View Full Version : GMC and Chevy rearends different?

Old Gold
09-04-2005, 08:01 PM
I just purchased a '58 1/2 ton GMC and I noticed that the rearend appears to be different than my '58 1/2 ton Chevy. The are both absolutely POSTIVELY the original stocker rear ends. Does anybody know if GMC used different rears in their 1/2 ton trucks? What about brakes, wheel cylinders, are they different? Any help would be appreciated.

Bill Hanlon
09-06-2005, 07:25 AM
Standard rear end for 55-59 GMC 1/2 ton pickups is the Spicer 45, which is definitely different than the Chevy rear end. Brake shoes are 11"x2", same as on the front of the GMC and I think same as the Chevy. All internal parts of the Spicer 45 are most likely different. I don't know if the wheel cylinders are different, but I have Wagner numbers at home if you need them.

A few minutes at your FLAPS with a counterman who still knows how to look things up in a paper catalog should help. I have purchased rear outer bearings, rear brake cylinders and a gasket for the rear cover gasket (NAPA # 18434) over the counter.

09-08-2005, 09:43 AM
to further confuse things, I had a spicer 45 that was original option on the 1959 chevy,but was positraction,which makes all of the internal parts different.Positraction brake drums are a different sort of convex shape than non-posi.

10-06-2005, 01:08 PM
If the GMC and chevy rearends are not the same, will ChevyDuty part#'s 93-513 and 93-514 work on the GMC or not? I have been looking at these kits for a while and I thought it was the right equipment.

I have a 1956 GMC 1/2 ton w/3.90:1 gears. (288 V8)

58 chevy
10-07-2005, 07:15 AM
I'm sorry those will not work on your truck, these gear sets and installation kits are for Chevy only, sorry.

Jesse Hazell Chevy Duty Tech.

Bill Hanlon
10-07-2005, 07:58 AM
I got lucky (for once). My '57 GMC V8 HydraMatic came with 3.08 rear end. 60 MPH = just a shade over 2000 RPM on 235/78x15 tires.

10-10-2005, 01:34 PM
So if I were to go to a local drivetrain shop and tell them I have a 55gmc with a spicer 45 rearend with a 3.90:1 and I am looking for some higher ratio ring and pinion gears could I assume they would think I knew what I was talking about? I don't want to disassemble my truck if I won't be able to find the parts.

10-10-2005, 05:37 PM

The Spicer 45 is already geared perfect for a 3 speed transmission in the 3.07 ratio. I run mine with a 350/350TH combo (originally 3 on the tree). It will go 100 if you are foolish enough to try that. If you want to run it with a 400TH or some other tranny with overdrive you will want a ration in the 4.?? range. The Spicer 45 came stock with ratios of 3.07 (mine), 3.9 and the Spicer 60 came with 4.56. There are a few workarounds to utilize Chevy drums but nothing satisfactory.

On another note: the Spicer company merged with the Dana company and the Spicer 45 very closely resembles the Dana 44. Most Dana 44's are actually used as the front differential on modern 4x4's. I have not checked yet myself with an actual gear shop but it would appear from the literature old and new that the inner parts ought to be interchangable.

A warning from experience: don't lose the drums or the e-brake cable housings for that axle as you will NOT be able to find replacements. You will have to have them relined when the time comes. If any one knows different please pretty please email me company name. I have been unable in 6 years to find the proper drums to fit mine. The Chevy drums work in the barest sense of the word and do not function w/ the e-brake.

John Dutton

10-11-2005, 05:39 PM
I have a 1956 gmc with a 288 v8 and 4 speed hydramatic trans. and 3.90:1 rear gears. I can go to a local driveline shop and ask them questions, but I want to know what I am talking about before I do. I have engine problems right now, but I may be able to do this in the meantime.

Bill Hanlon
10-12-2005, 08:03 AM
GMC Master Parts Book shows a different "Case - Differential, Less Drive Gears" for 3.07 and 3.92 Spicer 45 axles. The "Case" that they are referring to is the part that the ring gear bolts to.

3.07 case is part # 2346843 (2370713 if Power Lock [Posi-Trak])
3.92 case is part # 2354372 (2370714 if Power Lock [Posi-Trak])

So the change would not be as simple as just changing the ring and pinion gears.

By the way, the 3.07 and 3.92 are taken from the text in the book.
I had always referred to them as 3.08 and 3.90.

WAIT! Late breaking news. The GMC Master Parts Book also lists "GM SS H033 Hypoid" rear axle as the only axle available in 55-2 and as the standard axle in 56. The Spicer 45 is listed as an option in 56. Before we go any further you need to find out which you have.

The drawing of the GM H033 shows the casting that hold the pinion gear and U-joint can be removed while the Spicer 45 is one piece with the rest of the case. Maybe the H033 is the same axle as used in the Chevy????

10-12-2005, 01:11 PM
[updated:LAST EDITED ON Oct-12-05 AT 01:12 PM (CST)]Will this picture help any at all?


10-12-2005, 05:07 PM
Will this picture help at all?
My Vin# is 1028PY7655
According to the chart on this site
I should have a H033 Hypoid rearend.


Bill Hanlon
10-12-2005, 08:25 PM
That is definitely NOT the Spicer 45. Looks like the drawing in the Chevy Factory Assembly Manual, but I am no expert. Maybe one of the Chevy drivers can tell us. Question now is "Is it the same rear end that a Chevy truck uses?"

I'll ask the oletrucks e-mail list and see if anyone knows there.

10-13-2005, 08:45 AM
That looks like the h033 to me. It is definitely a Chevy-not Spicer/GMC-which means the Chevy Duty gear kit should fit. This is not uncommon on the GMC trucks.


10-13-2005, 02:07 PM
Thank you. It is amazing what a photo can do for some people.