In our past segments, we delved into Mark Williams Enterprise’s extreme duty street-strip iron case 12 bolt. We looked at the special housing they manufacture along with their custom axles as well as the custom 35-spline Detroit Locker that ties the pieces together. This time around, we’ll look at the little details found on the M-W 12 bolt. There are a lot of special nuances found here. And you might be surprised at what’s included. Check it out:
On the nose of the 12 bolt, M-W incorporates a forged 4340 steel pinion yoke. Typically, the pinion yoke is a potential weak link in the rear end chain. Most OEM and replacement yokes for the 12 bolt are fragile cast iron pieces, and for the most part, they can’t accept a large diameter, 1350 Spicer universal joint. That’s why Mark Williams includes a 4340-forged steel yoke on their extreme duty custom 12-bolt. Following forging the yokes are CNC-machined to exact tolerances, plus they’re heat treated to 200,000 PSI. Each yoke is symmetrical for balance and alignment. Special snap ring grooves also allow for easy u-joint installation. As mentioned above, they’re designed to accept a massive, almost indestructible Spicer 1350 universal joint.
On the outside, the axle flange (which we discussed in the second segment) includes dual patterns. In the case of the 12-bolt in our photos, both patterns are GM 5 on 4-3/4, but one is drilled to accept a conventional ½-inch stud while the other pattern is drilled to accept a set of M-W’s huge drive studs. The wheel studs shown in the photos are short ½-inch jobs, which the Williams’ crew designed to fit under a stock Chevy dog dish hubcap (think “sleeper”).
Internally, there are all sorts of details – some you can’t see and some you can. If you check out the photos, you’ll see that each ring gear bolt is safety wired. And each of those bolts is a high-quality piece from ARP. In fact, many of the fasteners used during the build are either ARP or high strength socket head allen-jobs. The same applies to bearings. Now one might think it’s pretty easy to nail down bearings for a 12 bolt. But you’d be wrong. There are huge numbers of offshore parts available today, and it’s often difficult to locate quality pieces. That’s why Williams makes use of Timken bearings in their rear end assemblies, and that includes the custom 12-bolt in this series.
But that’s not the end of it. With most popular rear ends (and that includes the 12-bolt), you have several options for rear end gears – street jobs and pro gears. The difference is in the hardness of the gear. Rockwell hardness tests of OE production line gears reveal a typical minimum Rockwell “C” specification of approximately 59-63. Many high performance “street” gears register approximately 60-64 on the Rockwell scale. “Pro gears” (9310 material) will register approximately 52-56 on the Rockwell “C” scale. Why incorporate a “soft” construction for race gears? The reason is impacted loading. When a relatively high amount of shock loading is introduced into a conventional “hard” ring and pinion, the gears can shatter. Softer construction allows for a certain amount of tooth “bending”, but this is certainly preferred over shattered teeth. On the other hand, due to the differences in Rockwell hardness, gears destined for drag race use only (“Pro” series) are not appropriate for street or street-strip use. The gears wear rapidly because of the softness. As you’ve probably gathered from this series, the custom brute strength 12-bolt shown in this series is a street-strip job and that because of that, Mark Williams fit it with a street gear set.
On the outside, all of the little details (which often go forgotten) are taken care of. Case-in-point, the housing includes a correct vent. The rear cover is chrome steel job and it’s even fitted with allen-head fasteners. You’ll note that the studs for the axle bearing retainers (and the brake backing plates) are milled on the backside in order to clear the huge wheel bearings. And each stud is zinc dichromate plated. The CNC machined bearing retainers are horseshoe shaped, which allow them to be installed or removed from the axle without having to remove the axle bearings. The actual axle bearings (discussed previously) are manufactured by Timken.
As you can see, there’s a lot to the custom 35-spline 12-bolt Mark Williams builds. The sky is pretty much the limit here, and honestly, everything is over-the-top when it comes to quality. This is quite likely the strongest iron case 12 bolt on the planet, bar none. For more, check out the accompanying photos.