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Old 12-07-2012, 03:11   #1
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Crankshaft replacement, does it need balancing?

Ahoy, I am rebuilding my Mercedes OM615 engine and discovered that the crankshaft was worn beyond its regrind limits. So I bought a new one from Mercedes and now they tell me it must be balanced, with the flywheel and pistons etc!!? Very hard to do here in Langkawi, Malaysia.

They say that it has been "nearly" balanced, but should be rebalanced with my components before fitting? I thought that this was only really needed in high performance engines?

Any advice as to fitting the crankshaft without balancing the whole assembly?
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Old 12-07-2012, 04:12   #2
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Re: Crankshaft replacement, does it need balancing?

The simple answer is "you cannot do it without balancing machinery and weights".

The complex answer is the same.

Do you have any machine shop services there? If so you can minimize the crank balance issue with other work.

In the past when I replaced a crank I also replaced the pistons and occasionally the rods for a matched weight set. This makes up one part of the balance formula. I would have the block bored over to round the cylinders, again making everything as close to uniform as possible.

The crank has been setup for typical block by Mercedes, you should have the matching bearings too. That is close enough to run without coming apart.
If there is a Mercedes shop somewhere around you could ask them what they do.

I would put it together and not worry.
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Old 12-07-2012, 07:09   #3
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Re: Crankshaft replacement, does it need balancing?

Thanks Me and boo, much appreciated. Yes, it is at a machine shop, with a lathe etc, but no balancing gear. The flywheel and damper plate are already balanced, the motor is 55hp, 4 cylinder diesel, crank has 5 main bearings, looks really solid and already bored on the cranks to balance. Have new bearings, con rod and mains and pistons and rebored block and added liners.

I will put it all together and hope it does not develop too many vibrations???

Cheers from Keith.
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Old 12-07-2012, 07:29   #4
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Re: Crankshaft replacement, does it need balancing?

In "the good ole days" motors were balanced by each individual part(s) The fly wheel, the crank shaft, the front pulley damper and clutch plate if applicable. And each piston with rod and bearing installed, weighing all the same.

I'm not sure how one could balance a motor once it's assembled.
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Old 12-07-2012, 07:41   #5
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Re: Crankshaft replacement, does it need balancing?

Ya can't balance an engine thats put together!! way back when, engines were balanced on knife edges, this was acceptable, but not as good as electronic balancing. you can only hope it will be ok and will not shake enough to tear it's self apart !! just my 2 cents. to bad the shop don't have an OLD shop hand who could have done the job the old way
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Old 12-07-2012, 08:14   #6
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Re: Crankshaft replacement, does it need balancing?

I have reconditioned 6 motors over the years, new pistons, bearings, sometimes new crankshafts, but this is the first time I have ever been told to balance the crankshaft with weights clamped onto the journals to represent the pistons and conrods etc?? Seen how it is done on utube, but those machines dont exist here in Malysia!!

Is Mercedes being to precious in insisting that I balance the crankshaft before assembly?? Will it really self destruct after a while, it does have a harmonic balancer on the crankshaft pulley?

What is the experience of other engine rebuilders out there?
A big thanks to those who have spoken up on this thread already!
Keith.
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Old 12-07-2012, 09:51   #7
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Re: Crankshaft replacement, does it need balancing?

Balanced is balanced, period. If you replaced the crank of an engine that was balanced, it needs to be re-balanced, period. That means the clamp on weights and all ! if ya cant do that then ya pays your money and takes your chances it wont shake to badly ! I wish I could be nicer about it but it's the real truth ! I don't see that you have any other choices left to you other then sending all the nessasary parts to a place that can balance it for you !! sorry
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Old 12-07-2012, 12:33   #8
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Quote:
Originally Posted by surfmachine
I have reconditioned 6 motors over the years, new pistons, bearings, sometimes new crankshafts, but this is the first time I have ever been told to balance the crankshaft with weights clamped onto the journals to represent the pistons and conrods etc?? Seen how it is done on utube, but those machines dont exist here in Malysia!!

Is Mercedes being to precious in insisting that I balance the crankshaft before assembly?? Will it really self destruct after a while, it does have a harmonic balancer on the crankshaft pulley?

What is the experience of other engine rebuilders out there?
A big thanks to those who have spoken up on this thread already!
Keith.
There are plenty of racers in Malaysia. I mean automobile racers. Look for a performance auto shop in or around KL.

You are spending the money on what is presumably expensive mercedes parts. It is like painting. 90% in the prep. You can buy all the new engine parts you like but that does not necessarily mean a great build.

In a 2 cylinder small diesel I might be tempted to skip balancing or do knife balancing. In a 4 cylinder engine at this horsepower I recommend doing it right. The haramonic balancer is sufficient to absorb power pulses. Imagine each ignition event putting torsion into the crank via the conrod that is what the harmonic balncer takes care of, not natural imbalance.

