symptoms of a bent crankshaft

1990XR7SC

Registered User
as the title states im looking for info on bent crankshafts. Im still digging into why i had 3 balencers break in the same place without the bolt breaking or the crank sensor breaking as well. i could do the dial indicator on the end of the crank. But also would pulling the broken parts of the balencer bend the crank and drilling out the broken bolts? And what the symptoms would be. Thanks.
 
Were the 2 replacment balancers that failed new or used ? Are you sure they were the zero balanced units that are specific to the SC 3.8 and not the NA 3.8 balancer that includes a counter weight ?

David
 
You couldn't have done anything to bend the crank. Were the replacement balancers new, rebuilt, or used? Most used balancers are junk which is why they generally fail rather quickly.

Is your car an AOD?
 
>what the symptoms would be

Of a bent crank? Well, to start, your fillings would be on the floor...
 
I've noticed on several of the used ones I have where there is a groove cut into the balancer hub. maybe 1/64 wide where the diameter is reduce by half. I'm not sure what causes them. I think the one I still have it from the used one David put on the new engine he built for me during transportation. It is just a matter of time if you get one like that.
John
 
to awnser some questions yes its a aod car yes one was used and yes one was new. my last one that broke was new same with the crank bolt. and yes it was a sc spec balencer. But looking back on it i never really had the crank pully checked out for any imbalance. But also i never had any reason to suspect that being out of wack till now since the weight was still on it. And today i did a check on the crank with a straight edge and everything checked out ok.
 
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1) AOD cars are harder on balancers than 5spd cars typically because the AOD tends to load the motor heavily in the 1100-1800rpm range and there really isn't much you can do about it due to the shift pattern of the AOD. The SC motor has a natural harmonic at about 1300-1600rpm that will cause a vibration that can be catastrophic to the motor if allowed to continue.

2) It has been my experience that once a balancer fails on a motor, it is very likely that it will happen again. This may be due to accelerated bearing wear from the harmonic vibration condition or it may be due to the original balancer failure resulting in a bent crankshaft that tends to break subsequent balancers. This is one of reasons that I tried so hard to get an inexpensive balancer option for us so that people could afford to simply replace an old balancer before it fails. The elastomer wears out, plain and simple, and there is nothing you can do about it other than rebuild or replace the balancer. Clearly the preference would be to replace the balancer.

3) Apart from all of these things it is also my opinion that some SC motors carry a poor balance right from Ford. I recently disassembled an SC motor with 200K miles on it that was on it's original balancer and the parts all checked out to within +/- 1gram. I've seen some SC motors off as much as 15-20 grams. What I'm saying by this is that just because your motor is breaking balancers doesn't necessarily mean anything is fundamentally wrong and needs rebuilding, but you could very possibly be faced with having to pony up for a BHJ.

The kind of bend in the crank that would break a balancer is on the order of .004-.006" so you have to realize that you aren't going to likely be able to measure that with a square of any kind. You'd need a dial indicator at least.

Final note, installation procedure is critical with these balancers. Being that they are aluminum, you cannot force them on! You must use anti seize on the snout and you must use the proper installation tool. A block of wood and/hammer will stress the balancer before you even get it on there making premature failure almost a given.
 
i only have one pic of the part in the motor broken in there. but what xr7 dave elaborated on gets me thinking yeah the bhj might be my only alternative. But i do want to get this right so i don't have this happen ever again.
 
i only have one pic of the part in the motor broken in there. but what xr7 dave elaborated on gets me thinking yeah the bhj might be my only alternative. But i do want to get this right so i don't have this happen ever again.

Can you confirm that the failed balancer is "netural balance" and does not have a counter weight ?

David
 
That would be your culprit, probably. The aluminum center scretches to conform to the crank diameter. Once it goes on it should never be used on a different crank.
 
That would be your culprit, probably. The aluminum center scretches to conform to the crank diameter. Once it goes on it should never be used on a different crank.

Ditto...

Get the BHJ this time, and either balance your stamped pulley or buy a netural balance aluminum one and your balancer problems will be over.

David
 
thanks guys. you just confermed my conclusion. But im working out a price on a used bhj. but i might just go and shell out the 400 bucks.
 
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