Rear Diff questions
 

Following this thread i started: http://67-72chevytrucks.com/vboard/s...d.php?t=793638..i have i think decided to go to a 5x4.75 bolt pattern for my truck. Seems to be a more common front end than the 5x5 i was going after for the drum to disc conversion kits i wanted.

I have sourced what is said to be a 73 Z28 rear end shell. No gears, axles, bearings, backing plates, cover..nothing bot the pumpkin and axle tubes. $20.

I have a rear end from a 75 C10 with 3.08 gears that I was going to use under my 56 but..see thread.

Will those gears fit in the 10bolt 8.5" from the Camaro?

PS thx to those who helped in that thread..mgchevyparts in particular..

Re: Rear Diff questions
 

Truck and car are different....dont go on what its "supposed'' to be..find out and know for sure... much easier to just get a complete axle...less headaches....to build that axle from scratch is gonna be big $$$....and if that housing doesn't have the carrier caps in it and marked its useless anyway....

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The seller says 73 Z28 so..thats all i have to go by.

Thx for the confirmation on the gears as well..i wasnt sure. Ive never replaced those so best to check.

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I'd pass on it...just get a complete axle..

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Another vote for passing and getting a complete rear with the right gears in good shape. That 20 dollar housing would end up costing you around 800 to 1000 to buy all the pieces for new and have it set up right and that is US dollars. A good rear end shouldn't cost over 200 to 300 ready to go and needing nothing. That's Canadian $$ If they say the "rare" word or the "Z28" word they want too much money most of the time.

Re: Rear Diff questions
 

regan56,
I assume you are trying to get the truck done a little cheaper than first planned. that's fine, no issues with that. it's called hotrodding for that reason, we do what WE think is good and affordable for OUR trucks. maybe not the style that is vogue but it is OUR style. anyway,
with that said,
the deal you got on the diff housing will ultimately cost you a lot more cash than you first may have thought. the housing without the bearing caps will require new caps to be purchased and installed and then line bored to specs. this is because when the axle was originally built that is what was done. for this reason no 2 bearing caps are the same as far as the bore in the middle to fi the bearing. it is also why the bearing caps need to be labelled or marked in some way before a diff is taken apart. for that reason alone the axle you have may be better off put back on the for sale list. it will simply cost too much to make it whole again. by the time you machine it, alone, I would bet you could grab a complete diff from pick n pull for less money. Kijiji would possibly be even cheaper.
so,
for a diff,
you should decide about the look you want and then get a plan going because depending on what you plan to use for tires and wheel offsets your wheel mounting surface to wheel mounting surface dimension could be different than the stock axle. if you plan on doing some deep dish wheels with some big fatty tires out back then the axle track width would need to be narrower than if you simply want a regular pick up sized tire. if going with fatty's you may also need to tub the box and will need to factor in the frame width. it could be in your best interest to do a quick drawing of the box width inside your wheel openings, the width between the box sides behind the tires and also the frame width. then add the track width of the axle you are looking at, including the brake drums or rotor flange thickness, so you know what the back spacing can be on the wheels. then look up to see if that is going to be a custom built wheel or is it is common wheel. custom always means money.
for a cheap axle that has most of the goodies you may want to look at the ford 8.8. they are pretty common in ford explorers and come with a posi and disc brakes. the ratio is commonly a 3.73 and they have pretty decent axles. maybe google it if that interests you. the explorer axle is offset but that can be overcome easily if you keep track of driveline angles. the same diff can be found in a mustang with no offset to the center section. they also have an easy to configure park brake set up which could also be had pretty cheap from pick n pull or kijiji. the wheel bolt pattern is wrong but that can be redrilled to what you want and also the wheel stud size can then be whatever you want it to be (I have done it at home with regular shop tools, drilled to 5 on 4.75 with S10 wheel studs). if redrilling you would need to also drill the rotors. again, not a big deal. the driveshaft connection is also wrong but fixable easily with the correct yoke on the driveshaft. you would likely need a new driveshaft anyway.
also keep in mind the ride height because if you plan to lower the truck then the axle center line may need to move on the front axle. these trucks have an angled wheel opening on the rear of the front fenders so as the body is lowered over the tire it looks like the tire is being stuffed into the rear of the wheel opening. that is why guys move the axle ahead on lowered trucks.
hope that didn't confuse you. I would just hate to see a bunch of money thrown after an axle when you could have easily had something useable for less.

Re: Rear Diff questions
 

thx for the input gang. i did end up nixing the shell and found a complete '79 Firebird 10 bolt w/3.08 rear end that im picking up.

yes cost is a thing, but only to the point where i know i can refurb certain items myself (engine, front end, rear end) with some help in the machine work if needed. i have a parts connection that i can get most OE stuff for cost, or theres RockAuto or the catalouge parts guys such as Speedway, LMC etc.. of course we all have our limitations on things we a) havent done before., or b) have done and vowed to never do again :)

always good to come here or places like this to get the opinions of others who have done and experience these tribulations. Thx Again..keep it comin!

