How does one find out if a 1969 Mustang Fastback has an 8" or 9" axel with 28 or 31 splines.

Asked by Nov 29, 2014 at 09:34 AM about the 1969 Ford Mustang Fastback

Question type: Maintenance & Repair

I am trying to order a barake line kit for the 1969 Mustang , but need these details to
make a selection of the appropriate kit.

2 Answers

5,035

The 9" rear end was and still is used in high horsepower applications, so if your Mustang has a 429, it's probably a 9". If it's a 6, then it's probably the 8". The differential cover gasket size for the 8" is 9-3/4" x 10-15/16" and gasket size for 9" is 10-3/4" x 11-5/8"

2,325

You can identify the 8-inch a couple of different ways. First of all, you know to look for a differential that unbolts from the front instead of the rear. That will narrow it down to two of the Ford Hotchkiss-type third-member rear ends, either the 8-inch or the 9-inch. Next, put a socket on one of the lower mounting bolts. If the socket will slip directly onto the bolt, you have found an 8-inch rearend. The lower bolts on the 9-inch are not accessible with a standard socket and extension because the areas around the bolt holes are reinforced for strength to support the larger pinion support on the 9-inch differential. If you are looking at an assembly without the carrier in it, measure the distance between the two lower bolt holes. The 9-inch will have a spacing of 10 inches between these lower bolts, the 8-inch will only be spaced about 83/4 inches. Another easy identifying mark on the early 8-inch rearends is the ribbing on the carrier housing. Those carriers having only horizontal ribs on the case were used in cars built between 1962 and '67. Pre-'67 differentials also will have a fill plug on the rear of the housing as you might see on a conventional rear cover, and the cover will have two dimples. Later model 8-inch carriers added a fill plug to the carrier itself and the rear housing had no filler hole or dimples. In the 1967-and-up vehicles, Ford added vertical ribs to the carriers to better reinforce the assembly.You my get lucky and the tag will still on the rear end it's located in the front of the housing, on one of the bolts. there should be a 4 or 5 digit #, if you find that, i can tell you what it is in a few minutes...hope this helps

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