I'm confused (12 bolt type O Posi)
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Registered User
Joined: Mar 2015
Posts: 230
From: Berwick, Pa
I'm confused (12 bolt type O Posi)
ok so I have 2 different posi rears. I bought one that I was told is 2.78. Got it home and then realized there's no brake lines or brakes etc so I bought another that had all the brake hardware and was told it's a 3.91 rear.
Fast forwards few years, I sell one online advertised as 2.78 posi. I double checked before loading into my truck and I think it's a 2.56 (2 turns of the drum gives me about 5-6 turns of yoke) so I check the other that's supposed to be 3.91 and get two drum rotations to about 8 turns if the yoke (2.78) Yes I know this isn't EXACT but it's close. The housing on the one I'm keeping doesn't have a 4 but it does have 0227 or whatever which is a later housing vs the other which had a different number making it older.
The older one said BS on axle tube if I remember correctly whereas the newer one says SJ.
The confusing thing is that I don't see BS listed anywhere and SJ is listed for older 60's rears. I see different ratios listed for it too.
What the heck do I have??!!! I thought I had this figured out when I got them but now I'm not so sure. So im
thinking the newer one with 0227 on the housing and SJ on axle tube is a 2.78 rear. Can anyone clarify? I think the older one was 31 spline and the newer one is 28. I dropping the older one off tomorrow 3.5 hrs away but I just want clarification with what I have. Want to keep everything olds.
Fast forwards few years, I sell one online advertised as 2.78 posi. I double checked before loading into my truck and I think it's a 2.56 (2 turns of the drum gives me about 5-6 turns of yoke) so I check the other that's supposed to be 3.91 and get two drum rotations to about 8 turns if the yoke (2.78) Yes I know this isn't EXACT but it's close. The housing on the one I'm keeping doesn't have a 4 but it does have 0227 or whatever which is a later housing vs the other which had a different number making it older.
The older one said BS on axle tube if I remember correctly whereas the newer one says SJ.
The confusing thing is that I don't see BS listed anywhere and SJ is listed for older 60's rears. I see different ratios listed for it too.
What the heck do I have??!!! I thought I had this figured out when I got them but now I'm not so sure. So im
thinking the newer one with 0227 on the housing and SJ on axle tube is a 2.78 rear. Can anyone clarify? I think the older one was 31 spline and the newer one is 28. I dropping the older one off tomorrow 3.5 hrs away but I just want clarification with what I have. Want to keep everything olds.
same but different
Joined: Jun 2007
Posts: 2,965
From: Central Missouri
A lot of things can happen in the last 40+ years. Like Inline said, you got to pull the cover. If its the factory GM gears, the numbers will stamped on the ring gear. Or..just count them. Never been a fan of chalk marks and count the turns etc. Open it up.
Registered User
Joined: Jan 2010
Posts: 6,104
From: central Indiana
Counting turns of the shaft is ok for a ballpark figure. It’s pretty hard to tell the difference between a 3.42 and 3.55 gearset just by a mark on the axle and turning the yoke. No way would I buy a supposedly desireable gear ratio advertised that way.
Registered User
Joined: Feb 2013
Posts: 15,409
From: Phoenix, AZ
A 31 spline axle shaft O-Type rear would have 3.42, 3.91, 4.33, or 4.66 gears (maybe 5.00 but those were dealer installed). If you THINK it has 31 spline axle shafts, pull a shaft and VERIFY. It's as simple as removing 4 nuts on the retaining flange and yanking the shaft out. Keep the end of the tube higher than the center and no oil will spill out.
If you have 2.56 / 2.78 ratios then it is not a 31 spline axle shaft rear.
If you have 2.56 / 2.78 ratios then it is not a 31 spline axle shaft rear.
Just an Olds Guy
Joined: Jul 2008
Posts: 24,528
From: Edmonton, AB. And "I am Can 'eh' jun - eh"
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