Coco,
A little clarification. The Mk3 has sherical inserts in the head to match the spherical end on the pipe. If you use earlier style (flanged) pipes you need to remove the insert. The 850 exhaust "rose" (not really a nut) is shorter than the 750 because the crossover necessitated the split collets to allow the rose to pass over the flange. 750 style pipes allow the rose to be slipped on from the muffler end of the pipe and do not need the collets. Hence, if you elect to use flanged pipes without the crossover, remove the spherical inserts from the head (slip in fit) and either use the 750, long style rose with copper sealing washer, or the 850, short style with collets and copper sealing washer. Alternately, you can use non-crossover, spherical end pipes (available from Raber's, among others) and either style of exhaust rose.
I do not like to use crossover pipes as it is very difficult to find a set that will mate up properly without undue stress on the crossover pipe. This results in cracking at the crossover in short order. I will concede that the power band is slightly different without the crossover, but I'll take the trade off not to replace the pipes every other season.
I tried to find a set of low mount crossover Interstate pipes for a friend and after returning three sets gave up and mounted non-crossovers. One set had the crossover at least half a pipe diameter out of alignment!
Also the spherical (belled) end, non-crossover pipes allow the pipe to be accurately aligned with the mufflers so they stay tight. Assemble the muffler and pipe and bolt up the mounts, then tighten the exhaust rose and they stay.
I have this set up on both my 850 cafe racer and my Interstate and have not had to tighten a loosened exhaust rose in over 10 years. Nor have I had a cracked flange due to misalignment.
