Hi all,
I just picked up a 1971 250c which I believe came with the 2.8 liter I6 (correct me if I'm wrong). The motor has been replaced with what I was told is a 1968 2.5 liter I6 (but I haven't confirmed this yet). The problem is the motor runs very rough and the previous owner gave up on correcting it. He was told the problem is that the older motors were externally balanced and the newer (1971) ones were internally balanced. The motor currently has the 1971 harmonic balancer and flywheel on it as the other flywheel didn't fit the '71 automatic transmition. I don't have the original engine or the '68 flywheel. My thought is to pull the motor, remove the crank, flywheel and balancer and have them balanced as an assembly.
Anyone have any insight to this situation before I start tearing things apart?
Thanks,
Darren

I have done this before. It is a bit tedoius, and requires pulling out the transmission.

You can balance the flywheel by trial and error if you can attach some test mass to the flywheel and start and run the engine without the test mass flying off the flywheel.

When I did this, I was balancing a manual transmission flywheel which has the friction plate surface recessed in the flywheel. This provided a cylindrical wall that made attaching the test masses (coins) easy. I could just tape them on to the inside face of the cylindrical wall.

When the engine was started, without the transmission, etc. connected, you can feel the imbalance quite easily as you gently accelerate the engine between ~1700 rpm & idle @ ~950 rpm. It will feel like a shudder.

By trial and error, you will quickly zero in a quadrant that works best for the first test mass, then you vary the number of coins and type until you can feel no improvement when you add or take away a dime or nickel.

Then you leave that mass there and by trial and error find which quadrant orthogonal (90°) to the first wants mass added. Then just as before, find out by trial how many coins are needed to get the best as possible result.

By now, the shudder should be quite improved. It may not be perfect, the harmonic balancer is also part of the dynamic balance, and you should be working with the one that came off the '68 motor if possible, but my experience has been that the flywheel has by far the largest effect.

Now, back to the balancing. Once you have the two orthogonal test masses on there and know there positions, all you have to do is duplicate their effect by drilling into the flywheel directly opposite their positions (meaning diametrically opposite on the flywheel). This again is done stepwise by trial. You will also note the drillings that were done in previous balancing. There will be holes that are positioned at 90° to each other.

This method will work to yield substantial improvement. You will want the car up on jackstands. You'll have to support the rear of the motor as the trans will be out. There may be enough of a lip on the automatic version flywheel to keep the coins from getting slung off. If not, there are some activated cyanoacrylate glues that sort foam up with the activator and harden almost instantly that might aid in keeping the test masses stuck on.

Good luck. The standard method of taking the motor apart and balancing the crankshaft assy alone is probably not what you would be interested in.

I've heard from various sources that all inline sixes made after the M180/M127 (2.2L-2.3L/2.2L FI) are internally balanced, this would include the seven main bearing M108 (2.5L), M129 (2.5L), M114 (2.5L) and M130 (2.8L); is this not the case?

I've got the motor running quite well and am pretty sure it is out of balance. The whole car vibrates and at certain rpms it is worse (I haven't driven the car yet, this is just running while parked). I did a compression test and got 135-145 psi in all cylinders. The previous owner tried changing the harmonic balancer to the '71 version and the problem got worse. I am going to change it back, luckily he kept the '68 one. He gave up on dealing with the issue and I'm having a hard time getting consistant information about which motors were internally or externally balanced.
Can any one help me verify the engine in the car is a '68 2.5L? Where do I find the stamped numbers? The car definitely had a 2.8L to begin with.

I've been doing research into the various sohc sixes because it's related to an engine swap project I'm considering, so while I've read a lot about them I'm far from an expert. I think it would be first priority to establish which engines are supposed to be internally balanced and confirm exactly what you have. Second, do you have any evidence of the current engine's history other than say-so? Aside from external issues mismatched internals could throw things out of balance as well. I know it's not much help but good luck.

Correct ....all 7 mains engines were internally balanced.

I assume that just because they are all externally balanced doesn't mean they are all balanced the same. ie. if the 2.8L has a longer stroke, the flywheel and damper will be differnent and this may be the cause of my problem?

Internal balanced simply means that the damper and flywheel are balanced by themselves before assembly. This means they are not considered in the engine balance and can be changed without regards to upsetting the engines balance , regardless of stroke or displacement. You are just adding weight that is already in balance...whereas externally balanced engines are balanced with the flywheel/damper bolted up before the balancing of the unit as a whole is done. In that case , one can not change a flywheel without re-balancing it to the original. This is done by bolting the old to the new , face to face, 180 degrees out and balancing as a unit. That way , when they are taken apart, the new will be balanced the same as the old [ which is usually out of balanced by itself] bc it is just a part of the crank unit on the original balance.
Internal balanced is much better when it comes to changing parts as they are not considered in the engines balance, ie-they are Neutral b/c they are balanced by themselves..

Forgive me, I'm new the the world of MB but this seems backwards to what my prior engine experiences have been. The mopar 360 v8 for example is externally balanced and has weights added to the torque converter to put the engine in balance. The same torque converter can be used on an internally balanced 318 engine if you remove the added weights (thus making the TC neutrally balanced)
My interpretation of internally balanced is the the crank counterweights are "adjusted" to provide balance and the flywheel and balancer are "zero balanced" (neutral). Externally balanced means that the crank is not fully balanced on its own and requires counter-acting balance of the flywheel and damper to put the whole assembly into balance.
Again, I'm just trying to get this straight.

That is correct ..my typo..and corrected

The '68 250 engine would be a 114.920 and that would be a seven main bearing engine.
The engine # should be on the left side of the block, down a couple of inches from the #5 and 6 spark plug.
You may also want to check the timing chain tension as that will set an engine vibration on these IL/6 engines. You can push the tensioner down with a large screwdriver. It may have to be bled..

The chain is very tight. Are you saying too tight causes problems? If so, can you be more specific on correcting this?

Too loose usually is the cause of vibrations, but I guess it could be too tight if the tensionor is frozen in full out/extend position.

Try this:

Turn the engine over by hand [ a socket on the crank pully,turning in the normal running direction only] until the index marks on the cam gear and cam front tower are perfectly aligned .. then look at the timing marks on the damper and see if they read zero or close ..if the chain is stretched , it will be off a few degrees there..that would indicate a loose/worn chain and will cause engine vibes..

OK, I just went out to the garage and check the block ID. It has a 180 then 9_ _ (I can't see the next digits but I assume 54). Also, I see a giant 230 cast into the block which through my brilliant deduction tells me this is a 2.3L motor!!!

Does this mean it is internally balanced after all?

My guess is a 180.954 from a 114.015 chassis 230/6.
I think that is externally balanced engine.