The Stovebolt Site Forums The Shop Area The HiPo Shop GM 10 bolt rear end question

I am a newbie working on a 55 2nd series.

I know that I am going to change from the original rear end to a GM 10 bolt. I have found the ideal donor years and models but have a question.

When they say it is a 8.5" 10 bolt, what does the 8.5" represent?

I hate to ask such a rookie question, but any advice would be appreciated!

Thanks!

8.5" is the diameter of the ring gear.

Thanks Bill!

are you changing the rear for different gear ratio or wheel pattern?

I am changing for both reasons. However, I guess the primary reason is because I am installing a No Limit IFS in June and want to match the 5 x 4.75 lug pattern.

I have heard the stock rear end is strong enough for the 350 I want to put in. But if I have to change the axles to get the right pattern I may as well go with an newer rear end as well.

It is a shame, the stock rear end is in GREAT condition.

That 10 bolt sounds good. I kept my stock rear and had the axles redrilled for 4 3/4 inch pattern.I am also running a 350 engine.As long as the width is OK you will be alright.

what year is rearend and what was it in?

Dale937 -

The one I am looking at buying has a 308 gear set. What do you think? Good ratio? I am planning on a TH700r for the tranny.

10 micron -

The rear end that is in the truck right now is the original (I believe) and it is in a 2nd series 3100. Previous owner even rebuilt the brakes.

1st project. I am running the 700R4 tranny with the stock 3:90 rear.It is a very good combination,for performance and economy,. 20 MPG.I dont know how the 3:08 will perform with the overdrive 700R4.A 3:73 ,3:90 or even a 4:11 will work out just fine with the 700R4.sounds to me the 3:08 will hurt a lot in performance.Perhaps someone else out there has used that combo,that can give you some more info.The 3:08 is a good choice for a 350,or 400 hydramatic.I had my axles plugged and drilled for 5 bolt 4 3/4 wheels,and also the brake drums, at Moser Engineering in Indiana. Cost was very reasonable.They have a web site with posted prices.

GM 10 bolt is about the Junkiest Differential ever built.
Most would not waist a dollar or an hour on one.
Try a f#*d 8.8

I've got a 10 bolt out of a Old's 442, you don't have to take the cover off the 3rd member to pull the axles. It has held up well in my Olds, I think it will be fine in my 53. I do not like the way the wheel cyl is mounted in the 78-87 Chevelles. They have a tendancey to rock back and forth till they wear out the mounting hole of the backing plate. This causes your cylinder pins to fall out and you will lose your brakes.Joy, when will GM ever recall that?

We have the GM 10 Bolts here, although getting a bit hard to find these days.
They only ever came out on our Chev V8 powered HOLDEN Monaro coupes in the late 60's.
They are a great diff if not too old or thrashed,so no matter if you use one of these or a f#@* 9" diff, they could both be as good or as bad as each other depending on how much of a hard life they have had.
I'm running a 3.08 ratio in my Torana 5.0 litre SS Hatchback and this is a great daily driver ratio and runs 14.7 sec quarter mile times and will still wind the speedo off the clock at 130 miles per hour. Not a bad ratio !!!!!!!!!

If you can get a 10 bolt cheap, have it reconditioned, and it will outlast the vehicle if maintained properly.
This would cost less than buying a 9", having it re-coed, modifying the axles and drums for the stud pattern and also possibly having to spend more money on making sure no brake modifications have to be made to run the F#*% drum brakes.
Hope this helps a little.....all the best, Doc Bob.

Thanks for the information.

A lot of good points were brought up.

I think I am still leaning towards the 10 bolt. I have heard the 8" is easier to repair and service, but I have got some leads to a used 10 bolt. I guess it also comes down to the fact that I would like to keep my Chevy as true to GM as possible. (I just can't give my father in-law the satisfaction of a blue oval part on my truck!)

Thanks to everyone.

Ross

In the off road circles, the 8 1/2" ring gear 10 bolt GM diff is considered to be as stong as the Dana 44.

My tranny builder (http://www.phoenixtrans.com/) talked me into changing out my 3.08 gears in my 10 bolt and replace them with 3.73. for a better ride.
You might like to know the 2002 Firebird disc brakes will bolt up to the 10 bolt without any mods.
I have bought two sets of gears on line at EBay. You will find plenty there and at a reasonable price.

Big chevy 3600 -

Thanks for the information. I did not know that about the brakes.

I have been looking on http://www.phoenixtrans.com/ for a while now at the TH700r4 tranny they sell that is matched for a zz4. I assume you like their work? Would you recommend buying from them?

Phoenix is recommended by many Rod publications and used by many rod shops. I heard of them through an article in Classic Truck mag.
They know their stuff...

rear end calculator

308 and a 700r are a bit much I think.

After looking around, researching, and speaking with the engine/ tranny builders that I want to use, I believe I will be running a 373.

Don't know if I can find a used one with this ratio right away, so I may end up buying a used rear end and have new gears put in.

Ross

That's what I've had to do with most work on my truck. Look on Ebay, you may find something close by you can pick up; that's what I did. Also it sure beats grazing through all the junk yards. Shipping on a rear can get costly.

At the end of the day and when all the bulldust settles, it comes down to what you want to do with your Truck.
3.08, 3.55, 3.73......the list goes on.
Want to cruise comfortably occasionally.....3.08.
Want to cruise occasionally with a bit of fun...3.55.
Want to get excited on a twisting back-road....3.73.
Want to run the Xmas tree down the 1/4 mile...4.11.

You'll be the deciding factor in the end!!!!!!!!!!!

doc bob -

That gives me a good perspective.

Thanks!

The 3.08 will give you plenty of low gear 3.08*3.06(700r4 1st gear)=9.425 final drive ratio whereas on the highway in overdrive you get 3.08*.7(overdrive ratio) =2.16 final drive ratio compared to 7.76 1st gear final drive and a 3.08 final drive ratio for a TH-350. I have to agree with underdog.... sort of. If you are planning on putting some serious hurt on your differential, or are going to be doing a lot of racing, that 10-bolt would not hold up to the pressure without some serious and frankly not worth it mods. A f*%d 8 or 9 inch would be better in that situation, but in a street driven truck, the GM rearends are fine, I am putting a 10-bolt in my 65 right now that i found in an 80 chevrolet truck that has some crazy 2.27 gears in it behind a TH-350.

use 3.90 or 4.11 with the 700R4. 3.08 will be fine with the TH350. The 10 bolt will be fine unless you plan on really giving the truck a lot of abuse - then I would use a 9" Ford, which is pretty bullet proof. My buddy has the 9"Ford with 3.90 and the 700R4 he puts his foot into it all the time and hasn't broken it yet.

My 1992 GMC shortbed came with 3.42 rear gears. I changed them for 3.08 and it has raised my gas mileage about 20%. I am running a 5 speed transmission and the .70 overdrive loves the high gears, runs just under 2000 RPM at 70 mph. The only change in driving is sometimes I have to shift into 4th on some steeper hills on I-5 at the ridge route. I still pull my car trailer with a Blazer on it with no problem.