You do not need to take the chain apart.

This is on a 693 but yours should be the same.

You might need to loop the chain around the top sprocket first and sometimes you need to remove the nut on the bottom sprocket and lift it off it's mounting bolt. Place the sprocket in the chain and then you can slide it back down on it's mounting bolt and tighten the nut up again.

The picture below is after the chains are on the sprockets but not yet tensioned.

Make sure your springs inside the tubes are not bound up with dirt and the springs are not broken. You push the bolt and compress the spring with hand tension enough to get the 1/2 jam nut started in position indicated by the red arrow. 

You hold the jam nut and tighten the bolt head which compresses the spring which extends the bolt through the hole in the  lower block. Once through the lower block you install a full nut (not shown in this picture) on the lower side where the purple arrow is.

You can then loosen the 1/2 jam nut and run it loosely down to the area of the blue arrow. Finish tightening up the bolt to the 3/16 gap between washer on bolt head and hollow spring tube. Then use 1/2 jam nut to lock everything in place.

When finished the full nut should be on the side of the purple arrow and the 1/2 nut should be where the blue arrow is. Both tight against the block one on each side. This jams the bolt to maintain the tension. 

It's a very good idea to use anti seize on these threads and nuts as over time they will seize up and are a bear to get off without cutting apart. 

Under spring tension you should be able to flex the chain to the side and it should tighten up when you release it. You should be able to see the lower strap the bottom sprocket is on sliding when you do this.

While the chain is off make sure your bottom sprocket turns freely and make sure your upper sprocket does not have any side to side slop in it. Were the top sprockets new along with the chains? If they were not replaced the teeth could be worn enough to have a hooked shape and are not releasing the new chain smoothly?

Also good idea to check the corn head grease level at the inspection plugs up top.

Be aware some of the older heads had cast plugs that can chip out while trying to remove. Best to replace them with new steel plugs. The zerks to fill the gearboxes are under the head. BE SURE TO USE YOUR SAFETY STOP on the feeder house before going underneath. Some shields may have been removed or drilled where you can access the zerks. My old 4 row you took off the tin shield to access the zerks. You need corn head grease up to just below the level of the inspection plug. There are springs in the gearbox that feed grease to those upper sprocket bushings. 

Hope this helps and let us know what you find out.

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Chain.JPG (69KB - 113 downloads)

Once the chain tension is released by it coming off the top sprocket the strap with the lower sprocket is pushed downward as far as it will go by the spring tension. 

I hope Maxx Catt won't mind me grabbing and using his picture to illustrate since his is tensioned up and mine is not.

The strap that the red arrow is pointing to is not threaded it just has a hole the bolt passes through and the spring pushes that part down to create the chain tension.

So once the chain is off the top sprocket the spring tension pushes that strap down as far as possible which is why the spring and tube end up down to the jam nut. The chain coming off allows the area illustrated by the yellow arrow to go to almost nothing. 

The bolt should not be spinning down to the jam nut it's the spring tension pushing the strap down since the chain is no longer on and no longer limits how far it can go down.

Edit: That might sound sort of confusing. Think of it this way if you cut the chain with a cutting torch relieving all the tension in the chain that strap part being pointed to with the red arrow would slam down to the jam nut. Same thing happens when the tension is relieved by it coming off the top sprocket.

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Capture3.JPG (72KB - 121 downloads)