My 400ex is making a horrible clanking noise in the bottom end of my motor.....I cranked it for a ride, rode it 5 minutes, then it started, so I shut it down.....I pulled the head and cylinder thinking it was the connecting rod bearing gone.....wrong, the connecting rod is fine, so I assumed I blew a head gasket....I replace it, put it back together, cranked it, and it is still there.....it shifts fine, runs fine for the few seconds I cranked it....I took the flywheel side cover off....nothing unusual, I have yet to take the ride side case off....It is a horrible clanking sound, anyone have an idea what this could be? The motor has 2 years of hard riding on it, with alot of top end work, never been into the bottom end....thanks for any help.

Why did you Assume that you blew a head gasket?? if a clanking noise was coming from the bottom end, then youd think you would tear the bottom end down... Check out your Crank bearings, clutch, everything, its better replacing one thing than half your motor.

Why would I assume it was the head gasket?......Well, as soon as I heard the noise I immediately shut the motor down.....So fast that I didnt have the time to examine and listen to the noise...I pulled the head,cylinder,and both side cases off.....main bearings and connecting rod bearings looked and felt good (I did not split the cases)...bike shifted smoothed, looked in motor with flashlight, crank and transmission gears looked and felt good....so, I ASSUMED I blew a head gasket....so I put it back together......cranked it, let it run for a few seconds...the noise was still there, this time I was able to narrow it down to the bottom end, transmission area.....but transmissions feels and shifts fine.....motor also ran fine when I cranked it, what else could the problem be? (Im splitting the cases in a few days, but am curious to what you all think it is)

can you say ......crank pin proply spun the crank ....thats what happens to modified 400ex's without the pin welded ...

I dunno, mines fairly modified and i havent even burnt up my stock clutch yet..... The oil pump does make alot of noise on the 400ex, did you check that out?? also, take the bottom line off the oil tank... dip your finger in there and see if there is any shavings in the bottom of that tank. If there is then thers something wrong for shure.

I would recommend taking the bottom end apart and checking your tranny. It may be that your second gear has blown up. It's a fairly common problem with 400's that are ridden/raced hard over a long period of time.

Regards,

Dennis

That's what I was gonna say, if the motor bearings are okay, it's probably missing some synchro lugs or tranny gear teeth ( If I'm not mistaken the ex's use a constant mess gears, a broken tooth on one will make noise in any gear. )

The verdict is in.....A bolt on the back of my basket, one of the bolts that hold the gear plate to the basket, backed out and was rubbing bad.....So I guess we were all wrong....haha.....thanks for the input.