The chain sprockets on the I-shaft are the drivers; they transfer motion via the chains to the driven cam sprockets.
Because the bottom sprockets are the drivers, they exert more force to move the whole cam-mechanism. That force acts on the sprocket-teeth.
The damage to the teeth occurs because the new chain will not seat into the bottom of the worn teeth; it rides somewhat more towards the top of the teeth stressing them and eventually breaking them. (In my case after ~10k miles)
You can tell in the picture that, as the sprockets rotates to move the chain, the top of the teeth first bend and then break.
Even though the pitch is the same, the wear in the old sprockets doesn't match the new chain link-dimensions.

Ok, lets see if I have this straight.. That is the drive gear mounted on the intermediate shaft inside the case. Because the gear was worn and the chain is new it doesn't get to sit at the bottom of the tooth and rides high on the gear. The gear shown is turning anti clockwise and the tooth is making contact at the top causing the thin part of the tooth to sheer or chip away..
Close??

It should never be a gamble. It's entirely dependent on the condition of the sprocket whether it should be replaced or not. Mileage is irrelevant - they're either servicable or they're not.

A hooked, damaged or mis-aligned sprocket will eat a new chain in very short order indeed.

I've replaced duplex and triplex primary drive chains on motorcycles where the sprockets on the crank and clutch basket were in good shape, but the chain was used up. Ask any Harley rider with a chain primary if they replace the clutch basket with the primary chain, and they'll likely look at you very oddly. Just doesn't happen. Just isn't necessary - until it is.

For those who were asking about gauging chain wear, it's kind of hard to measure with an endless chain fitted to a motor...

Chains don't "stretch", not really. At least, the plates that form the links don't. What happens in a correctly-installed chain wearing normally is that the 100's of roller bearings that link the plates together wear, and you get both end-to-end and side-to-side slop as a result.

The chain manufacturers publish a specification for the maximum permissable side-to-side slop in the chain, as a deflection distance per foot.

I think it's the end-to-end slop (directly related) which actually chews up the sprockets. Chains are not designed for, and can't tolerate, being run out of alignment for very long at all. That'll just end in a pile of shavings.

What if I keep the old chains, and put in different "used" sprockets? I bought a set of thick arms with spockets on them. I also ned to swap out one of the cam gears 'cuase it has a broken tooth.

thoughts?
oh and 80K og miles on the whole deal.

thanks
Chris

Agreed.

A gear is not a sprocket and vica versa.
The I-shaft gear is driven by another gear on the crankshaft by a 2:1 ratio.
Sprockets are driven/moved by chains.
Let's differentiate between sprockets and gears in the engine:

The chain sprockets are on the I-shaft, (Also called Layshaft)
The cam sprockets are on the camshafts,
The idler sprockets are on the idlers.
All are connected by a continuous dual chain.
The critical wear is on the I-shaft chain sprockets because they are the "Drivers".
Reading the whole thread and the posts will make it clearer.
With only 80k miles, the chain sprockets on the I-shaft, (and the chains), should still be good.
Swapping out used cam- and idler sprockets can be done provided they don't have excessive wear.
To break a tooth on a cam sprocket would require some force and I would wonder how and why that happened. :confused:

The broken tooth is ( I assume) improper tool usage by the PO. I found what looks like chisel marks on the (cam) gear and that is the one with the broken tooth. I cant imagine what they were thinking, maybe drinking and wrenching?? I have dismantled the entire engine and I never found any pieces, so I assume it was broken while the motor was open and the part was retrieved.
thanks for the clarification on my non differentiation.

Chris