28 or 31 Spline Rear

[franklinair]

'68, 302, 4V, "J" Code, 3.00 rear gear ratio (ring gear & pinion):
What determines the splines in a standard rearend axle, 28 or 31? I would assume the small block cars would use the 28 spline, while the big blocks would use the 31 spline axle. True?

Neil:undecided

 

[CougarCJ]

Neil, I am not aware of any 8 inch rear ends with a 31 splined axle.

All 289 and 302 equipped Mustangs (1964.5-1973) received an 8 inch rear end with the exception of the K code 289 HiPo's and Boss 302's which all received a 9 inch rear end.

I am only aware of 31 splined axles in Boss 302, Boss 351, Boss 429, 428CJ's, and 429CJ's.

 

[franklinair]

Got it. Thanks.

Neil

 

[robert campbell]

31 spline axels could come in a 9 inch behind any engine if you checked the right block on the order sheet. Typically behind the 4 pinion trac lock or Detriot locker. As these rear ends were common to the specialty Mustangs they are thought to only reside there.

I have seen 31 splines in many 68 and newer high performance Fords (not just Mustangs) with small blocks and especially with 351 Clevlands. 31 spline axels were very common in 68 to 72.

Rob

 

[franklinair]

Then I can assume my 302 with the 3.00 rear is a 28 spline?

Neil

 

[robert campbell]

Neil,
Yes. A quick way to identify a factory 28 spline and 31 spline axle is with the wheel off. The end of the axle that forms the center device for the brake drum is "cupped" or carved out. If it was standing on end you could pour about a 1/2 cup of water and the cup area would hold it.

On a 31 spline axle this area is solid except for two holes that were drilled into it.

If you have an 8 inch rear, you have 28 spline axles. The aftermarket world is offering all sorts of high zoot 8 inch rears with much stronger 28 spline axles that are designed for more abuse. I had a 3.80 ratio 8 inch rear I sold to a friend in the 1970's and he put it in an A code 67 Mustang fastback with a 4 speed. He loved to hammer this car. One day he hammered second gear and broke the 8 inch housing by actually yanking the entire pinion gear out the front. When he came to a stop we pulled the drive line out with the u-joint and pinion gear still attached to the drive line with nearly the complete front of the rear end housing!! I also have a couple ends from 28 axles that were twisted off.

Are you just curious or looking to make it stronger?

Rob

 

[franklinair]

Just curious. (Truth be known, I drive mine like a little old lady!) Whenever I see or hear discussions of engines running @ high RPM's (5,000/9,000) all I can do is visualize how fast all those steel parts are moving internally. It scares the yell out of me! Do the math: 6,000 RPMinute divided by 60 seconds=100 per second. :icon_pani

Thanks for the info.

Neil

 

[robert campbell]

RPM's equal "Rotating Parts Mangled or Missing"!! I spin the motor in my 67 to 7k all the time. She be very busy!!!! 64 Hipo 289 block and crank. ARP rod bolts and studs through out the engine. Soild roller cam, windage tray, forged pistons and so on. Built to spin, but still scary at times!! Not sure when you will need a catchers mitt to catch the items flying out from under the hood!!

I drive my cars like an 18 year old teenage boy who just figured out what the foot pedal on the right does. I may be getting older, but I refuse to grow up!!!

Rob

 

[joedls]

(Truth be known, I drive mine like a little old lady!)

Me, too. :wink:

 

[franklinair]

Now I REALLY believe all those things Donna told me about you guys.

Neil

 

[joedls]

Now I REALLY believe all those things Donna told me about you guys.

Neil

I wouldn't. Donna lies....:grin:

 

[murf104]

Do the GT cars with a 302 have a 9 inch or an 8 inch?

 

[robert campbell]

The GT option by itself has nothing to do with the rear end size. I would bet most came with an 8 inch rear. But I have seen others with 9 inch. It would normally be attached to the person ordering it with a posi unit, mostly with a 4 pinion trac lock or the detriot locker.

Again, Boss cars, Cobra Jet cars, and Shelby's were very loaded cars and purpose built for abuse and the 9 inch was a minimum in most if not all cases. The 9 inch 28 spline 2 pinion trac lock came in alot of "S" code 390 cars.

Bottom line is most 1968 302 J codes came with an 8 inch rear and 28 splines. But if ordered, they could have a 9 inch with 31 spline. I had a friend who had a 68 GT 302 Fastback with a 4 speed and 9 inch rear end. It was identified on the door tag as stock to the car. This was way back in the 70's before faking was big time. It was ordered that way. I bet their are people in this registry with J code 4 speed cars that have 9 inch rears.

Rob

 

[teamlo]

All 1968 J-code cars with the GT option had 9" rear ends from the factory. And it had EVERYTHING to do with the GT option. The suspension was a major part of that option, among other things. The 9" rear end was part of it. I've owned 2 1968 J-code GT coupes, and my cousin owns a 1968 J-code GT convertible. They all had FACTORY 9" rear ends installed. One was a 3.00 open and the other two were 3.25 open. All three 28 spline.

 

[teamlo]

The only Mustangs that came from the factory in 1968 with Traction Lock units and 31 spline axles were the 68.5 428 CJ cars. Everything else was 28 spline, and the differential was known as the Equa Lock. Not as durable as the Traction Lock.

