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The casting code on my rear end is 385914N

Put in in Google and everything points to a 65 chevelle

Would the GTO be the same?
 

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Thanks Goatroper. I suspected this was a chevy rear end,,,, don't really care that much, just curious
 

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The casting code on my rear end is 385914N

Put in in Google and everything points to a 65 chevelle

Would the GTO be the same?

Just to clarify. You said "casting code." The codes listed for the GTO's are the stamped letter codes found on the axle tube. This is different than the "casting code." I am no expert by any means, but it is possible the casting code on the housing center section may be the same as the Chevelle as it is a GM piece, but all else is specific to a Pontiac spec'd rear axle when it was built/assembled.
 

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So this stumped me for the better part of the day.

I spent quite a bit of time looking around. And I can not find a definitive answer.

To begin I think you are missing a zero in the cast code. I think it should be 3859140N instead of 385914N.

What I did find is that that differential cast code was used by GM on Chevelle 12 bolt rear ends. The N indicicates it was cast at the Neenah foundry in Neenah, WI. I found 2 guys that both have that differential, One with a 66 GTO and one with a 67 GTO that have 3859140N differential's in them. They swear up and down that they were original to the car. Both of their cars were built in Fremont California. Chevelle's were built in the same factory for those years. But what confuses me is that I keep reading about Buick, Olsmobile, Pontiac (BOP) specific parts. GTO's are supposed to use BOP parts. Now I called this old timer that I met at Good Guys in Pleasanton a while back. He used to work at the Fremont Plant in the 60's and 70's. He told me the differentials came to Fremont as empty housings and they were actually out together in Fremont. The guts that were put into the housing were BOP parts.

But this is from his mouth and I cant find anything supporting this claim.

What I would do is look for the 2 digget code stamped onto the axle and compare it to the list above. If it matches up then I would be comfortable stating that it was original.
 

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The only "Chevy" casting number rears installed in US built Pontiac musclecars were McKinnon plant built 12 bolts, used in factory '70-72 Pontiac 455 application A bodys as well as '70 GrandPrixs with 400 4spds. The center housing casting numbers on these were 3969278, 3969278N, & 3969278NF. These exact same center casting numbers were ALSO used on same era ('70-72) 12 bolts cast out of two other GM axle plants, but those axle plants cast the center housing differently with a large casting date in the RH lwr webbing.

Early Pontiac A bodys all received PONTIAC 8.2 10 bolts.
PONTIAC 8.2 10 bolts had their own center housings, own style ring and pinion, own design carriers, and own axle tubes that flared out to accept Pontiac bolt in axles.

Chevrolet 8.2 10 bolt had their own round 10 bolt rear cover with a splash lip on it. Chevy 8.2 10 bolt rears axle tubes neck down on the ends, as the axles are are retained with c-clips inside the carrier sand do not have flared out axle tube ends. Chevy 8.2 10 bolts took their own style chevy 8.2 carrier and ring and pinion. The chevy 8.2 10bolt pinion gear has less splines and requires its own pinion flange which uses the small Spicer 1310 ujoint.

The only uses for c-clip axle Chevy 8.2 10 bolts in US production Pontiac chassis were:
-multileaf 8.2 Chevy 10 bolt in base 1970 Firebird
-mono leaf 8.2 Chevy 10 bolt installed in all production '71 Ventura II's.

If a '64-72 Pontiac A-body has a c-clip axle 8.2 Chevy A body (Malibu-Chevelle-Elco) rearend in it, someone other than the original assembly plant has installed that rearend in the car. It's that plain and simple. I've done it many times in Pontiac partscars in order to be able roll them around. Across the country, many folks over the years have been happy to find the early configuration A body rear they needed to get what had been a bolt-in axle A body going. Original rears often spun an axle bearing, the early ('64-early 66) length bolt in axles have often been hard to find and thus the early Malibu 8.2 rear that was avail for $50 made it under another Pontiac. Other than installing an adapter ujoint in the rear of the driveshaft, the 8.2 Chevy rear was cheap and easy fix, and worked for them. That is exactly how 8.2 Chevy rears end up under early Pontiac A bodys.

As a GM differential builder, I personally do not like the 8.2 Chevy rears, as the lack pinion support and have cclip axle retention. I seldom build them unless a customer asks me to. Chevrolet deemed the 8.2 Chevy cclip rears weak enough that when a '65-72 Chevy product had more than 275 gross HP and aprox 325 ft lbs of torque, a standard issue 12 bolt was used. Most of the stock 12 bolts Chevrolet used also had their own weaknesses... am straying from the original topic.

Hope this helps.
 

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This is interesting stuff, thanks everyone. To Tony, you're right it is 3859140N. That was a typo on my part when I entered it.
 

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So this stumped me for the better part of the day.

I spent quite a bit of time looking around. And I can not find a definitive answer.

To begin I think you are missing a zero in the cast code. I think it should be 3859140N instead of 385914N.

What I did find is that that differential cast code was used by GM on Chevelle 12 bolt rear ends. The N indicicates it was cast at the Neenah foundry in Neenah, WI. I found 2 guys that both have that differential, One with a 66 GTO and one with a 67 GTO that have 3859140N differential's in them. They swear up and down that they were original to the car. Both of their cars were built in Fremont California. Chevelle's were built in the same factory for those years. But what confuses me is that I keep reading about Buick, Olsmobile, Pontiac (BOP) specific parts. GTO's are supposed to use BOP parts. Now I called this old timer that I met at Good Guys in Pleasanton a while back. He used to work at the Fremont Plant in the 60's and 70's. He told me the differentials came to Fremont as empty housings and they were actually out together in Fremont. The guts that were put into the housing were BOP parts.

But this is from his mouth and I cant find anything supporting this claim.

What I would do is look for the 2 digget code stamped onto the axle and compare it to the list above. If it matches up then I would be comfortable stating that it was original.
Tony,
The N or NF deal, I don't believe has ever been conclusively determined that the Neenah foundry cast the center housings. On several of the later McKinnon cast center housings they have he N at the end of the casting number but were cast in house.

Also own several Pontiacs built out of Fremont. From my exhausted research, not that much done differently on one other than different style bolt head markings, different placement of the partial VIN on the block, and a date code stamping on the side of the original hood. On a chevy 10 bolt being factory installed in a early GTO, GM didnt use adaptor style ujoints. Such a build would have required its own driveshaft, one with spicer 1310 ujoint ends on each end. There would be engineering documents supporting that use and the special application driveshaft would have made it into the '67 assembly manual, there would be a notation in the upper box. Short of that happening, I just don't see the chevy 8.2 rears being original to those two cars. Both GTO's in stock form also made way more torque than what 8.2 Chevy rears were put behind Over the years, I've across many owners that want to state something being original but there memories were often hazy... just throwing this out.
 

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Nodular iron was used in select Pontiac 8.2 10 bolts from '67-71. There is a big N on the nose of the center housing along witha specific casting number. The 8.5 housings also used nodular iron construction. No nodular iron in in low horse Pontiac 8.2 housings or Chevy 8.2 housings, both used gray iron.
 

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I tell you what. Just when I thought I knew something you guys drop more knowledge on me.

I bookmarked that page for nodular iron. Today I will spend more time on it when I should be working. Hope my boss doesn't check my internet logs....

So this is why I did not bring up Nodular iron. I understood the N had to be on top of the cast code to be the nodular iron. You know on a separate line. The reason was the cast code was actually part of the casting mold. And the differential could be cast in grey iron or nodular iron. When the cast was done in nodular iron they had this extra piece of a plate that they literally screwed into the cast. That is why when you look at the date codes on blocks they are usually adjacent to the block cast code and you see a screw impression to the right and left of the date code. But again I can not find anything 100% validating this. It is all coming from guys who worked at these plants and said this is how it was done when they worked there

And Pinion Head is correct. It was never confirmed that all the N of NF differentials always came out of the Neenah foundry.

So this old guy met at Good Guys who worked in Fremont has been a great help explaining things about how the Fremont plant was run. But what absolutely drives m crazy is non of these procedures or the way things were done is laid out consistently. Or even written down.

So yesterday he told me the differentials came to Fremont as empty housings and they were assembled in Fremont and then stamped in Fremont with what was inside them.

That got me thinking about the engines.

So lets say engine block cast code is 9773155. This is the engine block cast code cast code for a 64 GTO. In a GTO this engine produced either 325 HP or 348 HP. Depending on installation of tripower. But that same exact engine cast code is also used in 1964 B body Pontiac's and those produced 215HP. But the block cast code was exactly the same. But the stamps that were done with hand tools are different. For the 215hp it was stamped 01A and for the GTO it was stamped 76W or 76X or 77J or 78W or 78X or 79J or 769 or 789.

So it kinda sounds like what the old timer was telling me was right. But I really can not find any thing with 100% certainty that this is the case. Does anyone here know what the difference is between two blocks that both have a block cast code of 9773155 but one is hand stamped 01A and the other stamped 76W. Does the difference only indicate the internal components of the block. crankshaft, connectors, pistons, etc....... Did the bore size change?

What we really should do is try to find guys that worked at these places and just write a damn book on it. Or at least put it in one place where people like me who are confused can come and just read up on it. But then they will come across posts like mine and read something that can not be verified and pass it off as "the way" further adding to confusion.
 

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I tell you what. Just when I thought I knew something you guys drop more knowledge on me.

I bookmarked that page for nodular iron. Today I will spend more time on it when I should be working. Hope my boss doesn't check my internet logs....

So this is why I did not bring up Nodular iron. I understood the N had to be on top of the cast code to be the nodular iron. You know on a separate line. The reason was the cast code was actually part of the casting mold. And the differential could be cast in grey iron or nodular iron. When the cast was done in nodular iron they had this extra piece of a plate that they literally screwed into the cast. That is why when you look at the date codes on blocks they are usually adjacent to the block cast code and you see a screw impression to the right and left of the date code. But again I can not find anything 100% validating this. It is all coming from guys who worked at these plants and said this is how it was done when they worked there

And Pinion Head is correct. It was never confirmed that all the N of NF differentials always came out of the Neenah foundry.

So this old guy met at Good Guys who worked in Fremont has been a great help explaining things about how the Fremont plant was run. But what absolutely drives m crazy is non of these procedures or the way things were done is laid out consistently. Or even written down.

So yesterday he told me the differentials came to Fremont as empty housings and they were assembled in Fremont and then stamped in Fremont with what was inside them....
On the Julian date casting plate being screwed to the casting box, can definitely see that on the nodular 8.2's and I believe (need to dbl ck couple of my 12 bolts), the McKinnon cast 12 bolts. The nodular 8.2 Pontiacs have Julian based date coding (a number from 001 to 365), along with the big N cast into the nose of The housing. The nodular iron use began in late '66 for '67 model use, but only on select usage safe-t-track rears. It was basically a part of what was needed as there was a lot of pinion deflection under extreme loads with the small pinion in the 4.33 and 3.90 gear sets. '67 nodular Pontiac 8.2 A body rearends have always been the easiest for me to come up with, followed by the '68 nodulars. Every year '67, 68, 69, 70 the casting numbers changed on the nodular 8.2 center housings, so when identifying a bare nodular housing with no original dated gears, that best way to ID. Late in the '69 model year, '69 Pontiac 8.2 rears gained tapered bearing 8.2 Pontiac rears. I've never built an original tapered axle bearing rear out of a '69 Pontiac A-body, only the sealed axle bearing style. Have ran across late '69 usage Firebird rears that used the far superior tapered axle bearings. Eventually Pontiac drivetrain engineers found the limitations of the HD Safe-T-Track 8.2 rears and they began spec'ing the McKinnon cast 12 bolts and 8.5 10 bolts to go behind their highest torque load engines.

Tony, really appreciate your speaking with the former Fremont autoworker. Would love to hear if the practice of Fremont assembling their own differentials continued up through '72 models. My late '71 GT-37 has its original XV K coded McKinnon 12 bolt and the axle tube stamping is not picture perfect like many an earlier 10 bolt. Also have the original 12 bolts in two of my other HO cars, both built out of Pontiac MI, and they too, the axle code stamping string doesn't look like it is flawless, like out of one gang stamper. Similar deal too with a few 3.07 12 bolt cores I have from '70 and '71 455 D port cars. With you mentioning this, am going to take pics of all the McKinnon housings I have and compare to saved file pictures I have of others.

On the '64 GTO engines, I've ran across is a few tripower blocks coded 76 XW. I'm not a '64 guy, have heard at least one explanation, that the W was for wide ratio 4 spd. The M20 being the only 4 spd avail for '64 in GTO's, sounded good, but hard to believe. The latter W appeared to have been stamped later, possibly at the assembly plant, but that would mean the assembled engine was shipped from the Pontiac engine plant either as a 4bbl, or without the tripower set-up and some form of bolt down cover that totally covered the intake openings to protect it on the boxcar ride from Pontiac MI. Love to learn more!
 

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I just looked at a bop axle. it has a twelve bolt cover but the inside is a 10 bolt ring gear. also no C clips, axles are held in by a four bolt flange on each end
 

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I just looked at a bop axle. it has a twelve bolt cover but the inside is a 10 bolt ring gear. also no C clips, axles are held in by a four bolt flange on each end
Welcome Mel. What you looked at is a mid '67-70 model usage Type "O" Oldsmobile rear. Nothing "bop" about them. The Type "O" rears were made in several incarnations. For Cutlasses, Vista Cruisers, & 442's there were several different housings to accommodate different sized sealed axle bearings, & either 28 spline or 31 spline axles. Though quite a few 31 spline axle hsgs made it into the many lower ratio '67 & '68 442's & '68 RamRods, the 31 spline axle's large axle bearings are of the archaic sealed axle bearing design, & they eventually seize on the axle bearing inner race & chew up the expensive axle & have to be replaced. Have just such a '68 factory 3.91 Anti-Spin 442 rear.

There are a few ratios of new very expensive ring & pinion sets, as well as modified ford 8.8 traklock posi carriers that can be purchased to install in a single track type "O" rear. When all said & done, I only go through the Type "O" rearends for fairly high end restorations. The later 8.5 A-body rear is a MUCH better platform to build off of, has considerably more pinion support, as well as a large variety of aftermarket HD posi units, last all of the 71-72 8.5 A-body rears use tapered axle bearing retention.
 
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