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Old 03-03-2009, 11:14 PM
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Changing rear end gears

I have a 79 C10 (I'm sure you've seen my other posts) with a 3.07 standard 12 bolt axle under it. I'd like to change to a 3.42 or 3.73 gear, maybe even a 4.10 with positrac of some sort (am considering a 700r4 trans and *currently* the truck has a 305.) I am planning to rebuild a 454 that I have and drop in there down the road, or I may just settle for a 6.5 diesel with a turbo like my suburban has. (so many engine choices!)

Anyways, I digress. What all is involved in changing gears? How hard is it to do? What specialty tools do I need and would it be cost effective to buy the tools or should I pay to have it done? (I could use the tools to change gears in a couple other axles too if the same tools will work for an 8" ford and a 9.5" 14 bolt gm)

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Old 03-04-2009, 07:08 AM
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It's definitly not easy.I changed gears in my son's mustang (Ford 8.8,quite similar to a chevy 12 bolt).Prior to diassembly youll need to measure gear lash using a dial indicator and record reading for future reference.Also pinion depth must be measured using depth mics. I had to make a plate that stradled the end cap mounting bosses so I could position the depth mic. directly over the end of the pinion gear. The new crush sleeve must be torqued to achieve proper preload (you gotta use a an impact gun on this unless you have the strength of a gorilla)There is alot to it if you want to do it right. I would suggest reading as much as you can on this procedure before tackling it.If you have the time and ambition it's definitly worthwhile just for the knowledge gained...Good luck.... Ron
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Old 03-04-2009, 08:16 AM
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I just went from a 2.50 to a 3.50 ratio in my 9"....I had heard of so many horror stories about how if it ain't setup just right you'll end up with a Kaboom.......and I don't mean Billy May's cleaning stuff!
So, as piece of mind, I took it to a pro business and they installed the new R&P for $115.....I thought it was well worth it.
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Old 03-04-2009, 09:33 AM
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It's really not that big of a deal.You dont need a pinion depth tool, the gear pattern will tell you everything you need to know.Get a dial indicator and make some dummy Brgs.If your worried about doing the crush sleeve,there is some tricks you can do to help it crush,but you will need a 1/2 impact,or you can use a solid spacer.You could always post some pics of your gear pattern and me or someone else can get you going in the right direction.
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Old 03-05-2009, 02:07 PM
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I agree with prostreet6t9, I didn't have a pinion depth tool and my contact pattern and back lash all came out just fine. I used a big breaker bar and a 4' piece of pipe to crush the crush sleeve. Went from a 2.76 to 3.73 in my 80 C10. Good luck

Yukon or Randy's ring and pinion had good instructions you can print out.
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Old 03-08-2009, 08:41 AM
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Quote:
Originally Posted by 6426yy
I agree with prostreet6t9, I didn't have a pinion depth tool and my contact pattern and back lash all came out just fine. I used a big breaker bar and a 4' piece of pipe to crush the crush sleeve. Went from a 2.76 to 3.73 in my 80 C10. Good luck

Yukon or Randy's ring and pinion had good instructions you can print out.

Might you have a link to these instructions?
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Old 03-08-2009, 11:38 AM
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http://www.ringpinion.com/HowTo.aspx
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Old 03-08-2009, 12:36 PM
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Gear switch

I wish you were a little closer I would just swap out the whole rear end for my 3.73.
I am looking to go for better miliage.
I guess I'll be attempting a gear set change myself.
I may farm it out if te price is not too steep. For some reason this one scares me....LOL
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Old 03-08-2009, 09:47 PM
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hi there, well i am a '63-87 full size gm truck guru/nut ever since i was 16 when i got my first gm truck, a 1968 c-10 longfleet pickup all original nice. 307 2bbl, SM318 three on the column, 3.73 open 12 bolt with trailing arm coil spring rear. pic of it after a couple years of owning it and painting it and also a red 68 i pick-up next to it that i got later on


and my $.02 is the following:

youre better off swapping the entire rear for a 3.40 or 3.73 one if you canfind one but both were pretty uncommon in the full size 8.5 and 12 bolt axles

dont forget 63-72 leaf rear will work but the shock mount brackets are in a little different location is all,, relocate them or use from the old axle.

also 4x4 are 40" spring pack centers
2wd were 42.5"

all through 1987
so you need a 63-87 axle if you can live with '63-72 shock mount modification

also you could actually use a 63-72 coil spring rear if you simply weld the perches onto the tubes and modify the shock mounts or weld them from the old axle.

most common ratio were 2.73 and 3.07 in the full size trucks both axle models

'77-'80 could have either the 12 bolt or the 8.5, it was very random. same with the G series vans.


now, 14 bolt will bolt right in place of a 12 bolt or a 8.5 and billions of them were 3.73 and you could get an 81-87 9.5" which is a smaller 14 bolt and is semi float like the 12 and 10 bolts. and many were limited slip to boot 8 lug though but that will be ok
you can convert the front to 8 lug very easily with any GM dana44 or 8.5" front 3/4 ton spindles and rotors and calipers

a whole rear axle should be dirt cheap. if you know any of the local vehicle and scrap metal scrapper type ppl in your area then ask them to call you when they get a hold of an old chevy truck and so you can check the ratio code on the pass side axle tube to see if it is 3.73. they would sell the axle for what it is technically worth, unlike the yards. arund 75 or 100 tops.

in all reality unless you are gonna race track the truck or something crazy then i really would leave it be unless you really found a cheap 3.42 or 3.73 rear.

these trucks run good iwth any ratio without any very noticeable difference in ratios for the most part but that all depends on the power level of the engine you are running however.


i need to check the ratio in the 12 bolt under the 78 bonanza longbed fleetside pickup i recently adopted and am putting another sbc and a 3L80HD(th400 body but th350 tailshaft) trans into, from an 87 R10 pickup

cutting and welding the driveshaft 3" shorter because the 3l80hd are longer than the th350 truck '73-up 9" tail trannies, but are same 27 spline output yay
i am ready to weld the yoke back onto the shaft tube now, got that ready tonight.


hope i may have helped possibly

anything else then just let me know

Last edited by fast68; 03-08-2009 at 10:44 PM.
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Old 03-08-2009, 10:13 PM
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I would change the ratio and rebuild what you have! When your all done you know what you have and you have all new parts.
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