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?
Tony,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.
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.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....
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.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