Bob, your link is referring to a harmonic balancer, as all normal car engines have, not a counter-shaft.
Browsing thru the exploded diagrams for the F150 on the Yamaha parts catalog I find nothing that even remotely looks like a gear driven counter balance shaft. There is the crankshaft and two camshafts that run thru the block - no others.
Here is a diagram of the alternator and flywheel, and again, there is no referrence to any kind of counter balancer, gear-driven or otherwise.
It is common to have a flywheel keyed and weightet to act as a harmonic balancer for an inline motor, which looks to be the case here. I would like to know more about what's causing a shaft failure and motor explosion tho.
I'm not too worried. If it's a known issue with common failures they'll back it up. It still doesn't convince me to shell out $1200 or $1500 or whatever it is for an extended warranty. I'll take my chances. :wink: