Rear Axle and Axle Shaft Help

[drbeeper17]

I just started working on my project after it collected dust for 2 years and I have some questions about the rear axle. The truck is a hybrid of parts ranging from 67-72, which is why I do not know exactly what I have. My question is how to definitively tell if I have a narrow or wide 1/2 ton rear axle. It is currently setup with 5 lug axle shafts, but I have a strong feeling that parts have been swapped with it. It could be a stock 5 lug axle with all stock parts (which would be ideal) OR it could be a narrow axle originally equipped with 6 lugs converted to 5 lugs by swapping properly sized shafts (the lugs have not been re-drilled on the axle shaft) OR it could be a narrow axle originally equipped with 6 lugs with some combination of shafts and drums to convert to 5 lug.
I've read posts for about 3 weeks now and I can't determine where everyone is taking measurements from. I keep seeing that backing plate to backing plate of a narrow rear axle is 55.5" and the wide axle is 57". But depending on where on the backing plate I measure, I can get either dimension. The backing plate contours, so if I measure where it attaches to the axle flange, I get a wider measurement, but if I measure from the outside edge, I get a narrower measurement. Additionally, if I measure from the inside edge of the backing plate vs the outside edge of the backing plate, I get different measurements. With the axle widths differing by only 1.5", it's really hard to tell what I have. I want to rule out any measurements involving the axle shafts or drums since the ones on my truck may not be the correct ones for the truck. If I totally strip the axle down including removing the backing plates, is there a measurement for tip of axle tube to tip of axle tube? What about the width from the inner or outer sides of the backing plate flanges?

For additional background, I do not know the year of the frame, and the rear axle may have been swapped at some point. I do have 5 lugs front and rear, but the front has been changed to an all new 5 lug disc brake setup prior to me taking possession of it. This makes me think the rear was swapped as well to match the 5 lug front. The frame and axle have been painted in a thick black paint (also prior to me taking possession) and the numbers on both do not come up with any definitive results. I think the frame may be a 69 or 70, which means the rear end may be narrow or wide depending if it is a 70. The radiator mounting holes are about 1.75" and the bracket sits flush with the frame (making it a 69-72 frame I think). The front brake lines route through brackets attaching to the rear of the front cross member (making it a 69-70 I think).

Aside from providing some definitive measurements from definitive locations on the rear axle, I also have some pictures of what the axle shafts look like sticking out. Are they supposed to stick out this far? Since the axle shafts do not look aftermarket, my concern is that someone stuck longer shafts into a narrow axle and called it a day. I plan on running around 500hp, so I don't want to have a rear end that is not set up properly. The axle shafts measure 31.25" from inner tip to the flange where the brake drum sits (where the threaded portion of the wheel stud comes through the axle flange). This is once again a hard to know what shaft this is since some measurements I have found measure from the end of the splines and the shafts extend beyond that for the C-clip retainers.
Attached are some pictures. Hopefully someone can tell me what I have based on these pictures and the descriptions above. Thanks!!

[Ironangel]

They officially went to the wider rear end in 71 although some 70's had the wider rear. 63 thru 70 were 55-1/2" wide housings with 30-1/2" axles. The 71 on had 57" wide housings with 31-5/16"...Thats from "WMS" (Wheel Mounting Surface) to WMS...Thats the face of the axle flanges...These are 12-Bolt Truck dimensions...The 70 1/2 and earlier width is 62-3/8" from wheel mount to wheel mount.
The 70 1/2 and later (up to about 78 or so) width is 63-3/4" from wheel mount to wheel mount.

1/2 ton was the same width as the 3/4 ton. A GM12 bolt is a tough setup.

[drbeeper17]

Thanks for the feedback so far. The concern I have is that someone put longer shafts from a 1973+ truck into a 1969 axle. Measuring from WMS to WMS takes into account the lengths of the shafts plus the width of the differential center pin. It doesn't tell me if the shafts are the correct length for the axle.

I've read on this forum that you can put longer 5 lug shafts into an older, narrower axle and use a wider brake drum. The concern then is the fact that the hardened, polished surface of the shaft is not riding on the axle tube bearing properly since it is too far inward for the shaft.

I'm hoping to get a axle housing measurement vs shaft length measurement for a properly matched rear axle. I'm concerned that mine is mismatched.

[Ironangel]

I listed the housing widths, 55" on the pre 70 housings and 57-1/2" on the 70 thru 72 on to like 80 or 81 to where GM discontinued the truck 12's. As far as the bearing surfaces go, the later shafts are 31-5/16" long vs. the older shafts at 30-`1/2" long. Thats roughly a 3/4" difference in shaft length. Visualize that difference on the hardened bearing surface of your shaft and if your still on polished bearing surface, go for it. I would look in to the cost of having your existing shafts shortened at a machine shop vs. the wider drums. Or, you can buy a set of aftermarket 5-lug 30 spline axles. A machine shop wouldnt have to take a full 3/4" off and then cut a new C-Clip slot, very inexpensive endeavor. In fact, I would verify that the spline depth is sufficient to allow the axle to move in say, 1/2"? If it does and the drums fully cover the shoes, I'd cut the axles and machine the c-clip slots myself. But if you dont have the tools, let a machine shop handle it for what should be no more than $20 per axle, tops.

