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Old 12-08-2013, 05:14 PM   #1
frasermanx OP
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Loose crankshaft counterweights ?

Loose crankshaft counterweights ? Any experience with this problem ? I have never heard of that problem and am shocked that it''s possible, however if you look at a crankshaft carefully you will see plates 1/4" thick attached to the main counterweights. These are attached after crank grinding at the factory since the big ends can not be ground with them in place. Look carefully and you can see that they are merely pressed into the larger counterweights but not welded or bolted just pressed. Potential disaster waiting to happen !

2 years back I was shocked when I was shown that my ring gear and flywheel were Riveted together and the rivets had come loose, which had caused the terrible noise necessitating taking motor out, driving 200 miles, paying hundreds of dollars, to discover this rare problem.

Now I have another noisy problem. Loose crankshaft counterweights ? Another BMW makeshift factory corner cut ?

BTW apparently BMW racers have been known to leave off those extra counterweights so they are not absolutely necessary (but maybe necessary for a smooth idling engine.) ..

any comments, help, solutions, experiences ?

Here is pix that shows the attached plates to the main counterweights .. http://ecommons.net/~frax/BMW/bmw-broken-crank.jpg

thanks very much

frasermanx screwed with this post 12-08-2013 at 05:28 PM
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Old 12-08-2013, 05:32 PM   #2
DiabloADV
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Don't know if they come loose but if you remove them your bike and motor will tear themselves to pieces in short order.

I just had my crankshaft balanced for new lightweight pistons. The guy removed a total of about an ounce of metal.
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Old 12-08-2013, 05:43 PM   #3
frasermanx OP
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That begs the question. Which brand of piston ? Higher compression ? Nikasil barrel ?

Saturday I had a crankshaft in my hand ... it weighs a lot .. maybe 20 lbs. so taking an ounce off.... ?

my source, a BMW mechanic says racers dont use the extra weights ... I dont contradict him ...cuz I dont know different

..as I said all tips, info, experiences, theories are welcome

F
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Old 12-08-2013, 06:01 PM   #4
crazydrummerdude
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I have no experience, and this is just hearsay, but at the 2009 BMWMOA rally, I attended a seminar where the speaker said he saw a the crankshaft counterweight come loose, dislodge itself, travel up through the block, break apart the starter, and wedge into the frame.
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Old 12-08-2013, 06:01 PM   #5
bmwrench
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Find a good used crankshaft and be glad you caught yours in time. This failure is very rare.
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Old 12-08-2013, 07:24 PM   #6
frasermanx OP
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Quote:
Originally Posted by crazydrummerdude View Post
I have no experience, and this is just hearsay, but at the 2009 BMWMOA rally, I attended a seminar where the speaker said he saw a the crankshaft counterweight come loose, dislodge itself, travel up through the block, break apart the starter, and wedge into the frame.
Yikes that could be dangerous ! I hope he got satisfaction from factory.

As a former welder I would put my trust in welding the plates to the counterweights in two places and I doubt they would loosen ever. I am now shopping for a welding machine

thanks for all comments, tips, stories

frasermanx screwed with this post 12-08-2013 at 07:38 PM
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Old 12-09-2013, 09:59 AM   #7
DiabloADV
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Originally Posted by frasermanx View Post
That begs the question. Which brand of piston ? Higher compression ? Nikasil barrel ?

Saturday I had a crankshaft in my hand ... it weighs a lot .. maybe 20 lbs. so taking an ounce off.... ?

my source, a BMW mechanic says racers dont use the extra weights ... I dont contradict him ...cuz I dont know different

..as I said all tips, info, experiences, theories are welcome

F
Siebenrock pistons. They're lighter than stock...my point about the weight removal is that, even with lighter pistons/pins, a very little amount of weight was removed to keep the rocking couple in balance. If you lose the entire add-on weights, the thing is going to vibrate horribly.

An opposed-twin with weight-matched pistons/rods+pins will almost balance itself with respect to primary and secondary oscillations. The problem is the rocking couple since the cylinders are offset. That's why these cranks are so heavy, and need to be. If the cylinders were somehow directly aligned, you could have a rather lightweight crankshaft.
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Old 12-09-2013, 11:02 AM   #8
supershaft
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Quote:
Originally Posted by DiabloADV View Post
Siebenrock pistons. They're lighter than stock...my point about the weight removal is that, even with lighter pistons/pins, a very little amount of weight was removed to keep the rocking couple in balance. If you lose the entire add-on weights, the thing is going to vibrate horribly.

An opposed-twin with weight-matched pistons/rods+pins will almost balance itself with respect to primary and secondary oscillations. The problem is the rocking couple since the cylinders are offset. That's why these cranks are so heavy, and need to be. If the cylinders were somehow directly aligned, you could have a rather lightweight crankshaft.
How are you figuring that the thing is going to vibrate horribly without the add on weights? Removing them is a semi-common mod. I have worked on a bike that had them removed. It didn't vibrate any worse for it that I could tell. It did run like a mofo but . . . .