A crank this long can be out of balance in multi planes - i.e. the balance at the front different fron the balance at the back. This inputs torsion into the crank from the get go. I am betting the mercedes procedure is at least a two plane balance.

Get the service manual for balancing procedure, get to the race shop and make a deal to get the "heart" of your engine build right.
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Old 12-07-2012, 14:00   #9
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Re: Crankshaft replacement, does it need balancing?

Seems to me there is a bit too much paranoia here. You're not racing! This is a relatively slow revving diesel. What difference a no. 4 piston not the same weight as no. 2? or conrods each a little different in weight? When no. 1 fires, the "weight" on that journal from the explosion as it throws the piston down and the conrod off to one side is a great deal more than any other forces on the other cylinders which will be in differing stages of intake suction/exhausting or compression loads, so what need a balanced crank assembly? You'd be far better off checking clearances of bearings/ring gap/valve train etc. to be sure you've done a good clean rebuild. Fit a new oil pump if you can, and I would vouch you'd be fine. I owned and ran an automotive machine shop for years and never in all my engine rebuilds had the sub-assembly balanced, and never had a vibration problem.
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Old 12-07-2012, 20:48   #10
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Re: Crankshaft replacement, does it need balancing?

Ahoy shipmates, thanks for all your input!

It is so tempting to reassemble the motor without a final balance!!!
What is the most likely scenario, a cracked crankshaft?? Or just a guilty conscience??

I know that Mercedes is very anal about the tolerances and balancing of all their engines, but so far the advice seems to be balanced for and against balancing, ha!!

Anyone else with direct experience of assembling unbalanced engines?

It is just getting too hard with this flamin motor, the malaysian mechos have jerked me around for 2 months and I finally bite the bullet and buy the high end stuff from Mercedes and they want rocket scientists to assemble the motor and take the crankshaft to Canaveral where they will work their magic!!

Thanks from Keith.
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Old 12-07-2012, 21:19   #11
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Re: Crankshaft replacement, does it need balancing?

Well I worked in a balance shop in cali for a good while! and balanced many engines both gas and diesel. Mercedes were one of the most critical about balance in fact there cranks come un balanced and require a true electronic balance to be right! you might never have a problem, but for what your spending for the right parts I would sure look for a race shop as was suggested in an above post!! it would be money well spent!! just my 2 cents from a few years balanceing engines.
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Old 12-07-2012, 22:03   #12
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Re: Crankshaft replacement, does it need balancing?

Thanks guys........
I am not worried about the cost!!? Well... But carting a crankshaft and pistons through two airports and back and the week it will take is a daunting task.

I cant find any balancing machine shops on the net in KL, or Penang, any suggestions?

Keith.
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Old 12-07-2012, 23:09   #13
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Re: Crankshaft replacement, does it need balancing?

Im pretty sure RACE SHOPS were mentioned above might try that in the phone book or internet sounds like a heck of an idea to me ! it's the first place I would look !!
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Old 13-07-2012, 02:20   #14
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Re: Crankshaft replacement, does it need balancing?

Assemble the frikkin" engine already! You're using the same (balanced) flywheel, the same conrods, perhaps the same pistons. I never heard of a new crank so out of balance that it needed special attention. How did they get it ground to spec if it was vibrating so bad in the crankgrinder from out-of-balance when they machined it!? And if merc is so hot, why do they sell cranks that need user balancing? Don't get intimidated by merc's opinion that their engineering is any better than toyota or anybody else.
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Old 13-07-2012, 03:39   #15
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Re: Crankshaft replacement, does it need balancing?

Ahoy Holmek, just what I want hear, a hairy chested reply!!!Of course I dont want my motor to self destruct, but I just cant see how I can get it balanced without a huge expenditure of time and money and airplane travel on my part.

Stuff the precious merc engineers!!! I understand what you are saying mate. They just keep telling me that it MUST BE balanced, or the sky will fall!!??

Do toyota and mitsubishi and ford cranks need balancing with the attached gear before assembly? Will the bearings self destruct if I dont balance.

To summarise merc's advice....
"So I need to balance the crankshaft with the flywheel and damper plate attached"

But, the flywheel has already been balanced, but not as a whole, attached to the crankshaft.

The damper plate is simply metal round plates or two discs, one big to fit inside the flywheel recess and the other joined to the spline connection, all nicely machined and balanced already, that are joined together by rivets and coupled by flexible polyurethane, from R and D in England, looks very neat, no springs or rotating out of balance masses?

Not worry about the pistons, mahle pistons, brand new + new rings and gudgeon pins and bushes and connecting rods, con rods are the same from the old engine.

I asked them "Is the crankshaft itself already balanced, ex-factory? It is super hard to get the crankshaft balance here, but I will send it away again if it is essential???" They said that the crank was only "roughly balanced".

Thanks from Keith.

Cheers from Keith............
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