high speed axle

[Mike in Maine]

Some high speed rear axles were identified by a tag 7-36 attached to the axle. If this tag is no longer on the axle, is there a way to tell if it is high speed without taking the axle assembly apart? Some were also stamped.
Where would it have been stamped?

[Neil Wilson]

If the rear axle is still assembled then the following procedure can be used to determine the ratio. Turn a rear wheel (with the other held stationary) for one turn and count the number of times the drive shaft or engine turns over. If you are counting engine turns, then make sure the transmission is in high (which is the 1:1 ratio or straight through position for the transmission). The count of turns times two is the axle ratio.

The 30/31 bevel high speed axle will result in slightly over 2-1/2 turns of the drive shaft (2.57 turns) to make the 5.14:1 ratio. When the drive shaft turns over 3-1/3 times the axle ratio is 6.6:1 (a low speed).

Also, keep in mind, that if your truck has a Ford "dual high" or other manufacture's over/under drive, then this extra gear box must be shifted so that it is straight through (i.e. a 1:1 ratio through the extra gear box).

Ford's original identification of gear ratios is as listed below:

30/31 Bevel Axle - The number of teeth in the pinion and ring rear (7-36) are stamped in numbers 1/2" high on the axle housings to mark the high speed axle. The numbers are stamped sideways across the flanges of the two bells on the back of the axle. See the "Ford Service Bulletin" for March 1930 (page 438). (36 divided by 7 = 5.14)

A second method Ford used to identify the high speed axle was with a white metal tag attached by the rear housing bolt. The tag has the numbers 7-36. See the "Indianapolis Service Letters dated 8/18/30. The tag is shown in the current judging standards "Heavy Commercial Vehicle" section.