Through multiple examples on this and other forums we can come to the conclusion that although the XT660 engines are very robust the dogs on the gears are their weakest point. There have been examples of people losing multiple dogs from the gears and the engine keeps running until they pull it apart and realise what has happened. Or in my case I lost just one of the dogs and a metal shard got in between the crank and the crankcase and caused a terminal failure leaving me stranded in a country lane 137 miles from home in the blazing sun without any shelter! Not sure if it is advisable to pull the engine apart and check or not, as Pleiades said it kind of depends on the size and shape of the metal. It would also depend on how handy you are in a garage, how much money and time you have to throw about and how critical the bike is to you i.e a daily commuter or occasional toy. However if what happened to me had happened when I was doing 90mph+ down a dual carriageway instead of whilst accelerating out of a corner the consequences could have been far greater.

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Now have a 2017 S1000XR Sport SE

2006 XT660X - Cat D write off due to someone pulling out on me - Pipewerx carbon fibre exhausts - Snorkel removed and airbox drilled - DNA stage 1 + 2 air filters - Kevs fuel controller mod - Scottoiler V system - Oxford heated grips - HEL braided brake hoses - Wilbers 540 rear shock - Brembo brake pads - AFAM chain and sprockets - Michelin Pilot Street Radial's - Denali DX Xtreme dual intensity spotlights