[drbeeper17]

So I confirmed that my shaft measures about 31.375". Combine this with the other shaft and a 0.8" diameter center pin and I have about a 63.5" WMS to WMS measurement. This seems to fall in line with the later (70.5+) shafts.

When you provided the 55" vs 57.5" housing measurements, where are those taken from? Is it the inboard sides of the backing plate flanges or the outboard sides of the backing plate flanges? Or somewhere else entirely?

My brakes shoes are 2" wide, but the drums measure about 4.25" when I lay them flat on the ground. I don't have a good reference for a proper brake drum to brake shoe width comparison, but it does seems the shoes are almost too narrow for the drums.

I'm thinking I might have the setup described in this post:
http://67-72chevytrucks.com/vboard/s...d.php?t=215095

[drbeeper17]

So I just wanted to post a follow-up to how I resolved this situation and the information I found along the way. This should help those who want to do a 5 lug conversion on a 67-70 narrow ½ ton 12 bolt rear axle.

When I asked about rear axle width measurements, I was never able to get a clear answer. People provided a backing plate to backing plate measurement, but was that from outside edges of backing plate, or where the backing plate attaches to the axle, or some other flat spot on the backing plate? The backing plate is contoured, so depending on where you measure from you can get either the narrow or wide axle measurement.

So I too off my backing plates and found measurements from hard points on the axle housing. My axle measured 57" from the outboard edges of the backing plate mounting flanges (the square piece with 4 holes that is welded to the axle tube). Additionally, from axle tube tip to axle tube tip, my axle measured 60".

The 5 lug axle shafts that were in my axle were about 31 3/8" long, measuring from the tip of the shaft to the outboard edge of the wheel stud mounting surface (the surface that the wheel touches when it is installed). With these shafts installed, the stock rear drums would not work as they were too shallow. The brake pads were not fully inside the brake drum. However, these axle shafts have a pretty wide machined surface on them. This means that even though they may have been designed for a wide axle with the bearing further outboard, they will work on a narrow axle that has bearing more inboard. I checked and my bearing fully contacted the machined surface of the axle shaft.

I had a few options at this point. 1. Go back to stock 6 lug axles and have a narrow stance. 2. Buy 5 lug aftermarket shafts for about $260 and have a 5 lug narrow stance. 3. Find a way to make my existing shafts work and get a 5 lug wide stance to match the wide stance of the disc brake conversion front end. I chose option 3.

From what I have read on this forum and elsewhere, people swap newer axle shafts (from 1973 and newer trucks) into the rear ends of these trucks and then swap the associated brake drums and call it good. They think they are getting a cheap 5 lug conversion but they overlook some of the details. There are two problems with this. First the axle shafts are longer and are intended for wider axles. Therefore they stick out further on the 67-70 narrow rear axles. Secondly, the brake drums from the newer trucks are for 2 3/4" wide brakes that are actually 11 5/32” in diameter. The stock 67-72 rear end uses 2” wide brakes that are 11” in diameter. Mine was setup like this and I could tell that only one very small portion of the shoes were actually contacting the drum, which means I would barely have any rear braking. The 5/32” diameter does make a large difference in the shoe to drum contact.

You would think that you could just swap of the brake shoes to the 2 ¾” wide shoes, but they will not fit the stock backing plate due to the positon of the wheel cylinder and the length of the fixed upper stud. Perhaps you could use the 73+ backing plates, but I think their contour would be fitted for a wider axle, so the relation of the shoes to the drum would be off.

Another solution would be to make a spacer that goes between the backing plate and the mounting flange, which I have read that people have done. The spacer would need to be ¾” think and machined for the axle tube diameter and the 4 bolt holes. I did not like this solution for two reasons. First, I do not have access to anything more than a few power tools and hand tools. Secondly and probably more importantly, unless you weld these spacers in place you are introducing a greater shear stress over the 4 bolts that hold the backing plate onto the axle.

The solution I settled on was finding a drum that was 2 ¾” deep and 11” in diameter. These never came on trucks, but they were used in 1960’s car for front drum applications. After several purchases and returns from the local auto parts store, I found that Raybestos 2265R brake drums are the perfect fit. This way the additional shear stress is applied over the diameter of the drum, which they are designed to be loaded in this manner.

I know that was a bit long so here is a summary of what I did to get a 5 lug wider stance rear end using a narrow axle. I used 73+ 5 lug shafts in the axle, stock brake hardware, and Raybestos 2265R brake drums. You can get axle shafts from salvage yards for about $30 each and the drums can be found online for about $35 each. So for $130 you have essentially changed your narrow 6 lug axle to a wide 5 lug axle. I’m pretty sure the aftermarket shafts change lug pattern but do not give you the extra width. The only downside with the conversion is that it creates a greater bending moment on the axle shaft itself (due to the location of the bearing in the axle tube), but I don’t think this is a problem unless you intend to intend to carry a heavy load in the bed of your truck.

[drbeeper17]

Pics of finished product.

[drbeeper17]

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