Hey all! I know this subject has been talked about a great number of times but I have some info that might help other in the future. Keep in mind this info came from a person who owns his own shop and rear ends is all they do. They build rears for daily drivers, race cars, and restorations. They ship their products all over and are made to drop in and just plain work. Here is what Jim (the owner)has to say:
“Pontiac firebird axles are 29 ¾” – 29 7/8” same as the 68-72 GTO as I stated the firebird 67-69 rear is the same overall width as the 70-81 firebird BUT the spring perch on the 67-69 is 42 7/16” center to center and the 70-81 firebird is 45 3/8” center to center. Remember with a BOLT IN AXLE the axle shaft is about a ¼” away from the center pin they do not touch the center cross pin as a c-clip rear does. So we really do not measure the full length of the axle like we do with bolt in axles. GM has a 2.75 drum brake offset and the firebird 67-69 housing is 55.25 from axle end flange to axle end flange…then add the 2.75 x 2 and you get your over all width. This comes to roughly 60.75 overall, this is the reason for the Pontiac extra .25 backspacing on the rims. So they sit under the car with the little skinny tires they used back in the day.” The way he measures c-clip rear ends is he adds axle length x2 plus the width of the center pin and gets outside to outside (overall) width of the rear end. With this measurement you know exactly what you are dealing with because you are measuring through the center of the rear end and that will not change. Here are some common measurements that might help with determining what will work for your application: 8.5 (10 bolt): Center cross pin width 0.795” ; axle 30.062 Total width= 60.919” 8.2 (BOP 10 bolt): Center cross pin width 0.733”; axle 30.00 Total width= 61.233 8.875 (12 bolt): Center cross pin width 0.800”; axle 29.75” to 29.875 Total width= 60.3” to 60.55”
AS you can see there is .5 or less of an inch across all of the rear end measurements which opens up the possibilities a lot. I hope this helps someone.
I only know two things; I Love Pontiacs and a pair of TAs beats a straight flush every time.
Great write up! Thanks. By "Jim" are you talking jim mitchke also known as Monzaz on this site? Great guy and he really knows his stuff. I got my rear end from him and it was a simple drop in. Thanks for posting.
Great write up! Thanks. By "Jim" are you talking jim mitchke also known as Monzaz on this site? Great guy and he really knows his stuff. I got my rear end from him and it was a simple drop in. Thanks for posting.
You got that right! I also got my rear end from him and he hooked me up with a great deal and great rear end and he stands 100% behind his product!! Jim is a great guy to deal with, I would recommend him to anyone looking for a good rear end that built to your specs!
