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Taking a measurement for your new driveshaft is an important step in the process of building up your car project,  be it a restoration of an old classic,  or the manufacture of a full-on race or drag car.  Getting the measurement right is critical to the future performance of your vehicle.  Let's first look at the single piece driveshaft with a conventional slip yoke at the transmission and an 'open' yoke at the diff  (see picture 1).

Measuring a 1 piece tailshaft.

Firstly;  the installation of the transmission and differential must be completed,  then take a measurement from the output seal of the transmission to the 'face' of the diff yoke (equal to centre of the universal cap) as shown. That's it,  remember the carpenters rule;  "measure twice cut once" so make sure it's accurate.

Picture 1.

                                                             FORD 9" DIFF YOKES:
To identify yoke type & size refer to picture '1'.  Measure 'A' width between locating lugs and 'B' width from U'Bolt hole centres
'A' can be 82mm or 92mm and 'B' can be 35mm or 40mm.  This will identify the size of the yoke.  The universal joint is located in the yoke between the lugs at each side and retained in the yoke by U'Bolts,  see picture '2'

Picture 1:                           Picture 2:
Nodular Iron pressure forge                                                  'MARK WILLIAMS' Billet steel Yoke|
   cast 9" Ford Diff Yoke 

The main difference with these diff yokes from the Ford 9" is that the universal is located by circlips inside the shoulders of the yoke.     Vehicles in this group are all the original Holdens & Toranas up to the Commodore series. 
All Fords (except those fitted with the 9" diff) up until the EA & EB models when they changed to a flange yoke style.
Universal joints fitted to these yokes always had circlip grooves in the caps (see below)


                                     Measuring a 2 piece tail shaft.
First,  measure from 'A' - the transmission seal to - 'B'  the centre line of the centre bearing mount, 
     then from 'B' - centre of centre bearing mount to - 'C' - the flange face on the diff on flanged
     models,  or the 'face' of the diff yoke (equal to half way through the uni cap) on open yoke models.