 

[robert campbell]

Ah my favorite subject. People who deal in absolutes. Again I have to refer to the best of my knowledge and what I have read in documented sources. The 1968 “GT” option contained the following items. Fog lamps, dual exhaust with quad outlets, GT stripe (C stripe), heavy duty suspension including stiffer springs shocks and heavy duty sway bar, wide oval tires and style steel wheels, and a pop open GT gas cap. Even disc brakes were NOT part of the option.

Typically a 9-inch rear end was checked as part of the option package. But again it is NOT part of the GT package. The disc brakes were part of the big block package and were added automatically. But I would never say a big block GT never slipped out with drum fronts.

The above is document in “Mustang Does It” on page 202.

As far as 31 spline axles go, they showed up in 1968 as documented in the “Mustang Restoration Guide” on page 238. Any 1968 car could be ordered with a “I will say” posi trac rear end with 31 spline axles. 4 pinions and the term Trac Lock is better said for 69 and newer and I agree in the 68 ½ Cobra Jet.

I would not wager that a few of these “31 spline 4 pinion Trac Locks” did not find there way into other big block cars.

By the way, check out Jbarts Marti report on page 4 of the “Post your Marti report”. No GT option and a Trac Lock 3.00 ratio rear end as a check mark on the option list. A clear option check and not part of the GT group. She ain’t no 68 ½ Cobra Jet is she?

Never say Never

Rob

 

[CougarCJ]

Ah my favorite subject. People who deal in absolutes. Again I have to refer to the best of my knowledge and what I have read in documented sources. The 1968 “GT” option contained the following items. Fog lamps, dual exhaust with quad outlets, GT stripe (C stripe), heavy duty suspension including stiffer springs shocks and heavy duty sway bar, wide oval tires and style steel wheels, and a pop open GT gas cap. Even disc brakes were NOT part of the option.

Typically a 9-inch rear end was checked as part of the option package. But again it is NOT part of the GT package. The disc brakes were part of the big block package and were added automatically. But I would never say a big block GT never slipped out with drum fronts.

The above is document in “Mustang Does It” on page 202.

As far as 31 spline axles go, they showed up in 1968 as documented in the “Mustang Restoration Guide” on page 238. Any 1968 car could be ordered with a “I will say” posi trac rear end with 31 spline axles. 4 pinions and the term Trac Lock is better said for 69 and newer and I agree in the 68 ½ Cobra Jet.

I would not wager that a few of these “31 spline 4 pinion Trac Locks” did not find there way into other big block cars.

By the way, check out Jbarts Marti report on page 4 of the “Post your Marti report”. No GT option and a Trac Lock 3.00 ratio rear end as a check mark on the option list. A clear option check and not part of the GT group. She ain’t no 68 ½ Cobra Jet is she?

Never say Never

Rob

I would like to see a copy of an option sheet that shows a 9 inch being available as a stand alone option.

 

[teamlo]

The 9" rear end was NOT available as a "stand alone" option. In other words, you absolutely COULD NOT go into a Ford dealer and order say, a C-code 289 2V 1968 Mustang with a 9" rear end. If you specified that you wanted a locking differential, you could get that, whether the car had an 8" or 9" rear (I am speaking about V8 cars in this instance). Every single J-code 1968 Mustang WITH THE GT OPTION that I have ever seen (and I've seen a lot of them) had had a 9" rear end. Period. My car ('68 GT coupe, J-code, automatic) has the original rear end tag attached to it, which shows a 3.25 non-locking 9" rear. And guess what? No where on the Marti report does it indicate that it was an "option". I also checked the Marti from the '68 GT coupe I owned in the 1980s. Ditto, except that it had a 3.00 non-locking 9" which was actually an optional axle ratio that year. Notice I said RATIO. The standard axle ratio with the J-code 1968 GT was 3.25. My cousin's '68 J-code GT convertible.......3.25 non-locking 9". Nothing on the Marti report showing that a 9" axle was specified, because that was the STANDARD rear end for that car. Car I almost bought in 2006 before buying mine..........1968 GT coupe, J-code, automatic. That car had a 3.25 Equa Lock 9". It specified Equa Lock on the Marti as an option, and from the pics I have when I was looking at buying it, clearly a 9" rear end.

Everyone is entitled to his/her opinion on the subject. Mine boils down to this: With the GT option in 1968 on the J-code cars, you got a 9" axle. If the car was simply a J-code non-GT, you got an 8" rear end.

Incidentally, the door tag on the car does NOT differentiate between 8" and
9". It simply tells you what the axle ratio was, and whether it was a locking diff or not.

 

[teamlo]

If anyone can produce a copy of a Marti report for the 1968 model year Mustang showing that the ordering dealer specified an "upgrade" to a 9" rear end, or one showing an "upgrade" to 31-spline axles, I'll personally buy that individual a steak and lobster dinner (or a nice falafel if you're vegeterian).

Terry

 

[franklinair]

STOP ALREADY!!

I was only curious as to what the spline count is on my '68. Rob gave me the simplest answer - look at the outer end of the axle. If it's dished, it's 28 splines. End of story.
I feel like I asked what time it is, and I got instructions on how to build a watch!

Neil