Hey everyone, 
   Got some bad news on my rt2. The big end bearing on the connecting rod disintegrated :( needless to say I’ve got my work cut out for me. I’ve done absolutely everything on this bike- except split the cases. Any tips on this project would be greatly appreciated! 
thanks!

Sorry to hear that & also sorry to say you have a major job there & best undertaken by the an expert in that type of rebuild. In saying that if you have a press & general mechanical knowledge google dale sweger on youtube & scroll down, way down,  & I just found him doing a Yamy AT2 which is a 125 but the principle of building the crank is the same no matter what size bike. I haven't watched it. Only the intro, but it'll be good, love his videos. Best of luck. By the way I've just done the crank on a "54 BSA 350 I'm building out of junk parts & that crank weighs about as much as your complete motor haha. Trying not to drop it on my fingers while trueing it up. Used to rebuild these bsa cranks in the mid :60's so here we go again!!.

Splitting cases isn't very difficult, lots of YouTube videos on 2 stroke rebuilds to help you get a sense of what's involved. And of course there is much knowledge and help here on the forum. That said, I've never rebuilt a crankshaft, but I've sent several out to Bill Bune Enterprises for rebuilding. They do excellent work (boring cylinders too).

www.billbune.com

I've seen Dale's video and it is very good. I'd say unless you have some experience and feel good about your ability to rebuild that crank, I'd put it in the hands of an expert. The consequences of a mistake are just too high. It's your bike and it's your decision. Good luck!

Like the others have said get someone who really knows what they are doing to build the crank. I had a local shop build and install the crank on my DT400 and I believe they didn't get the crank trued very well since the bike vibrates more than I think it should or remember that it did prior to the rebuild, I'm afraid to even ride it...

That's not good but tight "very tight" engine mount bolts make all the difference. You could easily eyeball as to how close it is to be acceptable by watching the mag flywheel to see if it spins true, asssuming it has near perfect roundness, if that's a word. Watch the center & nut but that's not a perfect theory as threads can get bent for various brutal reasons--if only motors could talk & tell what been going on. Maybe watch the outer edge of where the puller threads in that'd be a perfect spot. If it's been trued using centers with a bent thread then it'll be reading all up the creek. But most people "except me lol" use knife edged rollers i think. Check those engine mounts. Do they run a head steady--ok maybe not. Just one more random now i realise its an RT2. My flywheel was loose on the rivets--RT1 road racing but!!--& had a stranger than normal vibe. Found 'that'. So riveted it up tight on the trailer drawbar & felt perfect & smooth for half a lap then the flywheel rivets broke & the flywheel whized across the circuit & came close to nailing another rider. I used to run without a cover for extra cooling. No damage to the coils haha, it just--left & kept going, went a long way at 7000rpm. Sorry, what were we talking about.

RT325, are you suggesting that loosening, if motor mounts are too tight, relax the crank? Not doubting, as have not experimented w/this.

Also, do you think those shock absorbers between the cylinder fins help much for vibration?
I see Yamaha installed them.

One " Bush Mechanic's " way of checking crank runout , is to have your ignition rotor uncovered , 
with the engine running , and gently hold a pencil , partially supported on the engine case ,
close to the outer circumference of the rotor until it just touches .
If you have significant runout , the pencil will only contact in a couple of places .
You could do the same thing with a dial indicator , securely mounted , but I suspect you may not have one of those available .

Sorry, hell no don't loosen them. Tighten them please. Sorry if you read it different to my intension. In fact on my RT2 or whatever mine is i run 3/8 bolts front & lower mount. Rears are both 10mm i think. I'll go check mine again. Cheers.
Ok had a look & looks like i have 10mm front mount bolt but thats too big. 3/8 high tensile would be best but maybe don't go there in your case as drilling required. Just use good original size bolts & tighten nice & tight. Be sure the motor is snug in the mounts before tightening. If at some stage in an earlier life its been jangling around with loose bolts it can wear the sides off the motor mounts plus elongate the holes. Hopefully not all that in your case. Must be my DT1 i have oversize bolts front & lower.
photos.app.goo.gl/SMBSVNLDPG1cQJ6c8

I'n not sure who i'm replying to now haha as post has warped into everyones vibrating bikes. Think it was AirborneSilva saying afraid to ride it now--hell that's a worry if its that bad. But keeping bolts tight applies whatever bike. Do the 400's have a rubbery bonded mount top back--i could never work out why. But bonded rubber on all mounts would work for vibration. Worked for Norton Commando's in the "70's--Uhh, bugger, i can't un-think that now--i hated having to shim them up at work.