Just a little more dialog concerning this.
The recess (on a SJ bulkhead for instance) isn't so much to facilitate a seal for the midshaft (which is what I thought it was really for) as much as the recess allows the midshaft movement towards the aft of the ski.
For instance if you have a flat bulk head and no spacer: You do a reverse lawn dart, back of the ski nails the bottom, the engine moves back via flex of the engine mounts. So the next thing to absorb the movement is the gap in the couplers, and then the bulk head itself or the engine bearings... couplers maybe... right?
The midshaft absorbs both radial impacts but also axial (aft movement) flex as well?
Sorry about beating this to death... Just want to understand HOW it all works...
The midshaft seal is on the outer edge of the rear of the housing and the recess has zero to do with sealing. The midshaft housing has an outer mounting flange (with rear seal) and an inner bearing carrier. The inner and out parts are attached to each other but the rubber center section. The inner and outer sections are flush with each other on the rear side. In use, the inner section is free to flex and move just like your motor mounts do, but only because yamaha superjets and blasters have a cup recessed into the bulkhead for this section to be able to move around.
When you mount a midshaft to a/m hulls with flat bulkhead, 2 things will happen- first you will get more vibration and driveline spline and bearing wear because of the inability of the housing to flex as designed and will eventually wear grooves into the bulkhead.
Cold Fusion noticed the issue and designed their mid housing with the inner section recessed inward to allow proper functionality on a/m hulls with flat firewalls. They also make the spacer for oem mid housings to work in a/m hulls as well.
When JetManiac first became a Cold Fusion dealer, Carl educated me about these designs and issues. I had seen the wear patterns deep into the bulkhead of my prebadass superfreak but didnt understand why. I told Tem at superfreak about the issue and he changed the bulkhead design when the badass was introduced to the same as oem bulkhead design.
I don't think many a/m hull builders still understand or have changed their designs yet.
