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  #31  
Old 08-13-2016, 07:09 PM
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I'm sure it doesn't matter on a low rpm diesel motor, but "evenly on each end" isn't correct...

What rods need for balance is... Even big end weight, and even little end weight... To other rods... So all little ends weigh the same, and all big ends weigh the same.


On high rpm motors, we used to massage the little end to the max, then adjust the big end to balance the throws...

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  #32  
Old 08-15-2016, 01:11 PM
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Once again demonstrating my lack of Education

How does one measure the weight of "Big"(Crank) or "Little"(Piston) Rod ends
individually?
Please don't be shy about LCD in descriptions!

thanks!
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  #33  
Old 08-15-2016, 04:36 PM
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Weigh the entire rod. Place it in this ( home made ) fixture and weigh again. ( and deduct for the holding fixture on the scale. )

Balancing an engine ( pistons / crank / rods ) is somewhat of a black art as the "perfect" balance at one RPM is terrible at another.

Also dig around on this site, lots of very high end engine builders. connecting rods weighing question • Speed Talk
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OM 617 Rebuild - Piston not perfectly aligned in Bore-rod_balance.jpg  
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  #34  
Old 08-15-2016, 05:02 PM
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I think one end on the same level as the scale being used and the other on the scale of course. Then reverse ends for a similar comparison. It has been a long time since physics but the un weighed end should be adjusted to keep the actual rod level for both weight measurements.

Also one can assume the engine ran previously to the rebuild with that bent rod. I would be all over that piston to see if some form of unequal wear exists because of that as well.. I believe it is only common sense to weight with the rod supported on the radiused sides..

These are just my thoughts though and I have never had to weigh rods myself. Chances are originally these engines may not have received the tight weight tollerances that Mercedes gas engines did. Or had perhaps a greater range of weight tolerance allowed. Simply because they do not rev that high in service but I may be wrong.

These are pretty strong engines and I was surprised to see the bent rod. They may be able to straighten it removing your weight concerns. Except this one may not be an original to the engine rod because of the markings you found.


The factory engine manual might very well provide better guidance. I do not have one. There may be one posted somewhere though.
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  #35  
Old 08-16-2016, 02:38 PM
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Thanks to '97 S L 320

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Propellers.

https://answers.yahoo.com/question/index?qid=20081229090156AAjAqFl
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  #36  
Old 08-17-2016, 06:32 PM
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Twenty grams overall weight difference was substantial.
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  #37  
Old 08-17-2016, 06:40 PM
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That was 20 G between two complete sets not 20 G within a set. And we don't know if the new rod set was a different design or not.
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  #38  
Old 08-18-2016, 12:49 PM
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The new rods were all more or less 20 grams heavier than the old one, but had all of the same casting marks. I can't access the FSM at the moment, but I recall reading that they are classed by weight from the factory and painted with a dot near the bottom based on weight class, and that no rod should be more than 5 grams from the others.

Based on the comments above, and seeing as I have 4 more of these, I am going to polish another one, using a jig like the one shown above to weigh the small end separately. The engine isn't back together, so there is no cost at this point in doing it right.

The procedure I intend to follow is to weigh the small end of both old and new, work on the new rod to get that measurement equal, and then weigh the entire rod and work on the bottom to equalize the total weight. We'll see how it goes ...
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  #39  
Old 08-18-2016, 03:09 PM
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If you need to take additional weight off after equalizing the small and big end, you can also remove casting seam material on the center I beam. That is if it has not been ground down already. It can be on one side as I've seen a few examples of these rods with one side ground down more than the other and a few that have equal sides ground down.
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  #40  
Old 08-18-2016, 05:41 PM
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Something that might not be completely apparent with the weight fixture I posted. The pivot points on each end must be free to rotate and reasonably in the center of the rod otherwise the weight will be affected. I'd also place the rod in the fixture in the same direction every time just to be consistent. ( RE front of rod facing you)
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  #41  
Old 08-20-2016, 02:41 PM
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UPDATE - Good as it gets for a beat up diesel

So I constructed a jig to measure the small end. I was advised elsewhere that for this application all I really needed to do was to get the small end weight right, then get the overall weight within tolerance. I realize this level of technology is far beyond the hopes and dreams of the non-nuclear scientists out there, but for future NASA engineers ... and after several beers ... I present the Pragmatic Small End Measuring Engine, Mark I (TM):



So for curious minds, I compared the old and new rods - identical casting marks, but the old, lighter one has clearly had work done on it. With calipers, I could tell it had been ground about a half millimeter on the top and sides.



So with a flap disk and a wire brush I ground the small end and got it within one gram. Yes, I realize my high-tech measuring jig isn't perfectly level, but the actual numbers are meaningless for this purpose, as long as the measurements were repeatable, which they were. Old:



New:



Overall weight came in ... after a bit of big end polishing ... within 3 grams of the old rod. And the finished, polished rod:



Installed, the piston sits perfectly:



Someone earlier had expressed surprise at the original bent rod. I'm going with the theory that the original was bored a bit off to account for a casting error and that caused it to continue to bend over time. It was definitely bent or otherwise out of alignment, though. You can see here how the piston was wearing at the top because it was canted ...



Someone asked above to show pics of the overall rebuild. So after fixing this I got the upper pan on, the crank seals in and timing chain guides installed. The machine shop did an awesome job with the boring and honing - I double checked with a bore gauge and calipers and all are dead on within the FSM specs, toward the lower half of the tolerance, which I requested. I'll post more when it is closer to done.

Short block rotating assembly:


Last edited by pragmatic; 08-20-2016 at 03:13 PM.
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  #42  
Old 08-20-2016, 04:11 PM
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Nice

Good Job !

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