View Full Version : Harmonic balancer trouble.
My truck has been idling a little rough and got worse lately. I looked at the harmonic balancer and noticed a piece of the rubber missing.
When I pulled it off, the rubber wasn't as bad as I thought it was but the balancer had spun on the crankshaft.
I thought it just sheared the key but when I went to put the new one on is when I found the bad part. The keyway is stretched about twice as wide at the top and I can almost push the key over sideways by hand. :(
Any ideas on what to do would be appreciated.
http://www.rsixray.com/junk/keyway.jpg
C.K. Piquup
07-23-2004, 02:41
I would talk to some machinists.Could a larger key be found/made?Then cut matching groove in new balancer.Looks like you just saved the crank.I`ve never seen this.Maybe you have found the missing link to crankshaft failure.It`s good you pay attention to how your engine sounds/feels.
G. Gearloose
07-23-2004, 04:19
When properly torqued, the keyway should be on no conseqence, in other words, if you rely on the keyway to KEEP things lined up, you get whats in that picture.
If the reluctor is undamaged, I would line it up and torque it down, retorque after one ride, and retorque a week later.
An oversized, 'stepped' key is another option, so the balancer can remain untouched.
So the bolt should be enough to hold it? That is what I want to hear.
The bolts was loose when I removed the old balancer.
rjschoolcraft
07-23-2004, 04:55
Sorry to disagree, but that key is there to transfer torque from the crank shaft to the torsional damper. I'm afraid that you've lost the crankshaft and need to tear down your engine to replace it.
Yikes!
Reminds me of an old Buick 350 that I had in high school. The bolt got loose and let the balancer slop around. I ended up with about a 1" wide key slot in the balancer. Crank was untouched. New (used) balancer and it ran a long time afterward.
What puzzles me here is why the press fit of the key into the crank didn't hold it snug. There's much more mass inside the crank than above the surface leaving the balancer very little leverage on the key.
I'd have thought the balancer would be much softer and take the beating. I'd suggest the possibility that someone put in a key that was too narrow...
I'm also assuming this was the 1993 truck?
Originally posted by kennedy:
I'm also assuming this was the 1993 truck? No, it is the 1995 truck. Is it supposed to have the long keyway?
eracers999
07-23-2004, 16:31
Amen, G Gearloose. A properly torqed balancer will not, tax the key way.
Kent
Originally posted by Kent:
Amen, G Gearloose. A properly torqed balancer will not, tax the key way.
Kent I guess I will see if you guys are right. I just put it back together with a new harmonic balancer and key.
It idles and runs much smoother than it has in in a long time.
rjschoolcraft
07-24-2004, 15:39
Prepare for disaster.
The bolt is much to short in length and large in diameter to get much stretch from the torque that is specified. Bolt stretch is what gives you clamp load (think cylinder head bolts). Clamp load is what gives you the friction that you want to count on to hold the damper... Not enough there.
Have you had a crank pulley failure?
Yes, I had a crank pulley failure about a year ago.
What would happen if I over-torque it? Will it hold better?
I intend to pull the pulley off in a week or two and see if it is coming loose. (or earlier if it starts running rough again.
rjschoolcraft
07-24-2004, 17:57
I suspect that the failed crank pulley probably hammered enough to get the keyway started failing. It's possible that you have a crank that is at the low end of the hardness spec also.
I doubt that you could overtorque it enough to do any good. Bolts (like head bolts) for serious clamping are usually necked down in the body so that they stretch before failing the threads. But, if you went 20% over, it probably wouldn't hurt anything.
Barry Nave
07-25-2004, 03:53
How about JB Weld around the key/crank?
RJ
When you say 'Necked down in the body', exactly what do you mean?
If it's 'UNDERCUT', I think it is done where an application has different bolt lengths as in a BB Chevrolet cylinder head. The shorter bolts(I believe there are 3 different sizes) are undercut to achieve the same stretch that the longer ones will.
Consistent clamping force if you will...
Or maybe that wasn't what you were referring to.
rjschoolcraft
07-25-2004, 05:58
No, that's not what I meant.
Many bolts are only threaded part way. The major diameter of the thread is larger than the main body of the bolt. That's what I meant by "necked down".
Your comment about undercutting a short bolt to get same stretch as a long bolt is valid. Strain is the amount of loaded deflection per unit length of a material. If you have a strain of 0.0002 in/in on a 5 in bolt, that equates to 0.0010 in total deflection, or stretch. That same strain on a shorter, but otherwise geometrically identical bolt, will produce less total deflection. On a 2 in bolt, for instance, 0.0002 in/in of strain will produce 0.0004 in total deflection. The two bolts will have roughly the same internal stress and close to the same clamp load.
However, the shorter bolt is much less forgiving. Now this "gets into some spreadsheet analysis and heavy duty math," as Hank the Cowdog says :D (thats an attempt at humor... if you havn't ever listened to Hank the Cowdog tapes, you should, but ignore that last statement). The torque on the bolt is a function of the clamp load and the coefficient of friction. The clamp load produces the normal force between the surfaces of the two helices as they mate and between the collar and the clamped part. Once contact between the head and the clamped part is made, the shorter bolt will require much less rotation to achieve desired torque than the long one will. Therefore, it's easier to over torque a short bolt than it is a long one. On the other hand, it takes much less reverse rotation to relieve the load on a short bolt than a long one. By undercutting, or necking down further, short bolts, you make the bolt stretch more, but the stress in the body is significantly higher.
i had the same problem on a 350 chev small block, i just used a small die grinder and cleaned ut the groove in the crank and used a larger key, then loctited it in. that was 100000 miles ago.
alot cheaper than replacing the crank :rolleyes:
oh yeah forgot to mention, i had to narrow the top of the key to the fit the balancer.
The washer is also convex although, I don't know how much it will deflect even with 200 lb/ft of torque.
Still troubles me how the crank was so shot at the outer surface, and how the key even THOUGHT about damaging it. I'd expectthe balancer to work on shearing the key, but the keyway is quite deep and snug so the key istelf shouldn't movein it much less fold over like it looks to have.
Is the key in the picture new?
Yes, that is the new key.
I lost the old one so I couldn't compare it.
rjschoolcraft
07-25-2004, 20:17
How long did it run with the failed crank pulley?
jspringator
08-08-2004, 14:26
This type of problem is comon to 1990 early 91 Mazda Miata. The Miata.net forum advocates the use of a "Locktite fix" which uses Locktite liquid metral to fix the keyway,and a clamping Locktite to hold the main bolt. This repair is known as the "Locktite fix". This application may be applicable to the 6.5.
Originally posted by whatnot:
Yes, that is the new key.
I lost the old one so I couldn't compare it. OK, that is what threw me...
I suppose I should pull it apart one of these days to check on it.
If the bolt is still tight, is it worth pulling the balancer off to see how the key is holding? I am not even sure if I would be able to tell my the time it is all the way off.
I have only driven it about 1000 (hard) miles since putting it back together but it seems to be fine so far.
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