[grahamp]

Looks as though I have a 69 Chevelle rear end manufactured by McKinnon in Canada. Cast number is 3959038, date code is 0325 and there is another number C 249, not sure what this is. 12 bolt posi unit, gear ratio unknown . McKinnon did supply axles to Pontiac I believe. Is there any possibility this could be original or just an imposter ?

[1oldgoat]

If it is a 12 bolt cover, it is definitely not original as Pontiac did not put a 12 bolt rear in GTO's until 1970 and only behind the 455 cars. All 69 GTO's had 10 bolt rears originally.

[grahamp]

Many thanks, I figured as much. Is it fairly common to find 12 bolt positrons on the Judge model?

[1oldgoat]

It was a fairly common swap as it was a strait bolt in. Do you know what ratio your original rear was?

[Judge273]

You got a 12 bolt on a 455 CID in 70 and 455 HO in 71 regardless of Judge and posi was an option on any GTO with a 12 bolt...

[grahamp]

Ratio on the window sticker was 3.55. I think the replacement is a higher ratio and closer to 4.00 since the rpms are very busy! This is what prompted me to take a closer look at the rear end. I tried to calculate by counting the prop shaft turns to one turn of the tire, came out as two to one which is certainly not accurate! Probably because I have a posi unit and had the other tire on the ground , I think both tires have to be free to turn? So will try that, and in extremis, I will remove the differential back plate

[grahamp]

Just a follow up, I would like to use a lower ratio number , seem to be several comments that a 3.26 is a better solution than a 3.55

[grahamp]

For Judge 273 - are you saying that a 12 bolt was an option for GTO’s but not before 1970.

[Judge273]

Yes, It was standard on 455 in 70 and 71 455. Posi optional.

[LPete]

Just throwing this out there, but with a regular posi you shouldn't be able to turn the driveshaft with one wheel on the ground. If you can it means the posi isn't working.

Originally Posted by grahamp

Ratio on the window sticker was 3.55. I think the replacement is a higher ratio and closer to 4.00 since the rpms are very busy! This is what prompted me to take a closer look at the rear end. I tried to calculate by counting the prop shaft turns to one turn of the tire, came out as two to one which is certainly not accurate! Probably because I have a posi unit and had the other tire on the ground , I think both tires have to be free to turn? So will try that, and in extremis, I will remove the differential back plate

[grahamp]

LPete - thanks, this is going from bad to worse! Am I correct in thinking that if both wheels are off the ground and you turn one wheel clockwise, the other wheel should turn anti clockwise? The casting number is said to be a posi unit, but maybe it isn’t - is there any other way to check without removing the cover plate. My Judge is a perfect and legit unit but this axle issue is a spoiler. Any ideas on where to find a correct axle ?

['ol Pinion head]

grahamp, if sourcing another rear for restoration use, it would be helpful to know if the Judge was originally ordered with Safe-T-Track, or if the original rear was a 3.55 Single track, do you want to go back that way? Listing an area of where you are located would also help.

Original '67-69 Pontiac A-body 3.55 STT rears will be built off a sealed axle bearing end usage nodular housing. Being an original nodular hsg, it will have its own model year specific casting number and will be julian dated. The common gray iron Pontiac 8.2 hsg for '69 Pontiac A-bodys which was used in singletrack & 2.56-3.23 STT builds had a different casting number housing which was alpha-numerically dated.

On your '69 cast McKinnon 12 bolt, the casting number on the hsg won't determine single track or factory posi. Have you examined the axle tube for the factory line stamping indicating factory code, assembly date? Totally agree a McKinnon 12 bolt is out of place in a '69 GTO, would however be interested in the stamped codes as ton discern whether it's original use was in a Chevelle/Elco/Malibu, or if it was factory assembled with the 3R pinion flange, non Chevy backing plates, and is coded for use in a '69 Buick Skylark.

[LPete]

If the differential is limited slip, and you hold the driveshaft, you shouldn't be able to turn the wheels. If it's not limited slip, yes, turning one wheel should turn the other wheel the opposite direction (as long as the driveshaft is held stationary).

Originally Posted by grahamp

LPete - thanks, this is going from bad to worse! Am I correct in thinking that if both wheels are off the ground and you turn one wheel clockwise, the other wheel should turn anti clockwise? The casting number is said to be a posi unit, but maybe it isn’t - is there any other way to check without removing the cover plate. My Judge is a perfect and legit unit but this axle issue is a spoiler. Any ideas on where to find a correct axle ?

[grahamp]

Old pinion head - here are some answers. Yes, my 1969 Judge came out of the factory with a 3.55 Safety Track HD rear axle and I would like to go back there with a correct date code if possible. Car was build at Fremont in April 69
Here are the stamps on the McKinnon 12 bolt Chevelle rear end - casting number is 3959038, the assembly date is 0325K and the date code is C249. I could not find a ratio code. I now doubt this is a posi unit, will try and do the wheel rotation test later today. The axle does have a GTO posi sticker XH which fooledme for a long time! I believe the McKinnon unit has a higher ratio than 3.55, much closer to around 4.00. The Judge is in Vero Beach Florida. It is a 2- Car, looks brand new and has won many awards. The axle imposter is a shocker so I want to fix it and get as close as possible to the original. Engine is original, 4 speed Muncie has just been rebuilt and has the correct casting and date code numbers. Thanks for any assistance . Chevelle unit looks as new.

[LPete]

Would you mind sharing your engine number? My Fremont July car’s WS engine number is 0698374, which seems like it should be in an earlier car. I’ve wondered if they had them sitting around for awhile.

[grahamp]

Old pinion - I am now pretty sure the McKinnon axle is not a posi but will make a final check in a couple of weeks when I get back from a trip. The ratio is about 3.90 based on the wheel turn method. Two anti clockwise turns of the driver side rear wheel equals about 3.87 turns of the drive shaft. I was very sure that this axle was a higher ratio than 3.55 which prompted me to take a closer look at the axle - surprise surprise! Many evil restorers out there!

[grahamp]

Old Pinion - my EUN is 0549408WS, the block number is 9790071 and the date code B189. Judge build date April 69.
How do I find/purchase a rebuilt rear end for the car?!

[Bigd400]

Originally Posted by grahamp

Old Pinion - my EUN is 0549408WS, the block number is 9790071 and the date code B189. Judge build date April 69.
How do I find/purchase a rebuilt rear end for the car?!

Not sure where your located but about a month ago I seen a complete rear end from a 69 Judge. 69 casting code, xh axle code and if I remember right around a 046 date code. No idea what’s actually in it though. I was gonna buy it but decided not to as I needed a xk.

[grahamp]

I am located in Vero Beach Florida which is 90 miles north of Palm Beach. Do you have a contact number for that 1969 Judge axle - sounds perfect. At the right price I would just buy it and have it rebuilt

[flyingn]

Originally Posted by grahamp

Many thanks, I figured as much. Is it fairly common to find 12 bolt positrons on the Judge model?

I see very few 12 bolts in 69 Judges. Very few. Its not a common swap from what I see