Mercedes Madness

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

[BillyH]

Dave: Can you give us a little diagram?

[Rick]

Or even better, a cut-away three view sketch?

Hi guys,

I knew this question would be confusing. I'm sorry, but I can't do a scanner posting, so I will try to key in a rough sketch.

This is the top view of one of the main bearings halves, as prepared by Mercedes, BEFORE they come back and widen the bearing. In this case it is the bottom half, and therefore the "O" in the center representing the oil feed coming into the middle of the bearing.

I I
I /I
Il lI
Il O lI
Il lI
I/ I
I I

The "I"s are the outer sides of the bearing.
The "ll"s are the inner edges of the "oil pocket".
The "/"s and ""s are the ends of the pockets.

Remember the oil pockets do not run the entire length of the bearing. The oil pockets are formed by machining the inside edge of the bearing at a 45% angle.

This would be a inside view of the above bearing as seen from the inside, side view.

______________
---------------/


______ ______

You can see the oil pocket at the top, and the opening for the oil supply at the bottom.


Now what they do is come back and scrape the bearing sides the entire length of the bearing and about an 1/8th of an inch lower than the oil pocket. This basically "relieves" the bearing and allows the oil to escape out the front and back of this "relief channel".

As shown below, they scrape about 15/1000ths wider on the inside bearing surface down about 3/8ths of an inch on both sides of the bearings. The oil pocket is still there, as it is much deeper, but it has been "relieved" on the ends by this additional scrapping.
______________
---------------/
----------------------

______ ______

Sorry for the crude sketches, and I hope it gets the idea across. If more clarification is needed, simply ask.

Best,
Dave

Hi Dave!
This sounds like an attempt, as you suggest, to eliminate overpressurization. Did you see my references to Caquot's resolution of the similar problem on the 220 H-S's? With 10,000(!) 220's being considered unacceptable for service, he was called in to find the problem. He tested a group of engines; "salade" is the interesting term used to describe the appearance of the failed internal components. He installed a relief valve to eliminate the fracturing of the transfer tube which fed oil to the reduction assembly. This thin-walled tube tube burst- when the engine operated in cold regimes- as the pump kept pushing the "tar" into the tube. I doubt if any of the modified engines arrived at U.S. units prior to the war's end. Thus Hartney's "Damn those SPADS!"
The same consideration plagued the P-38 in England: the oil coolers froze up creating "The Allison Time Bomb" and as a consequence they were transferred out of that Theatre and shipped to warmer climes in No. Africa and Italy. They re-appeared in England later- possibly with mods or perhaps they were limited to the Summer months? The P-40s and P-39s operated in ie.Russia, without notable problems so perhaps it was a Lockheed design flaw in the associated subsystems rather than a engine problem per se.
It appears as if the later designers- at least at Lockheed- didn't learn from the problems of WWI engine operations at significantly reduced temperature. Perhaps the information was something the French considered not to be disseminated?
Did you get your metric tubing? "Maryland Metric" in Baltimore supposedly has it in inventory.

Happy New Year! Lee

Hi Lee,

Thank you for the insight into the Hisso problem and resolution. This certainly supports my thoughts and makes sense. I am guessing the relief was needed for cold starts more than for high altitiude effects. Was that the case with the Hisso?

One other little fact on the Mercedes oil system; there is a "pressure regulator" on the oil pump. This "regulator" is simply a small piston with a coil spring under it. When the oil comes out of the pump it goes into a chamber above the "regulator" piston, and since the oil is coming in surges from the oil pump, the spring loaded regulator will "dampen" these surges and make the pressure more constant to the system. If the oil became to thick or the pressure too great, the regulator would become ineffective and "bury itself", and than cause damage to the bearings.

Gotta run,
Dave

Sorry, I was in a hurry, I should have been clearer in the fact that the oil pressure would "bury" the regulator piston, and then the high oil pressure would damage the bearings or burst a connection in the oil system and result in a damaged engine.

Best,
Dave

Dave:
The material on the 220s failures doesn't clarify whether the problems could be attributed to cold oil at start-up or high altitude operation at cold temperature. The problem was quite apparent there in the Summer and Fall of 1918 when Hartney "damned" the XIIIs so it was quite possibly a problem not of seasonal cold weather but of the nature of cold oil viscosity at start-up. The problem evidently existed only in the 220 (geared version) of the Hisso so apparently it was not related to the same problem facing whoever modified the MB: ie he had a concern with bottom end overpressurization whereas the Hisso only lost the lube to the gear reduction housing and no mention has been made- to my knowledge- of bottom end problems with the H-S engine of any model. I see some mention of the reduction assy. being removed by the Brits and an development of a cover plate and prop drive adaptor which enabled the engines to be run in a direct-drive mode. Too bad we can't go back a few years and ask some questions. I am always impatient with the emphasis on "Ace-istics" which has always cost us- over the last eight decades the knowledge we might have acquired from the technical types who waged a mighty- and most interesting- war of their own! Maybe it's not too late to get a N-28 wing into a wind tunnel? I'm going to comment on that thread when I get a moment or two. Have a great 2001!....... Lee