Mark Williams Modular Rear Ends Part 4

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Over the past three issues, we’ve examined Mark Williams’ Modular 9-inch rear end assemblies and, in particular, the street machine/street rod/street-strip versions. While M-W offers a wide range of choice when it comes to axles, the components fitted to our featured Modular 9-inch are Mark Williams’ MasterLine examples. According to M-W: “Owners of cars that turn the quarter-mile in 10 seconds or slower should consider the economical MasterLine series of axles. Anything quicker should be equipped with Hi-Torque series axles that are available in 4340 and 300M hi-strength steel.”

The Modular setup can also be fitted with Mark Williams Hi-Torque axles. What’s the difference? Let’s rewind a bit: The first thing the M-W tech will ask you when ordering a Modular 9-inch is the application of the car. The reason for this is because an axle engineered to withstand the punishment of a 2500 horsepower Pro Mod drag car would be total overkill for a street or bracket racing application. The amount of torque load to which an axle can be subjected at launch can be estimated by taking the engine torque, multiplying that by the transmission's first gear and then again by the rear end ratio. A Pro Stock car is capable of generating about 10,000 ft. lbs. of torque to the axles (5000 ft. lbs to each). That’s way more capability than you’ll muster in an 11-second street machine - hence the need for something like the MasterLine axle.

Designed for street and bracket racing, the MasterLine axle still provides many features of the premium "Hi-Torque" series axles. MasterLine axles are true custom axles, made to the exact length required and featuring hobbed involute splines to match stock differentials, posi-tractions and aftermarket spools (35 spline and smaller). The axles for the street machine modular shown in the accompanying photos are 31-spline.

Given the overall requirements for most street driven applications, MasterLine axles are forged from aircraft quality steel. Most OEM axles are made from carbon steel, (typically SAE 1055 or 1541) and they’re relatively brittle. When high impact torque loads are placed on them, they can snap. There are some who use those 1055 or 1541 materials for their so-called "high performance” axles. As mentioned above, Mark Williams Enterprises uses aircraft quality steel for its MasterLine series of axles and a more durable Nickel-Chromium-Molybdenum alloy for the Hi-Torque series. A 300M alloy is also offered for extreme duty use.

Yes, you can buy cheaper “alloy” axles; however, most of those axles are actually produced by OEM axle forging companies, whose main business is making axles for the high volume OEM car and truck markets. As noted above, the material used is carbon steel, common to OEM axles. It is not usually regarded as alloy steel, as advertised. The mass-market manufacturer produces the axle blank and then the “custom axle company” cuts the axle to length and splines it. These axles are made from the same material as stock axles and receive the same heat treatment. The only difference is that they are available in shorter lengths and with different splines (more on splines down the page). The quality of these axles is comparable to a re-splined stock axle.

The strength of a component is greatly influenced by the manner in which the material is heat-treated. A double heat treat induction hardening process is employed for the MasterLine axles (performed in-house at M-W). The axle is passed through an electromagnetic coil that excites a powerful current within the shaft, thereby heating it and simultaneously quenching the axle to full hardness. Following the heating and quenching process, the axle is tempered in special salt-heated draw furnaces to reduce brittleness and increase ductility.

The axles can be drilled with up to 3-hole patterns to match the most popular 5 on 4 1/2", 5 on 4 3/4" and 5 on 5" wheel patterns. The holes are tapped to accommodate 1/2"-20 wheel studs, with an optional upgrade to 5/8-18" threads to utilize Drive Stud wheel stud kits.

Some manufacturers, in an effort to cut costs, "gang run” quantities of axles with very long splines. Then they merely chop off the required amount of spline to get the desired length. Accordingly, some axles have very long splines (which weaken them) or very short splines (which compromise engagement). You should know that each and every MW axle (MasterLine and Hi-Torque) is individually made and has the optimum spline length for the application. No shortcuts are taken.

The flange is important too. Using a stock 12-bolt axle as a point of reference, it has a thickness of 0.320 inches and comes with sharp corners where it merges with the bearing surface. By contrast, an MW MasterLine 12-bolt axle has a 0.438-inch thick flange and employs a smooth transition to the shaft. This is obviously a much stronger way to build an axle flange. Increasing the strength of the axle flange area is a paramount consideration for racing axles, and is of greater importance when using increased-offset wheels. Additionally, a robust flange is extremely important when you’re in an environment full of curbs, potholes and general road debris.

As you can see, the MasterLine axles available in the M-W Modular 9-inch rear end assemblies are definitely up to the task of transmitting the power of your street rod or street machine to the ground. The accompanying photos give you a better look too.

When all is said and done, the idea of using a Modular rear end in a street rod or street machine certainly has a lot of merit. Not only are they robust, they’re light, and they definitely look uber-cool. Monster Modulars? We think so!

The axles installed in this particular Modular 9-inch are Mark Williams’ MasterLine pieces.

As you can see, the axles are different lengths. This is because the pinion is centered in the housing.

MasterLine high performance axles are forged aircraft quality steel. In contrast, most OEM axles are made from carbon steel, (typically SAE 1055 or 1541) which proves to be rather brittle. With the MasterLine axle, the flange is ductile, which is important especially when curbs, potholes and other “obstacles” enter the picture on the street.

It’s easy to see the taper in the axle. The triangular profile provides strength to the axle. M-W does not gang run axles. Each is built specifically for the application. This costs more, but makes for a considerably stronger axle.

A large diameter wheel bearing is fitted to the MasterLine axles installed in our sample Modular 9 rear end assembly. As you can see here, the bearing is fitted with an O-ring style of seal.

If you take a look at the axle, you’ll see there’s a generous, smooth taper machined into the transition to the wheel flange. This adds strength to the axle at a critical point.

The wheel bearing is pressed on the axle and held in place by way of this lock ring. M-W always uses a positive means of axle retention. More on this later.

Axles drilled and tapped for ½-inch wheel studs (5/8-inch Drive Studs) are optionally available. In addition, M-W can drill these axles for 4-1/2-inch, 4-3/4-inch and 5.00-inch bolt circle patterns.

Splines such as this are machined by way of hobbing rather than fly cutting. Hobbing is a machining process which incorporates a special type of mill. The teeth or splines are progressively machined into the component by a series of cuts made by a cutting tool, which is called a hob.

These are typical wheel studs used on Modular 9-inch assemblies. They’re 12 point, ½-inch examples with a bullet nose. ARP manufactures these exclusively for M-W.

As pointed out earlier, axle retention is always positive in a Mark Williams Modular housing. This example features Chevy housing ends; these hefty retainers are used to hold the axle in place. Note the aircraft quality studs and lock nuts used to install the retainers.