Well for what it's worth, the single rail was introduced as a
"commonization (is that a word??;-})" with some other models currently in
production, also referred to as the Marina box, since it was in production
use at that time. It was also an uprated, slightly more robust version of
the 3 rail. The basic box is the same (barring the obvious external
differences), sharing most of the major bits, but the mainshaft pilot
bearing (the needle bearing between the first motion, or input shaft and
the mainshaft) was made larger for increased load handling and longer life.
The older units are notorious for wearing these out rather quickly. Also,
the shifting is *slightly* more precise because all the selecting is done
with a single shaft instead of migrating through three to do the same
thing, and it adds the benefit of having the almost the entire shifting
mechanism enclosed (and lubricated). The problem with trying to use a
single rail on anything that was not originally designed for it is that
single rail actually protrudes into the bellhousing area during second and
forth gear selections. Later bellhousings have a "cup" cast into them to
accommodate this shaft, where as the older units do not. I have an older
aluminum thick flange bellhousing that I am going to modify by re-machining
to add this clearance by boring a hole in the proper location, and press
fitting a "cup" to be captured between the bellhousing and the
transmission, as in the later models for the shaft to enter. Should save a
little weight on the 70 project.
Barry Schwartz (San Diego) bschwart@pacbell.net
72 PI, V6 Spitfire (daily driver)
70 GT6+ (when I don't drive the Spit)
70 Spitfire (long term project)
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