matching diffs

[Tony Smith]

Originally posted by victor eaton
HELLO ALL
Help needed to determine ratio of back axle can it be done by counting revolutions of propshaft to revolutions of wheel? I have always thought it could for some reason but on trying it on our c 30 rear axle it doesnt to work out as the book states.

Yes it can be done. Firstly, you need to understand how a differential works and secondly, you need the diff ratios in a numeric form.
The spider gears in a diff mean that if one axle does not turn (ie that wheel remains on the ground) the other will turn at double speed when the pinion gear is turned.
The numeric ratios for large CMP diffs (C8A won't fit) are either 6.66:1 for C15, 6.5:1 for C15A and 7.16:1 for C30/C60.
Now, (with the vehicle chocked/secured etc) jack up one rear wheel. Put a chalk mark somewhere on the tyre to note it's position, and another on the pinion flange to help count rotations. You now count how many turns of the pinion flange are required until the tyre chalk mark returns to the start position, either 13.3 turns of the pinion for a C15 diff, 13 turns for a C15A diff or 14.3 turns for a C30/C60 (double the numeric ratio). You can either turn the pinion from the the vehicle or slowly turn the tyre while you count the pinion rotations. Don't lose count! You will see that the the number of rotations for the C15 and C30/60 diffs could be confused if you lose count.