I had a 61x cylinder and it got a hairline crack on the base of the bow/starboard side cylinder. I spoke with a local builder and he said that this is vary common. So I did some research and found that Yamaha corrected this design flaw with the 62t cylinder by a thicker wall and a gusset. (pictured below) Also, not all 62t cylinders have these additional gusset supports.
I am building a 760 out of an XL with a TBM lightened flywheel (-2 lbs) and Cold Fusion lightened couplers (-1.5 lbs) that is 3.5 pounds off of the rotating mass. I am hoping this will counter the port timing by allowing the motor to rev up just as fast or even faster than the 701 61x cyl/62t combo. I am also porting the 64x cyl. Which in theory will give the same throttle response / low end pop as the 701, yet with aproximatley 15+ more horsepower.
My 760 should be going in the next few weeks and I will let you guys know how the combo works out.
Just for clearification:
64x cases = 62t cases! (stamped right on them) with two brass pulse lines for carbs (=44mm dual Mikuni's)
62t cases = have only one brass pulse line (=dual 38mm Mikuni's), but have the indentation for where the second pulse line goes. In other words they are the same cases, except for the second brass line is drilled and inserted from the factory on what people refer to as 64x cases (can easily drill it out and press in another brass fitting) You would do this for dual 44mm or greater carbs.
61x cyl = 701cc
62t cyl = 701cc (sounds like it might also be a 760, not sure if it is has a larger bore from the factory -or- larger diameter sleeves?)
64x cyl = 760cc
