12 bolt ?

[sweet67]

Hey guys,
I have a 12 bolt from a 1980 pick-up I am planning to putting in my 67 camaro, but the gears are a little lower than what i wanted, they are around 3.50:1 ratio. Can i put a 3.90 or even a 4.10 in the housing i have?
Thanks
kev

[big gear head]

The truck 12 bolt gear ratio would be 3.42. This will be on a 3 series differential. You will need a 4 series to use a standard 3.73 or 4.10 in the truck 12 bolt. A thick 3.73 and 4.10 are available for this rear end so that you can use the 3 series differential. They are a little more expensive that the standard gears.

[chicane67]

The passenger car 12bolt is totally different as compared to the truck 12bolt. Not to mention that the truck differential is about as strong as the factory 8.2".

Available ratios are as follows:

3.08
3.42
3.73*
4.10*
4.56*
4.88
5.13
5.38

* = Available for use with a 3 series carrier.

[ProBell]

I have a friend that has a 70 Chevelle 12 bolt for sale. That is what i used to have in the car when I was drag racing it. I cut off the factory mounts and welded on some spring perches. Let me know if you want any info. I think it is a 3.75 .I am not shire on the width, but the one I used was the same as the 10 bolt I took out. Randy

[big gear head]

The '70 Chevelle rear end will be 1 inch wider than the '67 Camaro rear end.

[sweet67]

Thanks for the offer pro bell. I do have a 9" too, but was going to use the 12 bolt for a couple reasons. One it would be a little cheaper to build, second it doesn't take quite as much power; i know the difference is little but i am not going to be putting huge power to it anyway. Also gear head... why is the truck 12 blot weaker than the car 12 bolt? to be honest i would have thought the opposite! Thanks for the replies guys!
Kev

[big gear head]

The only thing about the truck 12 bolt that is weaker than the car 12 bolt is the smaller inner pinion bearing and the pinion shaft size. The truck differentials were weaker, but you will probably be replacing that anyway.

Here is a truck Eaton posi case. Notice that there is a large window on both sides of the case. The car Eaton posi only has one window. The arrow is pointing to the place where every one of these that I have seen has a crack. The new Eaton posi for the truck 12 bolt is made like the car Eaton posi with only one window. They are much stronger than the old original cases.

The truck Governor Lock shouldn't even be considered for a high performance car. There are too many small parts in it that will break when you abuse it. They were just barely strong enough for a stock truck.

[chicane67]

#2... Pinion shaft diameter.

[sweet67]

yeah i will be replacing everything, i plan on using the the housing only and getting all new guts.... this doesn't have much to do with the topic but i was also thinking about welding the axle tubes solid to the housing. thanks for the info!
Kev

[big gear head]

If you decide to weld the axle tubes you need to be sure that you use nickel rods or wire to weld them. Remember that you are welding mild steel to cast iron and the only way to do this is with nickel. The housing needs to be preheated too. Check this out. http://www.nastyz28.com/forum/showth...ing+axle+tubes

[MonzaRacer]

Whoa whoa whoa.
After 79 you will not find a true Truck 12 bolt in a C10 unless it was taken from a different older truck.
The big trick to tell is check the threads on the pinion,fine thread is true truck 12 bolt and the coarse thread is 8.5 10 bolt.
Now as for these being weak, just how you figure this?
The truck 12 bolt is the stronger of the 2 as the 10 bot and 12 bolt have the same spline size the onl;t thing is the 10 bolts got larger pinon seal diameter.
Now if you need a constant velocity joint from the 70s big cars (sayin a dropped or lifted truck with a 12 bolt is you have the companion flange turned down and the end swapped onto your drivesshaft.
As for my choice for a rear end for that car just get either a 10 or 12 bolt diff from a 70s Camaro, if its wrong width then switch wheels, or narrow it. If you can keep from narrowing it it saves on axle price as you can but them off the shelf.
Now Ill get a lot of flak for saying rec. a 10 bolt 8.5 but they are not that much smaller than the 12 bolt and can take a LOT of abuse if built right.
Therewas one of 10 or 10.5 in Fastest street car shoot out bunch that had a Camaro with a 10 bolt 8.5 and they stood that car on its bumper dang near every run.
I believe the name was Bob Joysey if I remember correctly.
Get a good center section, some quality gears, and good axles and clip eliminators and a preload cover and have fun.
Now one good thing to do is remember to break in your new diff with regualar dino gear oil for atleast 1000 miles.
When I build a diff I recomend followingthe breaking drive (50 miles ,vary speeds, pull over and let cool for a few hours, drive home then drain and fill again with non synthetic and drive another 1500 to 2500 miles at least I really like to see it broke in for at least 5000 miles then drain it and refill with synthetic fluid. I always use limited slip additive in dino gear oil as it had extreme preasure additives . Now synthetics dont need them.
Now some gear manufacturers will tell you to run as many as 10k miles before synthetic gets installed.
IF its drag only diff use the synthetic.
Lee Abel
AFTERMARKET PERFORMANCE

[big gear head]

The 12 bolt was used in trucks up to '82. Then they went to the 8.5 10 bolt.

[chicane67]

I'll second that Freddy...

The 8.5" bolt and the 8.875" passenger car 12 bolt are stronger than the truck 12 bolts. Peroid.

The failure point in the truck 12 bolt is the smaller, inner pinion shaft diameter. The inner pinion bearings of the 8.5" and 8.875" are much larger, albiet the spline size are the same on both. The 8.5" and the 8.875" passenger car pinions shafts have a taper to the yoke end spline.

The strength in and of any differential is an answer combined from many variables. From one point, I have seen an 8.2" go low 9's and 12" C12's go into the mid 4's. On the opposite end, I have seen 9" Ford rears get destroyed by 300hp street cars...