Rear Axle ID

Well, Andy, you already know about RPO codes. Google for it and it will tell you what you have in your truck. Why ask here if you know the answer?

I don't appear to have an RPO code for Rear Axle listed. Poly Wrap Air Cleaner? Sure. Also, I don't trust that the axle is original in a 30+ year old truck, and would prefer a way to identify by inspection of the axle itself if there is one.

Andy

Andy,

No RPO codes starting with GU or GT?

Best method I've found is to pop the differential cover and count the teeth. Some trucks will have the ratio stamped right on the outer surface of the ring gear. Your gear oil is probably in need of a change anyhow!

The axle did have a tag on it when it left the factory, if you can find that and get the numbers off it we might be able to figure it out. 99% of the time the tag is gone or so corroded it is illegible.

Doc

RPO Codes in the glove box VIN Base 1315 SE Model # CC20903 F51 L25 U37 V38 674 YPB 704 704 E63 K43 M20 V01 YJB XPB ZPB 704

All of them have understandable descriptions (shocks, fleetside, paint, etc.) except for (x/y/z)PB, which all say

7.50-16-6PR-HWY-NYL.

No GU or GT.

The only info I could get off the axle was 3977323, I didn't scrape off all the gunk though, so I will try again.

I definitely need to change the oil back there and in the transmission and also the coolant.

What I am hoping to do is get my RPMs down. I thought swapping out the back axle would be the most cost effective way of doing this. But in order for that to work I would have to be able to ID the gear ratios while I was at pick 'n pull.

Thanks, Andy

Inline...................

What year and model is this truck anyways?? 3-speed auto, 4-speed auto or manual trans? Engine? Mileage?

Interesting.....................

You do NOT want to grab an axle at the boneyard without popping the diff. cover anyways and checking things out. For all you know the ring gear could be missing 4 teeth and the axle could be severely compromised. No way of knowing unless you pop the cover, make sure there is still oil in there, no water in there, everything ain't rusted, everything is there and the ring and pinion teeth are in good shape.

Doc

4.53:1 was common as the base ratio with the M20 trans.

Did you mean 4.56? As far as memory serves, GM never made a 4.53 gearset.

Doc

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74 C20, 4 speed manual, inline 6. Records indicate 292 but I haven't verified that. No idea of mileage. Odometer says 87K but it rolls at

1. No vehicle info prior to July of 2004 when it was brought into California from Nevada. It appears to have had a 5th wheel installed at one point (it has a mesh tailgate with a cutout).

Andy

----------------------------------------------------- You're going to have to jack up one rear wheel and count the ratio to driveshaft revs.=F72. If it's any interest,I looked your truck up in an old GM "Data Book" and the standard rear ratio was 4.57.

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call parts dept at your dealer and have him tell you the what the code starts with then look in your glove box get him the code and he can tell you. There is an ID tag on the axle itself but it will only tell you what it is not the ratio>

Yep, 4.56 Bad fingers Badddd.

a 4.56:1 ration on a straight 6 4 speed? the thing would never even make it to 55MPH. I'd say it's either a 2.73 or a 3.08 given the 292 can't really rev and you have no OD.

-Bret

----------------------------------------------------- Can Anybody see my posts?I understand that A.O.L. customers can't see webtv posts. The 4.56 was standard with the 292. He would be at 2700 RPM at 55 MPH with 31" tire diameter.That's what he is complaining about--too high RPMs,and it is so bad he wants to change the rear=end. I guess that 4.56 was good around the farm.Not so good these days on the freeway! Brian(the invisible one) O.

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One has to understand that when that truck was manufactured it was as a truck. As such its got big heavy stiff ass springs, rides rough, can carry three elephants and pull a house. Personally I think he is going the wrong way. Instead of swapping the rear end, swap the tranny for one with overdrive. That solution retains the trucks pull power, and gives him some better fuel mileage. Whitelightning

This was my original thought. Some of my friends suggested this wouldn't be feasible from a cost perspective. The only overdrives I was looking into were add-on units that sat between the transmission and the diff, from the wording of your post it sounds like there are also transmissions with an overdrive built in, which I wasn't aware of. Any pointers and cost estimates?

Thanks, Andy

just about any 5 speed from a later model GM truck should work as long as you have a donor vehicle to scrounge parts from. look for a

88-up truck with a 5 speed. you'll either get a getrag, NV3550, or a NV4500. ( going from least to most desirable). NV4500's were in '91 up (IIRC) 8600# or heavier trucks. as long as you have access to the donor vehicle to get all the hydraulic clutch parts, the worst you'll have to do is find a clutch to work with the 292 and the transmission's input shaft, move the tranny x-member, and get a new driveshaft made.

hth, Bret

Ford and Chevy both had a "4" speed overdrive tranny in the early 80's manufactured by Spicer. Basically a four sped tranny, using 1st - 3rd gears from 3 speed set ups, and then 4th gear the overdrive. 1st gear was lower than a regular 3 speed, but not as low as a four speed, or as low as a granny gear 4 speed(which was really just a 3 speed with an "extra" low 1st that you only used when you were loaded to the gunnels, or pulling a trailer that mimicked a big rig) Both ford and Chevy 4 speed overdrives where top loaders. I do not remember if dodge offered this or not, but given they never produced a "big" six, I wouldn't trust it if they did. Another source would be C40 series medium duty trucks. Especially wreckers and moving vans, and don't over look old school buses. Because the 292 used a 12 inch clutch instead of the 10 inch used on V-8s in light duty trucks, it makes it easier swapping things around. You can use the stock flywheel and clutch plate, and then the disc for the application the tranny came from as well as the pilot bushing, as the medium duty rigs for the most part used a

12" clutch behind 6 and 8 cylinder engines.

International used a spicer tranny, 4753 that was an over drive unit and may fit the Chevy bell housing, (GM used a spicer 6853C with a 0.86: final drive ratio, as well as the 3153C with 0.79:1 final ratio). It was used in IHC

3/4 and 1 ton pick ups and travel alls and was refered to as the T-40. It may be possible to use the Fuller F-52 O/D referred to as a T-50. Be careful and make sure of what you are getting because IHC also used 5 speed trannies where 5th was direct drive, not overdrive, and 1st was basically a granny gear. GM also used a clark 5 speed manual, 264VO 0.80:1 finals. All of the above mentioned trannies were used in mid 60's trucks in the 4000 series and higher. there is also NP-549-GO The GO is very important as only the GO was overdrive, there was also a GL series of this tranny.

Now then if you really want to get crazy, you can(assuming this isn't a short bed truck) put a Spicer 5831 Auxiliary behind the tranny. There were

4 models with OD, the A,C,and E had 0.73:1 overdrive (this is available in all four trans forward gears as well as reverse) the B had 0.85:1. this gear box is sometimes referred to as a "brownie transmission". There was no "d" model, and there was an "F" model that had two under drives and a direct, instead of under/direct/OD. This would give you a "twin stick" set up in the cab and you could "split gears" to your hearts content.

For really go and strength the modern T-56 has two overdrive gears.

Whitelightning