rely on the manual technical aspect due that the parts have too much wearing.
I drain the oil then clean with kerosene. If I need to unscrew the sump plate, I clean it with kerosene then thinner to put silicone.
But mine for some reason, that still is a guess (I put a thread here few months ago about it, but I do not found it anymore...) leaks gobs of oil; in great % for the external oil passages and the other % for that unknown place near the base of the head; so after a long trip, like I did yesterday (800 kms )I need to refill the tank with about a liter!; so sometimes I do not do the oil change (draining the tank), only refilling... Im very proud of this motorcycle, no matter that big leak, because without any doubt it s one of the few pre units of the 40s (or may be the one)in the world that really keeps on putting kms and more kms in long trips at a strong cruising speed and still kicking.
--hey John Healy, why you do not answer my PM s? I tried to buy some parts from you...you know, may be it s good to sell parts to a real rider.
Almost a year on from this post, I picked up a LiquiVac based on leon bee's advice. You can order on-line and get free shipping, but I found that Menard's sells these in their big box stores for the same price.
I couldn't get the thing to develop vacuum at first, but the on/off valve just hadn't been pushed fully into place at assembly. After that, it worked very well. Thanks for the tip, leon bee!
'64 TR6R Plus some Twins from other countries (U.S., Germany, Japan)
I had a gmc 8v71 diesel in my boat and oil was sucked out at marina thru a 1/4 copper pipe thru dip stick hole. On my 4 cylinder small boat gas engine 4.1 . I just use a small bilge pump with 1/4 copper pipe thru dip stick hole. Have engine warm and pump for a few minutes or until pump signals all oil out. Same on my triumph.