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Old 04-14-2018, 08:07 AM   #1
epfatboy
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12 bolt truck rear.

Iím going to intall some 3.42 gears and a Eaton posi unit in my Ď73 C10 to support my 4.8 turbo build. Pulled the cover and found a good clean open 3.73. Has me wondering if I should keep the 3.73 and Just run a taller tire!

Are these 12 bolts all 30 spline? Think I can get enough of a look at the axle splines if I pull the pin and shove the tire in?.... being lazy!
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Old 04-14-2018, 09:03 AM   #2
sweetk30
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Re: 12 bolt truck rear.

with the limited info you provided i played with 2 options .

with say a 4l60-e in overdrive @ 70 mph & 28" tire .
3.42 = 2011 rpm
3.73 = 2193 rpm

then th350 for non overdrive @ 70 mph & 28" tire .
3.42 = 3035 rpm
3.73 = 3310 rpm

the overdrive #'s are in the sweet spot for most who want to drive and cruise . the non overdrive #'s are up there . . .

here play with this and get your combo in and scroll to the bottom for rpm results . and you can change just about any # you want in this calculator by clicking it . http://www.grimmjeeper.com/gears.html
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Old 04-14-2018, 11:29 AM   #3
Matt Man
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Re: 12 bolt truck rear.

Quote:
Originally Posted by epfatboy View Post
Iím going to intall some 3.42 gears and a Eaton posi unit in my Ď73 C10 to support my 4.8 turbo build. Pulled the cover and found a good clean open 3.73. Has me wondering if I should keep the 3.73 and Just run a taller tire!

Are these 12 bolts all 30 spline? Think I can get enough of a look at the axle splines if I pull the pin and shove the tire in?.... being lazy!
12 bolt truck rear ends are 30 spline, with a smaller pinion than a 10 bolt rear end but the pinion usually doesn't break. The ford 9" rear has a even smaller pinion than the 12 bolt! Great axle that was used up to the 1981 model year, my 81 k10 has a 12 bolt rear with 4.10 gears and a Detroit locker, great setup.
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Old 04-14-2018, 07:13 PM   #4
epfatboy
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Re: 12 bolt truck rear.

Yeah sorry about that.

Tranny is a 4L80E, the current tire is a 315/35R20 so its just a little over 28.5" tall.

Not worried about the pinion, it's the axles that let go first and honestly with a street tire I don't how I'd twist a 30 spline axle.

Seems like I should stick with the 3.73 and see what happens given the lack of difference in RPM. I know most of the turbo guys like a 3.42 but I'll give this a try and see how I like it. Plus keeping the gears will make adding the posi unit a lot easier.
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Old 04-14-2018, 09:15 PM   #5
sweetk30
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Re: 12 bolt truck rear.

@ 70 mph = 2294 rpm 3.73 gear.

@ 70 mph = 2104 rpm 3.42 gear.

reason is most likely some motor combos pull better with a bit of a drag load on the system . my 1 atv if i swap in a smaller engine sprocket the motor just winds up . if i go 1 tooth up in sprocket size on the engine she PULLS like a freight train
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Old 04-15-2018, 08:15 AM   #6
epfatboy
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Re: 12 bolt truck rear.

Quote:
Originally Posted by sweetk30 View Post
@ 70 mph = 2294 rpm 3.73 gear.

@ 70 mph = 2104 rpm 3.42 gear.

reason is most likely some motor combos pull better with a bit of a drag load on the system . my 1 atv if i swap in a smaller engine sprocket the motor just winds up . if i go 1 tooth up in sprocket size on the engine she PULLS like a freight train

Right and with an FI motor that is what Iím worried about. Just posted a question in the LS FI forum to see what I get.

Is there a decent site where I can get GM gears for this rear? Not a fan of Richmond for DDís they can make noise
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Old 04-15-2018, 11:02 AM   #7
andyh1956
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Re: 12 bolt truck rear.

My son ran a 3.42 against his LS & he was planning to install 3.90s before it got T-Boned...
The engine was 500 rpm off its low end curve w/ 3.42s & he had a 5.7, your 4.8 would want better than 3.42 s also I would bet..
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Old 05-01-2018, 03:08 PM   #8
epfatboy
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Re: 12 bolt truck rear.

So I'm keeping the 3.73 and made the investment on the Eaton Posi. So far the Posi is installed, backlash is at .008, and I'm "okay" with the gear pattern. Also threw in some new Axle bearings and seals... didn't need them but I figured while I was there. Now on to the rear disc conversion.


NOTE:
My Eaton Posi and Koyo Bearings would not allow me to re-install the original carrier shims. The orig shims were both solid .240's. Thankfully I had a super shim kit from a previous build and was able to get a .230 on the ring side and .220 on the other and got a good backlash. The wear pattern is stated as acceptable for used gears on a few sites but has me wondering why the drive pattern is Toe heavy and the coast pattern is heel heavy.

https://photos.app.goo.gl/ToNlkAXwzZyJf9nH3

https://photos.app.goo.gl/PWnOBVRlzUjbm0bl1


One more thing, I did try to use a .240 on the ring side but it would bind when I spun the pinion so I backed off by .010 on the ring side.
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