Year/Make/Model: 1999 FORD F150 SINGLE CAB LONG BED
Engine Type: 4.2L V6 FI SOHC
Mileage: 168,000
Original owner

Last October (2009) I was driving on the freeway, exited, came to a light, stopped, and boom, my truck died. I had no idea what happened. Had it towed to the mechanic and within a few hours he showed me the bolt, minus the head, that was suppose to hold on the harmonic balancer (crank pulley). Somehow the bolt head broke. No problem, he replaces within a few days and I’m good to go. A few weeks ago, heading down the highway and boom, the truck dies. With no power steering, I look in my rearview to check for clear so I can get to the shoulder and that’s when I see something bouncing down the highway. I climb out, pop the hood and sure enough the crank pulley is missing again. So, that's two broken crank pulley bolts in less than a year.

The first time it happened, there were no symptoms before the break. However, about 3 weeks before the last break I noticed a very fast vibration. Now, with my truck only out of the shop for a few weeks, I can hear the vibration again. The mechanic said the crank wasn’t bent and if it was, I’d have bigger problems. He said it’s possible that one of the other pulleys is slightly off and that the serpentine belt is pulling on the crank pulley. However after checking, he says that’s not the issue. Also, I verified he used a new crank pulley this time as well as a new bolt. And, he assured me it’s torqued to specs.

I’m at a loss for what could be causing this. And now with my truck vibrating again, it looks like it’s going to happen again.

Questions:
1. Has anyone ever experienced/heard of this?
2. What are some possible causes for the crank pulley bolt breaking?
3. Could the crank be slightly bent? If so, how could that happen?
4. Could the other pulleys be out of line and pulling on the crank pulley? If so, would that eventually cause enough problems to break the bolt head?

Thanks,
Shane

 

Pull the pulley and check the shaft REAL close, any burrs or wear will make it not fit properly and come loose. Also make sure there is no play between the key and the groove on the shaft and the pulley.

I had a dirt bike with a simular issue, couldn't keep the fly wheel and woodruf key on it. Had a machinist friend polish the mating surfaces and it never came loose again.

 

Thanks. I'll check this out.

Shane

 

Also make sure that the crank pulley bolt is actually seating all the way. You can tighten it and achieve torque but it may not be seating completely so you're not getting clamp load. This will allow the pulley to move back and forth on the crank and eventually break the bolt. A washer may need to be installed behind the bolt head to fix this.

 

Just lost the main pulley bolt (3rd time). This last replacement the engine was pulled and
crackshaft was examined, new pulley/harmonic balancer/sensor gear installed a new bolt
installed by torquewrench[no impact]. Spacing was checked.new key used. Lasted about
350 miles. mulling over sending this engine to its death. Tired of fooling with it. Now I've
got to figure out what to put in a bueatiful truck. Any suggestions ? One guy has already
voiced for 260/289 transplant. Did they ever make one of those in throttlebody ?

 

replace harmonic balancer if you haven't so it sounds to me like your harmonic balancer is off

 

8 year old thread sir.