31 spline vs 28 spline

[DuckRyder]

Think it is about time to build the old 9" for the F100.

Trying to dust to cobwebs off of the prior research.

To recap the truck:

390 - 30 over 10.6:1 with a Crower Street Roller. Desktop Dyno put it in the 450HP range that is probably optimistic.
C6 - ATI fuelmaster, Broader valve body, TCI soft kit VERY firm shifts

On to the rear end - it is a 3.50 open now with 28 spline axles. Plan is to build it with a 3.89:1 or so Detroit Locker. Because I'm really not looking to spend big money on custom 31 spline axles I'm considering using the factory 28 spline axles.

Is that crazy - If I'm not out there trying to drag race it on slicks - am I going to break the stock axles? I haven't broken the stand in Dana 44.

[1972hiboy]

oh boy I remember my detroit locker days. Sometimes they miss a spline and skip. you would think you just blew out the rear end. Mine was very street unfriendly in a 9-inch rear end with 3:50.1 , If your not after a smooth ride and not chirping tires around every corner then it will probably suite you just fine. They are supposed to be bullit proof. I think I had the 31-spline rear end and never broke a axle shaft but I wasnt throwing down the same power as you. I did manage to grenande my first DL and break the 9-inch housing. ( ripped the weld around the pumkin and into the axle tube.)

[Madman]

There are an arse-load of 28 spline equipped vehicles out there that are still on the road. I think you will be fine.

[DuckRyder]

There are an arse-load of 28 spline equipped vehicles out there that are still on the road. I think you will be fine.

That is kind of what I think too...

Not so worried about the Detroit Locker manners - truck isn't exactly subtle as it is...

[ultraranger]

I changed out the 28-spline axles and single track 3.50 geared 9-inch 3rd member in my '69 F100, for a 9-inch N-case 3rd member with a 4-pinion 31-spline Traction-Lok differential with 3:50 gears.

The 31-spline axles came from a 1971 Ford F100 that had a 9-3/8" rear end. 1968-1972 F100s with a 9-3/8" rear end had two different 31-spline axle bearing journal diameters, depending on what differential was in the 9-3/8" rear end.

If the 9-3/8" rear end had a Traction-Lok differential, the 31-spline axle bearings would have a 3-9/64" O.D. x 1-5/8" I.D.

If the 9-3/8" rear end had an open differential, the 31-spline axle bearings would have a 3-9/64" O.D. x 1-17/32" O.D.

The '68-'72 F100 31-spline axles are the same width as the regular '68-'72 F100 28-spline axles.

My stock '69 F100 28-spline axle on the left. '71 F100 31-spline axle on the right.


Stock '69 F100 C7AW-E 9-inch 3rd member with 28-spline single track differential and 3.50 gears on the left. '75 F150 4x4 Daytona 9-inch N-case with 31-spline 4-pinion Traction-Lok differential and 3.50 gears on the right.





31-spline axle installed.



'69 rear brake drum installed.

[DuckRyder]

Even though I don't really think I need it, I am willing to do a junkyard crawl for an N case and 31 spline axles...

Any way to tell if 9 3/4 - 31 without pulling an axle? (door axle codes?)

[ultraranger]

Even though I don't really think I need it, I am willing to do a junkyard crawl for an N case and 31 spline axles...

Any way to tell if 9 3/4 - 31 without pulling an axle? (door axle codes?)

Considering I currently just have a 240 and a C-4, I don't really 'need' 31-splines either but, I have them anyway.

An N-case may be difficult to find but, a regular Dentside 9-inch 3rd member with a 31-spline differential, Traction-Lok or not, would work with these axles.

First and easiest way to tell if the rear end is a 9-3/8" is to look at the face of the 3rd member. If the top rib curves downward on the left side, it's a 9-3/8".



I don't know if this is a definite identifier but, my stock '69 28-spline axles had two circular grooves on the center register of the axle flange and C8TW-D cast into it.

"Circular" grooves on my 28-spline axle flange.



Except for the casting number, the face of the 31-spline axle is smooth. Casting number on it isC8TW-E.

[mrtleavitt]

I have a 400 hp 428 and I love roasting the tires and breaking the wheels loose around 30 mph in my power band. I went through probably 5 or 6 28-spline axles. Then I went to a 31 spline rear end from a dent-side truck. I ended up breaking a stock 31 spline axle so I finally coughed up a little $$ and bought some 31-spline Yukon axles and I've never looked back. Now I am leaking oil out of the axle tubes where the leaf spring pads are welded on. Apparently too much torque/bad driving/clutch dumps/ etc. I need to strip it all down and weld everything up. Take it for what it's worth. Good luck!

[jrpumper]

im doing a 9'' build right now and im staying with 28 spline and i have the following,,,,.030 351 rollerwith comp cm and aluminum heads and a stealth intake and 650 demon and aluminum flywheel and a 3550 tremec i had a 3;25;1 rear gear with a open diff im gonna now have 4;11;1 with a spool

[RTG_RACING]

I'd stay with the 3.50 rear gear. I had a 3.73 and turned higher rpms than I wanted. I would think a 3.89 would be much worse.

[DuckRyder]

I'd stay with the 3.50 rear gear. I had a 3.73 and turned higher rpms than I wanted. I would think a 3.89 would be much worse.

With a 3.50:1 and a 215/75/15 it is below the cruise RPM of the cam @ ~60. With taller tires, which I plan to use, it is going to be worse.

The rev limiter on the engine is north of 6000 and the cam starts making power about 22-2500...

Still, since I'm feeling cheap, I may try the 3.50:1 I can always change it later.

[tnab1970]

Just for reference: I built a similar set up running a 429 and c-6 , was a " sleeper type" street race =truck. I ran 10.5" slicks on it most of the time and a bottle. truck ran mid to low 13's and was a daily driver. ran the stock 28 spline axles that came behind the 6 cyl that was in in when i got it. , In fact the only thing I did to rear end was remove pumpkin and install a " mini spool". These things are TUFF, Also ran a Mustang for years with 28 spline axles , running mid 11's. I think you will be fine.