The straight front axle on a Toyota truck or 4Runner can survive many, many trips over difficult obstacles rolling on 33-inch tires. But these days Toyotas all over are crawling extreme trails on 35-inch, and larger, tires. With this comes the need for sturdier axle hardware able to withstand the stresses of hard-core obstacles on big tires.
From 1979 to 1985 for U.S. models, Toyota used straight front axles that incorporated a venerable closed-knuckle design similar to what it had been using on the Land Cruiser. As with many of the components on these vehicles, Toyota axles are quite beefy and are far overengineered for most reasonable use. They feature beefy bearings, knuckles, axleshafts and housings. Inside each knuckle, they employ Birfield joints-one per side. A Birfield is essentially a constant-velocity joint that links the inner and outer axleshafts.
The Birfield joint consists of an outer steel bell that is part of the outer (stub) axleshaft, an inner bearing race, six ball bearings and a bearing cage. It is an efficient assembly, and combined with a water-resistant front-knuckle assembly, can provide good long-term service. But when pushed hard with larger tires, a bigger engine or lower gears, Birfield joints can be stressed beyond their limits.
Yukon Gear & Axle has come up with an upgrade to help fortify the Toyota front axle. Its kit, aptly named a Birfield Eliminator kit, replaces the stock Birfield with conventional U-joints to connect the inner and outer axles. The kit also includes new axleshafts crafted from 4340 high-nickel steel. These shafts mate right up to your existing 30-spline differential carrier and the stock locking hub assemblies. The standard kit includes a set of Spicer 760x U-joints. If you're looking to make the next jump up in axle durability, the 760x style joints can be replaced with a set of rebuildable U-joints built by CTM Racing Products.
Follow along as we hit the high points during reassembly of our Toyota axle and increase its strength, sans Birfields.
Straight-axle Toyota trucks and 4Runners use constant-velocity Birfield joints in the front closed-knuckle axle. The joint mates the inner and outer axles with a six-ball-joint assembly. These front axles are reasonably durable. | 
Pushing the Birfield's limits with tires taller than 35 inches can result in destruction of the bell of the Birfield, as shown here. | 
The conversion kit includes inner and outer axleshafts made from 4340 high-nickel alloy steel, new spindles, U-joints, bronze housing bushings and housing seals. |
To add ultimate joint strength to the heavy-duty axle kit we decided to use a set of CTM Racing U-joints. | 
The provided bronze bushings were installed into the axlehousing to prevent the axleshaft from coming into contact with the trunion bearings in the knuckle. | 
Each axleshaft was slid into the housing. Care should be taken to prevent damage to the seals. |
The new spindles were slid over the axleshafts and mated to the knuckles using the stock bolts and lock washers. | 
With the bushings installed, the inner axle seals were pressed into the housing. These are used, as in the stock setup, to keep the gear oil out of the knuckles. | 
The provided rubber boot should be installed on each of the outer stub axles. The rubber boot rides against the back of the spindle and keeps the axle assembly from moving side to side. |