I wouldn't worry about the crank weights coming loose and I regularly rev my airheads WAY above red line.
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Old 12-09-2013, 12:09 PM   #9
DiabloADV
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Originally Posted by supershaft View Post
I have worked on a bike that had them removed. ...
Just because it's done doesn't make it right. Removing counterweights is shade-tree mechanic stuff. Stupid, stupid.

Every engineered boxer motor ever built has counterweights to mitigate (but not eliminate) the rocking couple (more precisely, crankshaft precession). The crank journals are offset, and precession is a major problem without the counterweights. And precession will tear up bearings, even if the rider is too numb to detect the extra vibration.

The issue is worse on a boxer twin than on fours and sixes.
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1974 R90/6 w/1050 kit. Motor only. Seeking a frame.
1970 R60/5
1981 R80G/S Renovation under way. For sale Spring '14
'91 Bill Holland Steel w/Dura Ace

'01 Z3 3.0 Coupe. The Clownshoe.
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Old 12-09-2013, 01:04 PM   #10
supershaft
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Quote:
Originally Posted by DiabloADV View Post
Just because it's done doesn't make it right. Removing counterweights is shade-tree mechanic stuff. Stupid, stupid.

Every engineered boxer motor ever built has counterweights to mitigate (but not eliminate) the rocking couple (more precisely, crankshaft precession). The crank journals are offset, and precession is a major problem without the counterweights. And precession will tear up bearings, even if the rider is too numb to detect the extra vibration.

The issue is worse on a boxer twin than on fours and sixes.
Just because it was done and worked doesn't make it right? Who is calling who stupid? That bike won a lot of races and set a lot of airhead lap records. It didn't vibrate horribly. It didn't tear up bearings. I believe Moorespeed does it among others.
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Old 12-10-2013, 06:44 AM   #11
ericrat
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Frasermanx,

I suspect the only thing to do with you knocking engine is to tear it down, inspect and measure everything. Loose counterweights is pretty uncommon, but obviously possible and easy to fix. We have beat the main bearing measurements thing to death, etc.

You can tell it is winter, we are all on here worrying about crankshaft balance factors and other incredibly pointless wanderings.

I might just be cranky because there is a couple of inches of fresh snow and I have to go walk the dogs.


Eric
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Old 12-10-2013, 07:25 AM   #12
Cogswell
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Quote:
Originally Posted by ericrat View Post
Frasermanx,

I suspect the only thing to do with you knocking engine is to tear it down, inspect and measure everything. Loose counterweights is pretty uncommon, but obviously possible and easy to fix. We have beat the main bearing measurements thing to death, etc.

You can tell it is winter, we are all on here worrying about crankshaft balance factors and other incredibly pointless wanderings.
Eric
Agreed....

Can you not remove the oil pan and check the weights to see if they are properly secured ?


How was the previous loose ring gear repaired, re-riveted or new part ?


Mike
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Old 12-10-2013, 08:38 AM   #13
frasermanx OP
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I did take sump off but did not check the weights yet. Ya cant get at the weights from the sump but need to access weights from the cylinder hole/s

I also saw a low mileage R100 engine in a parts bike this weekend that I am considering. Getting a trailer, storage, workspace arranged for the 100 mile tow/drive .

The ring gear was replaced with a slightly used unit... not expensive

I will take the main housing off to check the front main bearing ... and I will take a barrel off to check the weights before I buy the parts bike ... ya never know how good a parts engine is ... so not a sure thing ..
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Old 12-10-2013, 08:55 AM   #14
DiabloADV
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Originally Posted by frasermanx View Post
... ya never know how good a parts engine is ... so not a sure thing ..
I bought a short block that looked perfect; everything spun easily and smoothly. Got it open...the crank journals were ruined. Surfaces were cracked and split. Looked like a bad factory part, since there was no sign of overheating.

But, I got some other good parts from it. Oil pump, crankshaft sprocket, etc.
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1974 R90/6 w/1050 kit. Motor only. Seeking a frame.
1970 R60/5
1981 R80G/S Renovation under way. For sale Spring '14
'91 Bill Holland Steel w/Dura Ace

'01 Z3 3.0 Coupe. The Clownshoe.
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Old 12-10-2013, 02:37 PM   #15
pommie john
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It's my understanding that when you remove those weights the crank needs to be rebalanced. I'm not sure if extra weight is added in a different way, but I have heard of British twins being balanced by the addition of " heavy lead" ( forged lead) .

I have not looked too closely but I thought they were rivetted on rather than pressed in.

Other common knowledge is that you should never regrind an airhead crankshaft because it mean removing the weights and many crank grinders simply don't refit them as well as the factory . A friend of mine did this and the weights came off around turn three at Phillip Island ( look at turn three on youtube, flat in 5th on my airhead). The weights came out of the engine completely along with the oil and various other bits.
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