I modified a Viking R2R  deck with 2 Nortronics 4 channel 1/4" tape heads. Each played  2 pairs of stereo tracks (4) the other head played the alternate channels. 
Nortronics made a head mounting jig with tape guides on both sides of the head to assure tape path alignment.. the jig also provided accurate asmuth adjustment.
With the advent of opamps it was easy to construct 4 stereo preamps eliminating  switching the  head leads for each pair of tracks. 
8 track tapes were recorded in one direction not back and forth. The player would move the head up or down to play each pair of tracks. The head assemble was on a ratchet the turned or other wise caused the  head to raise and lower across the tape.
The metalic sensing tape ran across a contactor that would activate the head movement ratchet. After it played the last pair of tracks the heads would return to the first position. 

  

--- On Fri, 3/12/10, Steven C. Barr <[log in to unmask]> wrote:

> From: Steven C. Barr <[log in to unmask]>
> Subject: Re: [ARSCLIST] 8-track cartridge degradation problems
> To: [log in to unmask]
> Date: Friday, March 12, 2010, 7:33 PM
> ----- Original Message ----- From:
> "Ted Kendall" <[log in to unmask]>
> > I just hod-rodded one of those Fostex 8 track things -
> surprisingly good!
> > 
> "8-track" tapes used 1/4" tape, but arranged in an
> "endless-loop" format;
> they actually, being stereo, used all eight tracks (.25"/8)
> on the tape. Metal
> (IIRC) portions at either end of the cartridge signaled the
> player to play
> the next set of tracks...this would continue (as I
> discovered one drunken
> night...?!) for as long as the player was operative...?!
> 
> I would guess that the tape, if removed from its cartridge,
> c/would be
> played on an r2r machine, providing it was capable of
> playing 8 separate
> tracks on a .25" tape (standard r2r "stereo" on a back
> & forth format...?!)
> 
> Steven C. Barr