Frame cracked!

[Speeddog]

The frame is dependent on the motor mounts being sound; both that the bolts are tight, and the engine cases aren't cracked.
As long as that condition is maintained, the frame is triangulated by the motor itself, and it's not going to crack there.
Both of those tubes are then only loaded in tension and compression, and they can handle that with no problem.

With motor mount bolts loose, or broken, or engine cases broken, the frame is no longer effectively triangulated.
That section then has to carry load in bending.
Those tubes aren't anywhere near big enough to do that, and the welds in that area compromise the bending strength.

[Dmitry]

I would not go any further than nursing it home if I didn't have to go far.  Once it cracks through you will be on the ground.  Also, do follow Speeddog's advice to check the motor mount bolts and case.  Looseness there can cause that crack.

You're right! Motor bolts have been loosened! I tightened them. Is it true that frame could crack because of this?

[Dmitry]

The frame is dependent on the motor mounts being sound; both that the bolts are tight, and the engine cases aren't cracked.
As long as that condition is maintained, the frame is triangulated by the motor itself, and it's not going to crack there.
Both of those tubes are then only loaded in tension and compression, and they can handle that with no problem.

With motor mount bolts loose, or broken, or engine cases broken, the frame is no longer effectively triangulated.
That section then has to carry load in bending.
Those tubes aren't anywhere near big enough to do that, and the welds in that area compromise the bending strength.

Thanks for the reply, Speeddog!

[Dmitry]

Mine has been cracked at the same site on both sides for years. Probably pretty reckless on my part but I still ride the piss out of it. I'll have to pull the engine to gusset weld it.

Show your crack I want to see photos.

[Howie]

You're right! Motor bolts have been loosened! I tightened them. Is it true that frame could crack because of this?

Since you read Speeddog's explanation...  I would still fix it before riding though.  Then again, I am very conservative on the advise I give to others.  Closely look at the rear engine case where the motor mount bolt goes through to confirm there is no crack. 

[brad black]

bolt it up before welding it too.

bob brown ran a new rail above the v, pretty much continuing on the horizontal rail from the front when he was racing an 851, and he says it stopped the cases cracking.  which they did a lot of.

[BK_856er]

Reminds me I need to check my monster frame bolts again!

Monsta's excellent reinforcement looks a bit like what's on my 749s frame from the factory (i.e., original).

BK