T SERIES GEAR RATIOS
                             Part II

                          by Ben Prince      
             
                                          
     In the last installment, the desirability of changing rear
end ratios in T series MGs was discussed.  Conversion to an MGA
4.3 ratio was recently completed in my TF.  I am exceedingly
pleased with how much better the car seems to drive since
changing ring/pinion ratios.      
      
     Before you embark on a rear end change to your TD/TF, it is
a good idea to determine what ratio you currently have.  Even
when you find manuals and markings that indicate a particular
gearing, something different may have been installed since the
car left the factory.  The sure way to tell is by direct
measurement.

     It is easy to figure out the rear end ratio without removing
any parts.  The first step is to place chocks on both sides of
the front wheels and jack up one rear wheel.  Since you will be
crawling under the car, insert a jack stand or a block of wood to
make sure that the car will not fall.  The other rear wheel
should remain in contact with the floor.  The gear box should be
in neutral.  

     Make a chalk mark on the floor directly below the tire that
is jacked up.  Also mark the edge of the tire right above the
mark on the floor.  Next mark the drive shaft and establish a
reference point next to this mark.  Have someone slowly turn the
tire 16 complete revolutions.  Keep track of the number of turns
of the drive shaft.  If the drive shaft rotated 41 times, the
rear end ratio is 5.125.  If the drive shaft turned 39 times, the
ratio is 4.875.  If the drive shaft turned 36.4 times, the figure
is 4.555.  And if the drive shaft turned 34.4, you have the MGA
4.3 ratio.
           
     Suppose that the above test shows that your ratio is 5.125,
with the drive shaft turning 41 times for 16 tire revolutions.  
Note that 2 x 41/16 = 5.125.  We multiply by 2 in the calculation
because one wheel is on the ground and the differential gear
allows the jacked up wheel to turn twice as fast, compared to
both wheels turning equally.  If you were to push the car in a
straight line (both wheels turning the same), then the drive
shaft would turn 5.125 times for each tire revolution.

     Once the ring/pinion swap was initially completed in my TF,
I ran into a minor snag.  The hub at the differential protruded
5/8 inches farther than the original setup.  The drive shaft was
now too long-- it was up against the stop on the spline.  With no
slack, the drive shaft will bang on the back of the gear box and
damage it.                        
            
     Shortening the drive shaft is not a problem; there are a
number of establishments listed in the yellow pages who routinely
do this.  While you are at it, also have them balance the drive
shaft and check the universal joints.  Shortening and balancing
should cost about $50.
             
     The proper way do determine how much to shorten the drive
shaft is as follows.  First, push the drive shaft all the way in
(on the spline) and measure the overall length.  Suppose, for
example, that this distance is 37 inches.  Next extend the drive
shaft to its maximum length, say 38 inches.  Then average these
two numbers (to get 37.5 inches in this case).  With the new rear
end installation, measure the distance from the face of the rear
end hub to the back of the transmission.  Suppose this distance
is 37 inches.  Subtracting 37 from 37.5 results in 1/2 inch, the
amount to shorten the drive shaft.

     In Part III, we will discuss how to calibrate your
speedometer gears.