Sandy, that's a good one... I am taking this one to work tomorrow and let my other friend listen to it. He think it might have to do with cam chain tentioner.

Seattle_Savage wrote on 09/10/08 at 13:37:27:
No, not in the Savage unless you have just rebuilt it with the wrong piston or conrod.  
There is 3.5mm of clear space between the top if the piston and the cylinder head.
Forget that line of enquiry - the guys on that forum must be smoking the same stuff as the mechanic you took the bike to.

You're only going to get the piston hitting the cylinder head in one of two circumstances:

A -  Your conrod had let loose, and the inside of the motor is an expensive mess of hot oily chunks of useless metal.  It wouldn't even turn over if the engine had blown like this.
B - The engine has had a custom rebuild using aftermarket performance components and somebody got his math wrong. Unless the bike has just been rebuilt in this way, this is not the case.

As for piston hitting the valves, B also applies.  
Piston/valve contact can also happen if a standard engine is assembled wrong with the timing chain out by a few teeth so the valves are opening at the wrong time, or if the chain has 'jumped' on its teeth (can happen on some motors).  But, if this were the case, the bike would be very difficult to start and run like an asthmatic donkey.  

If it runs fine apart from the knocking, the cam chain is in the right place and the valves are NOT hitting the piston.
If it's not just had some fancy but badly-planned rebuild, the piston simply CANNOT hit the cylinder head.

Trust us here.

Seattle_Savage wrote on 09/10/08 at 13:56:31:
Now that does sound like a bearing, what we call the 'little end' in the UK, or as Verslagen says, the wrist pin.

Fingers crossed for you that if it's a bearing it's not the big end of the conrod - that would need a bottom end motor strip.


Possibilities:

Wrist pin
Big end bearing (mains tend to rumble, not knock)
Worn cam/followers
Cam chain tensioner
Broken piston ring (less likely)
Decompression mechanism

As others said,  it would be a lot cheaper to open it up and look than to have it go bad.  

Going bad usually takes out parts that can normally be re-used.    Also, if it goes bad and parts break apart, the engine should be torn apart and checked for metal fragments.    That is very labor intensive and if you have to take it to a shop, would mean the bike is totalled…… .

First and foremost,  do a compression check on the engine.  

Next, what to open....  all these can be done in one day.....   first, right side cover.   Measure the cam chain tensioner (how far the rod is sticking out).    You will need a gasket to put it back together.    Also oil change is invoilved.

Next,  the Head cover (requires tank removal, but only uses gasket sealer, no gasket).  Look at rockers and the cam lob surfaces.  Also, if you can, look close at the cam journal (the surface the cam rides on).  It's only aluminum and if the Prev Owner mistreated the bike (oil wise) it might have excessive wear.  This manifests itself as a burr/bump of aluminum on it's edge by the chain sprocket. Will be hard to see.   I'm not sure if that would make it knock since the cam chain is tight enough to stop it from bouncing (normally   ).

If they are good, then the connecting rod would be suspect....  but it's very robust.   If your oil is so bad as to take out the rod bearing,  the cam journal would have been disintergrated long before.