FYI: '83 OM617a flexplate - not neutral balanced - PeachParts Mercedes-Benz Forum

KAdams4458 · #1 04-13-2013, 12:12 AM

Just picked up my flywheel from having it match balanced to the original flex plate from my '83 motor today. it turns out that the flexplate was several grams away from being neutrally balanced. I other words, my motor is externally balanced.

Folks, it is vitally important that you get flywheels and flexplates match balanced to whatever came off the motor. I've heard it repeated a few times that later model motors were internally balanced, but that certainly didn't prove accurate for my motor.

It's just not worth the risk to blindly swap these things. For the $50 - $75 it costs to get something match balanced, you get real peace of mind. Vibrations are annoying, but destroying a motor will really screw up your day. I hope this helps someone else on the fence about having stuff balanced.

/public service announcement

t walgamuth · #2 04-13-2013, 08:05 AM

I second the above post. I have had at least five benz motors apart and four were neutral, one not. It appears the factory built them with the crank balanced internally when possible but if a crank was too far out to balance alone they balanced with the flywheel, so its nothing to fool around with.
1. Mark your crank and flywheel when disassembling.
2. Check your old flex plate or flywheel for balance. If its neutral you are good.
3. Check your new fw for balance. If its good you are now good.
4. If either is not neutrally balanced the new one must be match balanced to the old one.

The checking costs even less than balancing.

Shortsguy1 · #3 04-13-2013, 08:47 AM

As a newcomer, I was confused by something in the previous two posts. I made the mistake of thinking that "flex plate" is the same thing as "flex disk." In case there are others like me with limited knowledge, these are not the same.

t walgamuth · #4 04-13-2013, 10:00 AM

We are talking about the flywheel like part that is used with an automatic transmission, the presumption being an automatic to stick conversion. The above tasks must be done also if a flywheel from a stick is moved to a different motor with a stick flywheel as well if that should occur.

barry12345 · #5 04-13-2013, 10:21 AM

I wonder if one of us should contact the classic centre to see what was the situation during production of the 123 diesels. A few grams might be within the manufacturing tollerances of the flexplate. There are about 28 grams in an ounce. I knew or believe during the earlier gas era the balancing was done including the flywheel.

JB3 · #6 04-13-2013, 10:26 AM

x3. Another example in 3 stick conversions ive done for my DDs, 2 were neutral, one was weighted for 617s. For the 616's though, it seems much more likely they all are neutral, at least I haven't come across one that wasn't yet. Worth testing on those as well.

Its a roll of the dice gamble thats not worth it for all the PITA involved to pull it back out and correct the issue. There also really is no way to track what engines have an external balance, and what engines have an internal balance by external part numbers or anything unless the classic center has some way to track they can tell us

t walgamuth · #7 04-13-2013, 11:30 AM

The one I had that was externally balanced was my euro 280e. There were husband marks on the fw and crank but they were very very light like a hologram and it took the young sharp eyes of my favorite machinist's son to spot them. I could hardly see them at all even after he pointed them out to me.

KAdams4458 · #8 04-13-2013, 12:31 PM

For what it is worth, my 240D flywheel was apparently not neutrally balanced, either. However, it had already been hung on what was reportedly an '80 300D motor, so it might have been rebalanced as part of the prior conversion. I could find no witness marks on it, so I suspect it may have simply been part of a hang it and hope for the best conversion.

barry12345 · #9 04-13-2013, 01:43 PM

If the flexplate is neutral the flywheel should be made simular unless only a gram or two is involved I suspect. Again there wil be production tollerances that wil have to exist in manufacturing. Actually the balance should be with the clutch and presure plate involved as well technically . We take it for granted that even new components will be balanced properly.

97 SL320 · #10 04-13-2013, 09:38 PM

There is some terminology that is getting foggy here, in the USA engine machine shop / engine build world, the terms mean:

Internal balance: The crank can be balanced with no other attached parts during the design process. The vibration damper and flywheel are neutral balanced ( no counterweights on them. ) This is the _design type_ and not the _balancing process_ or where balance holes are drilled.

External balance: There isn't enough space in the crankcase to add more weight to the crankshaft during the design process to bring the crank into balance. Weights must be added to the vibration damper and flywheel to accomplish this. This is the _design type_ and not the _balancing process_ or where balance holes are drilled.
( 360, 400 , some 440 Chrysler, 400 Chevy , 302 Ford to name a few )

Balanced assembly: The vibration damper , crank and flywheel are all bolted together and balanced. This is a must do on external balanced engines unless you allow for the missing counterweights. It is a can do on internal balanced engines. The " proper " way to do this is to balance each item by it's self then bolt together for a final check. In an production environment this added work gets expensive.

I'm guessing MB decided that the balanced assembly was a better route to go and this is sometimes incorrectly called external balance.

KAdams4458 · #11 04-14-2013, 12:11 AM

Sounds correct, but I'll be darned if engine shops around here use the terms as intended.

For giggles, I would say that I maintained my balanced assembly by having the new flywheel matched to the original flexplate. (Okay, not really the flex plate, because that actually refers to the thin steel triangular bit bolted to the back of the cast monster with the starter ring gear on it.)

The shops around here go full racehead when you bring up a balanced assembly. They want it to turn 10k and have the friction disc and pressure plate matched up, too. Then again, they still don't seem to own any metric tools. Ah, America. LOL.

4x4_Welder · #12 04-14-2013, 01:37 AM

The weights talked about here really don't sound like enough to make a difference- The weight I see thrown out there the most is 12g, but it hasn't been made clear if this is a flat total static weight or the dynamic which should be oz/in or g/cm. 12g/cm is right about 1oz/in, and the back of the hand calculations show a tolerance of about .9oz/in for that engine (if someone can give me the actual crankshaft weight I can give a more exact number). If it's 12g/in, then that is well within tolerance.
This isn't a racing engine and doesn't see high enough rpms to require such careful balancing.

Stretch · #13 04-14-2013, 02:59 AM

Quote:

Originally Posted by 97 SL320

...
Balanced assembly: The vibration damper , crank and flywheel are all bolted together and balanced. This is a must do on external balanced engines unless you allow for the missing counterweights. It is a can do on internal balanced engines. The " proper " way to do this is to balance each item by it's self then bolt together for a final check. In an production environment this added work gets expensive.

...

The process can indeed get expensive if every part that is made is balanced and checked and re-checked when fitted. But I think this expensive method can be made to be a cheaper method by ensuring that parts are made to a decent standard in the first place. If you have good solid - homogeneous - lumps of metal then you are half way there!

Also as mentioned here =>

Quote:

Originally Posted by 4x4_Welder

The weights talked about here really don't sound like enough to make a difference- The weight I see thrown out there the most is 12g, but it hasn't been made clear if this is a flat total static weight or the dynamic which should be oz/in or g/cm. 12g/cm is right about 1oz/in, and the back of the hand calculations show a tolerance of about .9oz/in for that engine (if someone can give me the actual crankshaft weight I can give a more exact number). If it's 12g/in, then that is well within tolerance.
This isn't a racing engine and doesn't see high enough rpms to require such careful balancing.

<= and where the extra weight is - is also important.

Manufacturing is a compromise.

Stretch · #14 04-14-2013, 03:04 AM

Quote:

Originally Posted by 4x4_Welder

... (if someone can give me the actual crankshaft weight I can give a more exact number)...

I measured that once - I'll see if I can dig it out.

t walgamuth · #15 04-14-2013, 07:05 AM

I have lifted the 617 crank and would guess about 65#. You can pick one up if your feet are planted well but you wouldn't want it to fall on your foot!