700r4 bolt up to a 350 in a 1971 chevy PU

ltrrtl

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I recently purchased a 1971 Chevy C10 long bed with a rebuilt 350, 3-spd auto trans. I have not run the #s on the 350 and I am told the trans is original. I do not know what the reared is but, the truck has 8 Lug bolt Hubs (front and back) which seems strange to me that it is a C10. I thought on C20 (3/4 ton) came with 8 lug rims. When I am driving and let off the gas the truck goes into a dive which is telling me it is geared LOW!!! I am looking to either replace rear end with 3.55, 3.70 or replace the 3-spd auto trans with a 700r4

Looking for recommendations for the least expensive, least complicated and best bang for the buck!
 

hurst01

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OK, :twisted: , lets see what we can come up with. First of all, if the truck has 8 lug wheels it is not a C10 half-ton. My suggestion would be that somewhere along it's life the front fenders might have been changed :eek: , or could be some other reason such as just pitting the C10 emblems on the truck, or, someone could have put the 8 lug set of spindles and axles on the truck, who knows for sure? You can look at the VIN number to tell for sure, check the 3rd digit. 1=1/2 ton, 2=3/4 ton, 3=1 ton.

Being a 3/4 ton, it probably came with 4.10 gears. You didn't mention it being a 4X4, so assuming it is a 2 wheel drive, a 700 R4 will bolt up. You might have to change the rear transmission mount location because the the 700R4 is about 3 inches longer than the TH350. I really like the 700R4 because instead of being a 1-1 ratio in 3rd gear like the 350, the 700R4 and a final ration of around .7 to 1.

Another thing you will have to do is shorten the drive shaft 3" to make up for the longer length. You can have it shortened or stomp around the junk yard looking for one that is about 3" shorter. I have done both. Contrary to the belief of some, the 700R4 does not need a computer to operate. If you are looking for one I would suggest 1985 or later. The 700R4 transmissions came in cars up to 1993 and then changed to the the 4l60 and then 4l60E. The 4L60 is a beefed up version of the 700R4 and the 4L60E requires the use of a computer. You can now purchase kits that will allow you to use the 4L60E without a computer. The 4L60 is stall a beefed up version of the 700R4 but electronic.
 

1977 chevy trucks

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This might help figure out what gearsyou`ve got,if you can find the service parts ID sticker or plate,in glove box on later chevys,and go to GM TRUCKS.COM/RPO CODE LOOKUP.Hope it helps.
 

hurst01

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I tried that link and all I got was a site that sells garbage trucks.
 

ChevyHiPro

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Hey ED, great info on the transmissions!!
I have a couple of both laying around the shop, TB350 and 700R4!
I think I'm going to scrounging around our local junk yard for a 4L60, just because!
i had succumbed to the 'needs a computer' myth and avoided them!!
I have a '85 Chevy 1/2 ton, that's my first hot rod project after I retire the end of this year.
It has a worn out 302 and a 4L60, with a crap rear end, that's like 3.08 non-posi.
Gotta find a rear end for it, looking for a 3.55 - 3.88 or similar with posi for it.
Gonna have to just check around junk yards, prop up the rear end and turn a wheel, while counting the turns of the driveshaft to check the ratio.
Thanks again for sharing your most impressive knowledge!!
:D
 

hurst01

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You can't get a good reading on the rear end gear by turning the wheel unless it is posi. The spider gears will let it turn any given number of turns with inaccurate results. It is difficult to do and requires a helper, but if both wheels are up and you can ly under the vehicle and using both hands, turn them both together in the same direction while someone watches a pre-determined mark on the driveshaft or yoke and counts the revolutions per one full revolution of the rear wheel.
 

ChevyHiPro

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Thanks for the heads up on checking the rearend ratio!
I've actually only done it a couple of times and I got the right answer, probably by accident!

Now, another question; anyway to tell if a rearend is posi without taking it apart?
I understand if both wheels are off the ground and the trans is in neutral, turning one wheel will turn the other one the same amount, right?
A non-posi rearend may 'drag' the other wheel around a little, so if a guy was to hold against one wheel and turn the other one, on a non-posi, the other wheel wouldn't pull, on a posi both would turn, right?
Of course, this would take two people, or one guy with chimpanzee arms!

I'm going to have to get at least a couple of posi rearends for my up coming projects.
So, as soon as I can get around to a junk yard, I want to start shopping for them.
Thanks for the help!
 

hurst01

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There are a couple of different ways to go about this. You can jack one wheel up with the transmission in neutral. The one wheel that is jacked up will either not turn or will turn with a good bit of resistance. By jacking both wheels up and in neutral, by turning one wheel the other should turn in the same direction at the same speed. Or, you can look the information up be the information on the differential tag, providing it has not been removed.
There are exceptions to this. On some later model differentials, they have a clutch inside that will allow one wheel to turn until it gets to a certain RPM and then the clutch grabs and locks the other wheel.
What kind of rear end are you considering? I might suggest a place such as Pull-A-Part, if you have one in your area. The basically sell their parts by weight. I have purchased differentials for less than a hundred dollars. About mid-last year, I bought a 5.7 350 engine coupled with a 4L60E complete with wiring and accessories for something like $156. About $30 more will buy a warranty. You go in with your tools, pull the part and take it to the cashier. Core charge involved is more.
 

ChevyHiPro

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"What kind of rear end are you considering?"
Either just the 3rd member I can put in the '85 Chevy 1/2 ton pickup rearend (don't know the size of the rearend or the number of bolts yet, but I'll find out and probably a couple of complete readends.
I want to find rearends with posi and a ratio in the mid to low 3.xx:1, 3.55, 3.88, like that.

I live about 90 miles west of Houston and about 80 miles east of Austin.
Either is big enough to have a Pick-A-Part, I think I remember Houston having a couple of yards call something like that, or Pull-A-Part.
Seems as I remember they work like you were saying.
I don't remember that they sell by weight.
It's been a long time since I've been in a junk yard with a tool box!
But, I'm looking forward to doing it!
As we approach the end of March here, we'll be getting more warm weather, but some spring rain.
I'm going to have to load up some equipment and start cruising the junk yards.
I should be able to find out on the internet, if there is a Pull-A-Part in Houston, sounds like the best place to shop.
 

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