Page 1 of 1

differential gear ratio

Posted: September 7th, 2017, 9:50 pm
by wild4679
I've learned AA's came with different gear ratio's in the differential. The higher speed rear ends were stamped on the outside. How does one find these stamps? I have three complete rear ends and two of them have stamps on the right side. I have included a picture of one of them. At this writing, I can not read the stamp, but believe that with some elbow grease, I can interpret what is stamped.

Can someone help me learn if this is the gear ratio stamp and what it means? My goal is to install a rear end in my
31 to obtain the maximum comfortable highway speed.
Rear end stamps.JPG
Rear end stamps.JPG (900.96 KiB) Viewed 736 times

Re: differential gear ratio

Posted: September 7th, 2017, 10:06 pm
by Chris Haynes
Easiest way is to remove the speedometer drive from the drive-line. The ratio will be stamped on the gear.

Re: differential gear ratio

Posted: September 8th, 2017, 6:50 am
by wild4679
Thanks Chris. Only one problem: the speedometer cables are gone. So not only do I need to know the gear ratios of the three rear ends so I can choose the two I will use (two trucks to rebuild), but I'm assuming I will need to know the ratios to order the correct speedometer cables.

Now what?

Re: differential gear ratio

Posted: September 8th, 2017, 8:42 am
by Bob C
Like Chris said you need to remove the driven speedometer gear from the torque tube not the speedometer cable.

Bob
driven-gear.jpg
driven-gear.jpg (61.04 KiB) Viewed 711 times

Re: differential gear ratio

Posted: September 8th, 2017, 11:16 am
by 1crosscut
Block one drum / wheel from turning. Mark the other drum / wheel. Clamp a vise grip to the end of the drive shaft and see how many turns it takes to rotate the marked wheel one full rotation. You can compute the gear ratio from that.

Re: differential gear ratio

Posted: September 8th, 2017, 12:29 pm
by Farrell In Vancouver
You are looking for a tag on one of the diff bolts that reads 7-36 which should denote the 5.14 gear set for high speed.

Re: differential gear ratio

Posted: September 8th, 2017, 2:59 pm
by Chris Haynes
Farrell In Vancouver wrote:
September 8th, 2017, 12:29 pm
You are looking for a tag on one of the diff bolts that reads 7-36 which should denote the 5.14 gear set for high speed.
My Early 1930 has High Speed gears. But NO TAG on the rear axle. A tag is easily removed.

Re: differential gear ratio

Posted: September 8th, 2017, 5:45 pm
by Hayslip
The images on the housing as shown above are hallmarks and will tell the restorer NOTHING. They may refer to suppliers of housings or they may indicate batches of stampings in case of defects; a mystery yet to be solved. Obviously not all AA housings were made in the same casting mold; maybe they indicate which mold in case of a problem.

Re: differential gear ratio

Posted: September 8th, 2017, 8:12 pm
by Neil Wilson
1crosscut wrote:
September 8th, 2017, 11:16 am
Block one drum / wheel from turning. Mark the other drum / wheel. Clamp a vise grip to the end of the drive shaft and see how many turns it takes to rotate the marked wheel one full rotation. You can compute the gear ratio from that.
FYI - You need to divide the number of drive shaft revolutions by 2 to determine the gear ratio.

Re: differential gear ratio

Posted: September 8th, 2017, 8:15 pm
by tiredtruckrestorer
The lower triangle in the picture (although upside down to read the letters in it) is the Timken trademark. The big "T" down the middle is for Timken, the "D" and "A" on either side of it is for Detroit Axles, and the "CO" beneath it is for Company. Can tell what the other triangle means.

Keith

Re: differential gear ratio

Posted: September 8th, 2017, 10:16 pm
by Stakebed
tiredtruckrestorer wrote:
September 8th, 2017, 8:15 pm
The lower triangle in the picture (although upside down to read the letters in it) is the Timken trademark. The big "T" down the middle is for Timken, the "D" and "A" on either side of it is for Detroit Axles, and the "CO" beneath it is for Company. Can tell what the other triangle means.

Keith
So since its timken logo embossed does that mean its one of those Canuck rearends? :mrgreen:

Re: differential gear ratio

Posted: September 9th, 2017, 4:15 pm
by tiredtruckrestorer
No, the Canadian Timken rear is completely different than the '30-'31 "AA" Timken used in the trucks in the US.

Keith

Re: differential gear ratio

Posted: September 10th, 2017, 10:01 am
by Chris Haynes
Once again. Take a wrench and remove the speedometer drive. The gear ratio is stamped on it.