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I hope everyone had a merry Christmas.
This question may seem a bit beyond the resources of most, but possibly someone knows of an old motor guru that is able to "solve this riddle".
I recently picked up a friend's Mercedes D.IIIa, for rebuilding/overhaul. The motor is completely original with all matching parts, (other than the carb). Upon inspection of the main bearing, I first thought someone had ruined the bearing with some strange idea, but upon inspection of my old original main bearing and the connecting rod bearings, I concluded this "damage" was actually an intended "modification" by Mercedes.
The "modification" I refer to is a widening of the bearings on the sides.
Before I go into detail, first let me clarify the standard D.IIIa design.
The D.IIIa crankcase is split or divided horizontally into a top and bottom half cover, with each cover containing seven "half" bearing shells to support the crankshaft. That way when you put the two halves together with the crankshaft in the middle, you have the basic crankcase assembled.
The design of the main bearings is; two bronze half shells with a babbitt lining. The main bearings are feed oil through the center of the bottom bearing shell. The bearing shells or halves have 45% beveled "pockets" along each side of the bearing shell half. Where the two halves come together it forms a large oil pocket. The important thing about these oil pockets is they do not extend the length of the entire bearing. Say, for example, the bearing is two and a half inches long, the side pocket would be two inches long centered on the side of the bearing. This is normal standard practice for bearings of the period. Today, they did away with these "pockets" and they have the inside of the bearings, a perfect circular surface.
Now for the strange "modification". For some good reason, our friends at Mercedes decided to add a special "touch" to the bearings. As you recall, we have the side "pockets". They come down the side of the interior bearing half shell surface about 1/8th of an inch, and do not extend the length of the bearing, so as to form a "pocket". Well, after all of this was done a workman would come back, and hand scrape all of the bearing halves wider. By this I mean, on the sides where the "pockets" extend about an 1/8th inch down, the workman would scrape down about 3/8ths of an inch down on the entire bearing sides at about a 70% angle, right over and including the oil "pockets". Normally, this would be the opposite of what you would want to do with a motor, because you are now giving the oil pressure an easy way to escape from the main bearings and the crankshaft. In principle, you would be lowering the oil pressure to the crankshaft, but you may get more volume of oil to pass through the bearings. The connecting rods have this same widening of the bearings.
To further explain the oil system, there is an oil pump, with no pressure relief valve. Each main bearing is fed oil by pipes coming from the oil pump. The oil goes into the main bearing and the main crankshaft journal fills up with oil, and there is a passage going from the main crankshaft journal to the connecting rod journal on the crankshaft. So each main feeds a corresponding con-rod. The oil than passes up a tube on the con-rod to the piston wrist pin, and passes out into the crankcase sump and back to the oil pump.
I do not know of any good reason for doing this widening of the bearings, but Mercedes did.
I know this is way out there, but I would appreciate any input.
My only guess was it had to possibly do with oil viscosity during the winter months, or at altitude, and it allowed easier movement of the oil so the pump wouldn't be over stressed.
By the way, all but one of the bearings in my friend's motor appear okay, the number two top half bearing shell is scorched, and the babbitt began to blister. This is exactly what I would guess would happen from the widening of the bearings, the top half of the main bearings would get very little pressure. Especially the front bearings since the oil pump supplies the bearings from the back forward, and the number one bearing may get the least pressure.
Thanks for any assistance,
Dave Watts
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