1979 Chevrolet C70 Ring and Pinion Upgrade

[Airdirect]

I have recently upgraded my 1979 C70 Chevrolet from a 5:29 ratio Eaton 2 Speed rear end to a 4:56 ratio. I did the work myself. This was my first time to setup a rear end. It was a simple and straight forward process, the only element that made it difficult was the weight of the assembly and parts. This truck was factory equipped with a Eaton 17201 rear assembly with a 16 spline pinion and yoke. The later series was 17220 which had a 32 spline pinion. I have listed below the available ring and pinion sets for this series.

[Airdirect]

I used an online gear ratio calculator "http://spicerparts.com/calculators/transmission-ratio-rpm-calculator" to compute the desired ratio. I was targeting a top speed of 70 MPH. The original 5:29 ratio yielded 59 MPH. This was appropriate in 1979 with the national speed limit of 55, however I wanted to be able to cruise at 65 MPH. This truck is equipped with a Detroit Diesel 4-53T engine which produces 170 HP at 2,500 RPM. The engine is coupled with a Clark 390V 5 speed transmission and Eaton 2 speed rear end. I wanted to have max power available at top target speed of 65 MPH. The 4:56 ratio should achieve this goal.

First step was to drain the 32 pints of 80-90 gear lube. Once this was drained I removed the nuts and locking collars from each of the axle hubs (12 each). I was able to strike the axle and free it from the Dayton hub. I then extracted the axles just enough to allow the rear end to clear. Use a catch container under each axle to collect the lube that drains from the wheel bearing areas. I then removed the electric shift unit which is held with two nuts, and the drive shaft which is held by four 12 point socket head bolts. My truck did not have a bed so I was able to lift the rear end using a cherry picker. I am not sure how much it weighs, but I suspect it is around 500 lbs.

[Airdirect]

Once the third member was removed I recorded the pre-load settings on the carrier end caps. If you are reinstalling the same ring and pinion these settings would be beneficial. Given I was changing the gear set, they were of no value. If you look close you will see the numbers on the end of the lock tabs of the bearing retainer.

[Airdirect]

Next step was to remove the ring gear and 2 speed carrier assembly. First step in the process is to remove the lock wire (not shown) which prevents the cap head bolts from backing off.

[Airdirect]

Next step was to disassemble carrier unit and check for wear and damage.

[Airdirect]

Next step was to remove the pinion carrier assembly. It is held in with several large bolts. I then used a large punch and hammer to drive the pinion shaft assembly out of the housing. Once removed I used an impact to extract the pinion nut allowing the yoke to be removed. Carefully remove the taper roller bearing and the spacers behind it. These spacer(s) set the pre-load on the pinion bearings. This pre-load will be checked during re-assembly. Inspect the pinion assembly for damage or bearing wear. Mine had a defective race necessitating a new timken bearing and race be installed. Re-assembly is the reverse of removal. Once the pinion shaft and cage are assembled, use a wire to test the pre-load on the pinion bearings. I used mig .023 welder wire wrapped around the pinion cage. It should take 6 to 8 lbs of pull to make the cage turn. Clamp the yoke in a vise when doing this test. If it spins to easy, reduce the amount of shims between the pinion bearings. If it spins to hard, add shims to get it within range.

[VOLKSWAGNUT]

Nice write up and pictures..
Where did you buy/find the gear set?
I want to do the same thing.. it's just nuts, bolts.. and time..


.

[Airdirect]

I bought the ring and pinion set from LKQ heavy truck parts (goodies) in Ohio. If I had it to do over again I would go with 4.11 ratio. I believe my engine has the torque for 4.11 and it would give me a higher top end or better economy.

[sweetk30]

why not the whole used 3rd member swapped in ?

and watch the top speed . . . as these old trucks and there old design tires might NOT be speed rated for the speed you can now go .

bad things happen when over the speed rating . . .

friend years ago did it in a car . way over speed rating and front 2 tires shifted the belts so bad the next day he almost could not drive it to the tire store .

my 88 gmc 7000 366/5 speed / 2 speed rear has 6.57 aspr the rpo code sheet in it. and 10.00x20 tires.

[Airdirect]

I considered replacing the whole third member, however my truck only had 45,000 original miles and I wanted to keep the low mileage components.

One of the big differences is the 4-53T Detroit Diesel only turns 2,500 RPM. To achieve 70 MPH you need a much higher rear end ratio. With the 4.56 gears I now govern out at 67 MPH. If I had 4.11 I could run 70 MPH and not be against the governor. If I were selecting a new ring and pinion set I would try the 4.11. Regarding the tires, I am planning on putting new tires on the truck, still trying to decide between original 9.00 X 20 or 11.00R X 22.5. To go with the 22.5 I would need new tubeless rims.

[Marrowthirst]

I agree this is a nice write-up and some good solid pictures.

I have a 1978 C65 453T with the 5 speed Clark transmission. I believe the only difference is I have a tag axle.

I will send you a PM on where I have researched to get the demountable tubeless rims. I believe it's an 8.25 rim width x 22.5 that you will need.

[Airdirect]

Very cool, I would appreciate it. Do you have pictures of your truck? What is your top speed?

[Marrowthirst]

I haven't tried the top speed yet. The brake booster is bad and going to get that fixed.

[Airdirect]

Wow, looks really sharp with the tag axle.

[VOLKSWAGNUT]

Mine is not a C series.. but a 77 Chevy H70.. with a 6V53, Spicer 5252A short 4th... working 5th. -n- 2 speed ..... tops out around 60 in a hurry.. at an ear ringing 2925 Rpm's..
I just want a more user hearing friendly top speed and RPM option.. and like for it run 65-70..

[sick_46]

I have a 70 c50 with a 4 speed and a 2 speed rear. do you think its the same as yours? How can I figure out what gears it has in there now?

[Airdirect]

You should have a equipment list located on the inside of the glove box door. It will show you the rear end ratio. Another way to tell is to look on the end of the pinion shaft, it will be stamped as a fraction on center of the pinion shaft were the nut holding the yoke screws on.

[sweetk30]

when its a 2 speed rear what does the split make it / cut it to ratio wise ?