Nathan, I'm not totally certain but I've been told by folks who are are far more knowledgable than I in these matters that the cylinder is vastly improved over that of the A model and that there are various other changes. Having raced both models myself back in the day I know for certain just from my 'seat of the pants' instincts that the B is vastly superior to the A in every performance aspect. Of course it can be argued that the A cylinder can be ported to B specs or in reverse, an A head can be fitted to a B and all is well. If the latter was allowed in pre '75 it'd open up the old chestnuts of allowing Mk 8 250 Pursangs, VA125 Montys and all of the other borderline machines whose owners feel victimised because their bikes were available in 1974.
The factory designated 1975 model stanza was included to cut these disputes off at the pass. Remember that it's not just the MX B engine that's not allowed, it's the whole bike. The whole engineering concept of the MX B belongs in the next generation of motocross history, purely for its suspension package*. December 31 1974 was deliberately chosen as a cutoff date as it marked the end of the 7" and4" suspension travel era. Even though the MX B engine may be considered an update of the A model, the bike as an entire package is a whole new deal that launched Yamaha into its next phase of development. You can't seperate the engine from the whole unit and call it a flow on while still disallowing the monoshock frame. It's all or nothing.
*Before somebody enters the discussion with the notion that the YZB is allowed in pre '75 and it features a similar monoshock suspension system to the MXB, an explanation is in order. In the first few years of the sport the YZ250/360 B was not allowed, a situation accepted by most in the sport. Then along came a Victorian named Jody Mason who challenged the validity of that ruling and lobbied to be allowed to race his YZ 250B. I got the job (along with Peter Drakeford, I believe) of researching the history of the YZB for the commission. Despite my and other delegates objections on both moral and engineering grounds for the same reasons I have laid out in the previous MXB paragraphs, Yamahas own paperwork proved that the YZB was indeed a 1974 factory designated model and for that reason alone we concluded that it'd be discriminatory if we disallowed the YZB because of its advanced suspension travel yet allowed 74.5 Maicos, CCMs and other bikes with more than 4" of rear travel. We voted to allow the YZB on the condition that the suspension travel be restricted to 7" and 4", as we'd insisted the Maicos, CCMs and others do. Ironically, Mason, the guy who bought this whole situation to a head, never did race his YZ250B or any other vintage motocross bike to my knowledge. He did ride some speedway and classic dirt track on Jawas. He was last spotted punting a Charger around Eastern Creek in the Biante series.
