[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.