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Discussion Starter · #1 ·
okay, my parts book tells me the rear end in my 1975 lemans is a 8.5" 10 bolt. the cover is identical to the cover shown for the 8.5" 10 bolt; however, when I opened it up the ring gear has 12 bolts and measures 8.875". now I always thought that "10 bolt" ment 10 bolts on the cover AND 10 bolts on the ring gear. the cover does not match the cover pictured for a 8.875" 12 bolt. the axles are huge and don't come close to fitting into the posi unit (yes, I bought all my parts before opening anything up; yes, I realize this is moronic). maybe someone put a 8.875" ring and pinion in? is this even possible? and what exactly am I supposed to do with all these 8.5" parts that I bought?? :pain:
 

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Return the parts to the place of purchase and ask to exchange for parts to fit the "P" axle 10 bolt, also known at 10 bolt cover 12 bolt gear.
Most GM diffs are identified by pinion spline count and and bearing journal diameter....
 

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Discussion Starter · #3 ·
okay I'm looking at my parts catalogs (jegs & summit) and I can't find anything that lists a "p" axle or a 10 bolt cover 12 bolt gear. the gear that came out of it is 8.8", not 8.5" like is supposed to be hiding under that 10 bolt cover. plus the axles are huge (gonna go measure them in a sec) and I thought I counted 33 splines on the axles but I cant find any axles listed that have 33 splines so I'll recount them. if I bought new axles could I use my 8.5 set up? I was going to send everything back, but I bought the posi on ebay, so no return there, and I just stepped on the styrofoam package the gears came in, so doubt they'll take them back. and I thought this day couldn't get much worse....
 

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Save a horse, Ride a Cowboy.
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Be thankful you have the big one. It's significantly stronger.

If you just hate it, I bet there are some people out there who would trade you even-up for their real 10 bolt.

What about the ID numbers on the housing? Use that to order parts.

FYI there are several GM 10 bolts that are 10 bolts, and the parts are not necessarily interchangeable from one to the other, either.
 

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The "10 bolt" and "12 bolt" designations refer to the ring gear. On Chevy ("corporate") axles, this also happens to correspond to the number of bolts on the cover. This is NOT always the case for the less common GM axles, as you've found out. The one to really be wary of is the Type O axle used on 67-70 Cutlass/442. These axles were unique to Olds and used an 8.5" 10 bolt ring gear despite the 12 bolts on the cover. I frequently see these advertised as 12 bolt axles. The sellers are either unaware of the difference or are lying.
 

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Discussion Starter · #7 ·
any time I read old posts on troubleshooting, hardly anyone ever posts what the final solution was. its like reading a book then having someone rip out the last page. so....

after much research I have found what is going on. the rear in question is what is known as a 8.875" mp series rear, used from 1971-1976. it has a 10 bolt cover that is almost identical to the 8.5" cover with the )( on the 3 and 9 o'clock positions. 12 bolts hold the ring gear on; it has a very large 1.8" diameter 30 spline pinion; 31 spline "c" clip axles. they were used on pontiac station wagons, grand villes, and bonnevilles (why its on my lemans I have no idea) as well as some full sized chevys and buicks (not sure about olds but I would imagine). it came in 2.56, 2.73, 2.93, 3.23, and 3.42 ratios, with 2.73 being by far the most common and 3.42 being very rare as it was only available as a mountain grade or towing option. no one makes aftermarket gears for it and no posi type carrier was ever made. seems like a waste since everything is so much more massive than the 8.5".

I'm off to find a GM 8.5" 10 bolt so maybe I can use all the parts I bought after all. this isnt exactly great news, but at least I know what I'm dealing with now. thanks guys for all your help.
 
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