Int. 650 Forage Harvester

Greetings all,
I am flummoxed, and hoping that someone can help me.  My International 650 forage harvester (with 2-row corn head) is breaking shear pins.  The shear pin is located where the main drive is for the gathering chains on the corn head.  These chains are the only thing being driven by this drive line. 
I cannot find anything binding or catching.  I did see that one of the sprockets that drives the lower gathering chains is missing two teeth (rust patterns tells me they have been broken for a while), but other than that I see no obvious things wrong. 
Bearings are tight, shafts have no play, chains are not catching, I am stumped. 

Anyone have any experience with these?  Any advice?  Thanks in advance.

John

3/8 bolts 2.5 inches long.  

Are you sue the broken sprocket isn't the problem?

I would think when you get up to speed that broken sprocket would be the trouble.

I'd say if you replace the sprocket with new you might solve your problem. Make sure your gathering chains aren't so worn that they are jumping the sprocket . If that's the case replace.

That is a head scratcher. When you turn it by hand, do you turn it far enough to pass the gathering chains all the way around once or twice? Just wondering if there's a link or 2 in there that are just iffy enough when it hits the missing teeth in the sprocket, that's when it binds. Or, I have lots of experience working on old choppers, but not this model, does this drive line also drive the feed rollers? Bad bearing or something in the rollers? Seems like a bit more of a long shot.

you don't have a 1/2 link in the chain that's causing it to lock on that broke sprocket?

Run it empty while watching it closely. When whatever binds up hard rough to shear the pin you will likely see a momentary stall or hard load up on that component and what it's catching on.  

All,
Thanks for the great advice. 

Tbone:
"When you turn it by hand, do you turn it far enough to pass the gathering chains all the way around once or twice? Just wondering if there's a link or 2 in there that are just iffy enough when it hits the missing teeth in the sprocket, that's when it binds."
I can turn it by hand around and around for many revolutions of the gathering chain system.  All the chains are snugged to the manual specifications. 
Shameless:
There are no half-links. 
Tbone: " does this drive line also drive the feed rollers?"
The driveline for the feed rollers and chooper mechanism is separate from this shaft and shear pin.  The only items run by this shaft are the gathering chains on the head. 
Allis Dave:
All the bearings are in good condition.  They have no noticeable slop in them when grabbing pulleys, or shafts.  I may see something different when I take it apart.
JC(WI) We broke a shear pin when we pulled a branch into the pickup, jamming the chains and the knives.  Had to replace the sickle.  Since then, we are breaking shear pins.  Now, like said above, I can turn it by hand, and run the chains all day long.  But when I run it at low idle on the PTO, It can turn for 5 minutes or 5 seconds before it breaks a shear pin....but it always breaks.  The machine distinctly goes bang when the bolt breaks.  I am using just simple machine bolts for the shear pins. 
The shear pins are located on a shaft coupler.  The shaft has a dog eared, spring loaded coupler on the machine side, and a dog eared pined coupler on the gathering chain side.  It is just barely in view in front of the gear box on the third picture you posted.  I will see if I can get a photo for people tonight.  All shafts are tight. 
TcmTech:
I have watched individual components for 10 shear pins, and nothing moves or jumps, or changes when the pin breaks.  I think JC is right, that I might find the problem when I replace the sprocket
JohnCO:
I will call the local dealer today to verify the bolt type.

Thanks to everyone.  Will keep you posted.
John

Dad was helping a neighor once and had same troble .Chains must be same length and same no of links between dogs. Need good sprockets.If you have an extra link one chain will gain on other and the dogs will evently run togather and break something.