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04-14-2018, 08:07 AM
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#1 |
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Registered User
Join Date: Mar 2018
Location: Barrington nh
Posts: 101
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12 bolt truck rear.
I’m going to intall some 3.42 gears and a Eaton posi unit in my ‘73 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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04-14-2018, 09:03 AM
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#2 |
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Registered User
Join Date: May 2012
Location: horseheads,ny rust belt
Posts: 2,794
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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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77-k30lb BIG truck build . 87-k30lb budget beater build . 85-k30lb the plow machine build . 85-c10sb summer fun toy . ----------------------------- HOLLEY SNIPER efi = worst case of p.i.t.a i ever had . EDELBROCK pro flo 4 = best deal going so far . love my setup works great. |
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04-14-2018, 11:29 AM
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#3 | |
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Registered User
Join Date: Oct 2009
Location: Buhl, Idaho
Posts: 1,438
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Re: 12 bolt truck rear.
Quote:
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86 c30 crew cab dually 454/400 4/6 drop SOLD 1-8-11 1981 chevy K10 short Bed Scottsdale SOLD 10-26-2020 67 GMC Stepside Straight six/3 on the tree "The Peach" 2020 Silverado Double Cab LT Z71 5.3 8 speed Daily Driver K10 build http://67-72chevytrucks.com/vboard/s...d.php?t=671934 The Peach build http://67-72chevytrucks.com/vboard/s...d.php?t=733903 
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04-14-2018, 07:13 PM
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#4 |
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Registered User
Join Date: Mar 2018
Location: Barrington nh
Posts: 101
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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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04-14-2018, 09:15 PM
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#5 |
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Registered User
Join Date: May 2012
Location: horseheads,ny rust belt
Posts: 2,794
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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
__________________
77-k30lb BIG truck build . 87-k30lb budget beater build . 85-k30lb the plow machine build . 85-c10sb summer fun toy . ----------------------------- HOLLEY SNIPER efi = worst case of p.i.t.a i ever had . EDELBROCK pro flo 4 = best deal going so far . love my setup works great. |
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04-15-2018, 08:15 AM
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#6 | |
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Registered User
Join Date: Mar 2018
Location: Barrington nh
Posts: 101
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Re: 12 bolt truck rear.
Quote:
Right and with an FI motor that is what I’m 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 DD’s they can make noise |
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04-15-2018, 11:02 AM
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#7 |
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Registered User
Join Date: Mar 2010
Location: thomastown mississippi
Posts: 1,261
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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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05-01-2018, 03:08 PM
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#8 |
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Registered User
Join Date: Mar 2018
Location: Barrington nh
Posts: 101
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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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