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are you planning on doing a lot of highway driving? 308 is a pretty tall ratio. depending on the tire size and trans i guess.
this site has some good info on tire size comparisons and also rpm/gear ratio. check the different tabs for all the info offered there.

https://tiresize.com/comparison/

another for simple gear ratio/engine rpm

https://www.randysworldwide.com/calculators/rpm/

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and another

http://www.csgnetwork.com/multirpmcalc.html

Re: Rear Diff questions
 

would be interesting to know the plan as far as engine/trans/diff/brakes/suspension. lots of guys on here have been there so can give some pointers to possibly save you time and money.
daily driver, trailer queen, weekend warrior, carbed/fuel injected, low rider/stock height?
it's cold here to, eh!
happy thanksgiving.

Re: Rear Diff questions
 

327 w/700r4 transmission. 4bbl carb. 15inch tires somewhere around the 255 60 size. Summer day driver maybe the odd day trip

Straight axle with drop springs lowered rear to match (flipped axle).

Re: Rear Diff questions
 

thx for the links too.

i double checked my inputs and with the 3.08 and 700 trans i should be ok, for around town (in 3rd gear)@1500RPM @40mph and in the hwy @1900RPM @ 70mph.

just have to match my converter, but thats another day

Re: Rear Diff questions
 

at that setup you will never use overdrive (4th gear) I have a 3:55 gear behind the zz4 in my 34 Chevy and it is a little boggy in overdrive at 55-60 miles per hour. If you are going to use the 3.08 rear you'd be better off with a turbo 350 trans. Especially with 15 tires which are usually around 29" tall. My 35 chevy has a 700r4 in it with a 4:11 gear and 29" tall tires and it is much happier the other. Fun out of the hole and 2400 rpms at 70mph right in the power band so it doesn't bog. Good luck





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Re: Rear Diff questions
 

255/60/15 is 27" tall, would be better with 3.55-3.70 rear gears and a 700R4.
27" tires with a TH350 would be drivable ok but on the edge

Hows the rear end width of that 79 Firebird to fit under the truck?

That’s pretty much what I went with. 10 bolt 8.5” limited slip out of a 76 TA. Only 1/2” different between backing plates from the original. Had zero issues getting it to fit.
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ogre paid $200 for a complete 86 trans am rearend with disc, posi and 3.27 gear
i did replace the two wheel bearings and dif lube as it was an 86
62" wide

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interesting...assume modifictions to the perches tho?

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I had to change perches for the 76 rear I used. I wanted to keep mine in the stock position. So the perches has to be changed

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is yours the 3.08 w/4speedOD tranny too?

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how much on the edge? i may have a deal to swap my 700r4 for a TH350

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if you plug in the numbers you will get the outcomes in rpm for whatever speed you want. roughly though, a 3.08 diff, with a 27" tire, with a non overdrive trans, at 65 MPH, would give you 2558 rpm. that might eat some gas depending on the cam etc. an overdrive would give you 1744 rpm. waaay better on gas if you do a bunch of highway driving.
now change that to a 3.73 ratio with overdrive and get 2112 rpm. or non overdrive and get 3100 rpm.
use the link above to do some of your own figuring but keep in mind the engine set up, cam, carb etc. figure out the beast rpm range for your purpose and then do the appropriate gear ratio. obviously a 3.08 gear would be better on the highway with a non o/d trans because the rpm would be lower than with a 3.73. now if you plan to do mostly city driving and like to take off fast...….you figure it out.

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LOL. i had used the calculator you provided and thought i had it nailed. but i guess what i didn't consider was the gearing as i tour around town or up the hwy. 1900 RPM or 1500 RPM sounds good while your cruising on the hwy or around town but if ya need to get up and go i might have a dog on my hands. this is where the voodoo enters, that i dont have a good handle on.

im not doing 1/4mile launches so i dont need to burn tire on every gear change, but i dont want a dog either. from what i can read on the I-Net the TH350 with the 3.08s arent terrible for everyday but do leave some to be desired when hwy driving, which wont be alot anyway. Can anyone confirm my synopsis?

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700r4 transmission. That has OD. I believe the gears are currently 2.73? I’ll change them out when I get it closer to being on the road

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your 3.08 rear end gearing will cruise the highway quite easily.

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700 R4 or th350? I guess what I'm down to is the 700R4 or the th350 I'm okay with it 3.08 gears just trying to determine what the best transmission for me is going to be I won't be doing a lot of highway driving mostly in town and with the 700R4 I can just drive it in 3rd gear