Probably a stupid question, but is there a brand/type of key that is stronger than normal?
I picked up my last one at a local machine shop for my sbc. It worked fine for the past 7 years, but this past weekend it sheared itself in half. Half stuck in the crank and the other half in the balancer.
It's a basic NA small block. I've never had a key shear like that... anything I need to check on other than buying a new key. I've already ordered a new SFI balancer as this one is 17 years old.
MIght get a new fancy bolt as well and torque it accordingly.
Not just to that feels about right.
I see fretting where it meets the timing gear which means it was working loose via not enough torque, not enough press fit, and possible detonation.
Could be all of the above.
Might double check TDC when you get the new balancer and review your ignition timing.
7 years. Might want to check the timing chain while you have a chance.
Dave Koehler - Koehler Injection Enderle Fuel Injection - Nitrous Charger - Balancing - Nitrous Master software http://www.koehlerinjection.com
"Never let a race car know that you are in a hurry."
I bought the short-block used. The PO had the timing chain break.
I installed my old parts from my hurt 383. I did install a new timing chain setup when putting it together last early fall. I used my old key as the crank didn't have one in it. The balancer bolt was new, it came with the stub and pulleys when I went with the Ron's toilet system. I don't think I torqued the bolt... I will this time.
I found Summit carries the Pioneer alloy key... have to buy a 25 pack for about $5.00, and wait for shipping. If I can't find one local I guess I'll order it from them. We lost pretty much all of our local owned speed parts stores over the years... I miss being able to support them.
Dave check Home Depot or Lowes. they usually have them. Mom and pop hardware stores are famous for that type of stuff. I hope your key way slot did not wabble out. Mine did years ago and took out the radiator water pump chunk of block and put a big dent in my hood. was the end of my new crank. I thought about filing an over sized key but don"t know if that works. Good luck you:ll get it fixed.
Something is wrong with the way the damper was functioning. You have a crankshaft torsional vibration problem that is not being controlled, and this will put very high vibratory stress on the crankshaft. It's the very best way to break a crank.
This torsional (twisting back and forth of the crank) vibration can result in timing chain damage and crank fatigue. Either the damper was tuned wrong for the combination or someone honed it to reduce the press fit and it could no longer function. That vibratory load is what fretted the timing gear contact surface and sheared the key. Normally the key is only there to align the damper, the tight fit is supposed to carry the load. The broken key is the result, not the problem. If the key is seeing load the damper is no good.
It's a very complex topic and most aftermarket damper manufacturers do not even know what a torsional vibration damper is, let alone how to design one. The factory engineers know, so a stock damper will be properly designed, and if you are not that far from stock and the rules allow that is usually best. Otherwise ATI does actual twist deflection testing and with experience have a pretty good handle on what is required for many engines. BHJ has a consulting engineer who actually knows what these things do and has written a very good paper.
Sorry to be giving you a painful suggestion, but it really would be a good idea to pull it down and crack check the crank.
The balancer was a loose fit and slid on without much work. When I went to remove it, it was really easy to remove. The balancer was thrown in the garbage yesterday after pulling it off. It was an old TCI Rattler. This engine setup has only made 7 passes since it was thrown together, but has been on 3 different engines including a 355, 383 and an internal balance 406 before this current engine.
I have to have a SFI balancer for the ET's I run. They don't worry about age at my track, but it has to have a SFI one.
406shark wrote: ↑Tue Jul 09, 2019 10:44 pm
The balancer was a loose fit and slid on without much work. When I went to remove it, it was really easy to remove.
You might get lucky but I can almost bet money that the crank snout is shot too. I just measured three that I have sitting here and they are all 1.247" I am pretty sure that ATI will make one with a smaller id but you probably should have the snout of the crank ground to make sure it is round and straight first.
Bill Koustenis
Advanced Automotive Machine
Waldorf Md
I just measured my crank snout, 1.248" per my dial indicator. The balancer measures 1.240" at the inner edge of the balancer, not the chamfered area, and measures 1.218" at the outer edge of the balancer. By hand I can put the balancer onto the end of the crank approximately 1/4" before it stops.
Certified key steel is typically a 1045 grade, which is reasonably tough as far as mild steels go. Ungraded keys from a general store can be in the 1020 range which is about as basic as a graded mild steel gets.Drive keys from an automotive supplier for a specific task will typically be of the correct grade for the job.
Keys are not designed to drive by themselves [they do provide a torque transfer shoulder to bear on hence the condition of the groove being paramount] they are designed to locate in the application you have much like a key on a taper, if it ends up driving something, the retention has come loose, as pointed out, the cyclic load placed on it by a faulty harmonic balancer will eventually exacerbate the problem you have encountered.
I think you're right about the rivets. The balancer is in the garbage.
I believe that the old balancer wasn't properly torqued in place. It worked its way loose cause the bolt worked itself loose. When installing the new balancer it went on nice and tight. When I checked the end of the bolt it appeared that the bolt only went in about 1/4" of the thread. I called my friend that helped me throw the engine together and asked if he remembered anything about installing the balancer last year. He said, I just put the bolt in and tightened it. I attempted to install the bolt... once the thread started there was barely 1/4" of bolt showing under the head. I'd guess the combination of not torquing the bolt, and the bad balancer led to my problems.
I'm picking up a new grade 8 bolt that's 1/2" longer and test fitting it before putting everything back together. If it's slightly too long I'll add a washer... and I've already got my torque wrench out.
I thought about filing an over sized key but don"t know if that works. Good luck you:ll get it fixed. 
