my gut feeling from the start of this thread is the pair valve.
i have full titanium exhaust and was warned to do the pair valve mod right away to prevent backfiring. Mine did not backfire so I didn't do the mod... yet (even tho the parts are sitting on my microwave just waiting).
The Pair valve introduces fresh air into the exhaust system for a 'cleaner' emission. Hmm.... extra OXYGEN mixes with any unburned fuel.... Hmmmm..... what do you expect to happen?!! BOOM!
what happens during a huge backfire is a large explosion in the header pipes. if this happens at the wrong time - during exhaust stroke when a piston is going up with the exhaust valve open, the explosion from the header creates so much pressure on top of the piston, you may as well shove a rock in there. Therefore, breaking rods, bearings, crank, etc.
If your bike backfires alot or even just once in a while, fix it before it fixes your bank account!
thanx big FatSac for being the first to mention the pair valve!