I have a 70 GS455 that when got it has a non-original 8.5" open rear in it. I have no idea and no way of telling what gear is in it. Is there some way can tell without having to pulling rear cover off ?? By the way it goes down highway would say it is is 2.56 -2.93 rear range But car has no tach ( not yet) to even give an idea of what RPM is revving @ 55mph Building motor to 425-450 HP and I want to go to posi rear and 3,42 gear. What is most simply way to tell if it is series 2 or 3 rear-end ? When jack car up and rotate wheel the number of revolutions is not making sense as rear has much play in it now ? Or maybe I need better explanation on how it is done. Would like to get a posi and gears soon. is stock posei find or will need Eaton or Auburn ? I only run car on track at most once a year. Thanks very much !
well on the 8.5 , 2.41's and 2.56's had one carrier and 2.73- 4.?? had another . where if you are going to change to lower gears the 2.73- ? carrier is what ya want , dont run a ring gear spacer on the odd carrier . as for aftermarkets , eatons are good , yet ive always favored auburns
Jack up one side of the rear. Mark the driveshaft and tire with chalk as well as a reference point for both. Spin the one tire 2 full revolutions. Count the number of times the driveshaft makes a revolutuion. Two and a half driveshaft revolutions means it's a 2.56, etc. For more accuracy, Spin the tire 20 revolutions, and divide the driveshaft revolutions by 10, i.e. 29 revolutions means 2.93, etc.
Ahhh , that may make more sense now then !! Because when turned right wheel 1 full revolution , driveshaft only turned about 1.5 turns. I thought it was only 1 rev of wheel to driveshaft revs ?? I like the 20 turns of wheel. I really hope a see about 29 revs of driveshaft then would be 2.93, If have 2.93 I guess then I have a series 3 then ?. Also if I find cast number on housing will that help determine if it is a 2 or 3 Hope this will be me some day.... ------> :3gears: Thanx guys so far Dave
Since you are sure it is an 8.5 rear....the two most common gear sets were 2.56 and 2.73. If you can find the axle code on the pass tube.....2-4" from the edge where the tube enters the pumpkin...that will tell you the original gear set. For example....if you see LKB.....that means LK is the gear code for 2.56 and B stands for Buick. Since you are going to install a posi...the current gears don't make a difference since you will need to change the open carrier to a posi carrier.
Ok, Going to try tonight. 1 good turn deserves another. And also try to see the casting number on housing. Stewart I am just north of Chatham about 30 mins. Here is pix of rear. Has u bolts on yoke not the straps with 7/16" head bolts. Is it 8.5" then from pix?
Yes, you have the most common housing & axle version of the '71-72 8.5 A-body rear. Has the A9 axle bearings vs the larger A10's. Usually the ratio on single track '71-72 8.5 A-body rears is 2.56 or 2.73, that is on factory a/c equipped Skylarks & Cutlasses of that era. Got posi? :3gears: Roger
Not what I wanted to hear.. In turning tire find that with 2 turns that yoke turned approx. 2.5 turns and 20 turns of tire yielded 25.5 turns of yoke. So I have a 2.56 ..I guess it would be a Series 2 ?? :ball: :ball: What is needed to make this into a 3.42 posi rear ?? Or can I do it easily or properly ? Should I find a Series 3 rear ? Building my motor now for the 450-470 HP range.
As others have previously mentioned, you will not need your existing carrier or gears. To do the job properly you'll need an experianced diff. rebuilder to set up the new limited slip (POSI) and ring and pinion set. Yes, you will need a new Eaton, Auburn or rebuilt O.E. posi unit. A master bearing kit with adjustable shims and while you're at it you could have your axle bearings and seals replaced while their out. While the driveshaft is out check the u-joints as well as the trans rear seal. Just killing a lot of birds with one stone (or uzi) :blast: :laugh: Personally I repair them in the vehicle but most of you live too far away from me for me to be of any assistance. Good luck and ask more Q's here for additional help. :TU:
Sorry to hear all this Dave. Maybe I'm losing my mind, but I could swear that when I let that car go all those years ago, it had a 3:42 one-legger in it.
parts for your 8.5 rear Well You can shoot me a pm and will discuss your options I can supply parts no problem. It is your call. Jim
