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Toyota Axle Tack Tech:

The Birfield joints in Toyota LandCruiser and Hilux (aka mini-truck) axles has been getting a bum rap for years. Many people, when their truck evolves from stock to modified, begin to have reliability problems with Birfields. To address the strength issue of Birfields, a few Toyota truck experts started to experiment with making the stock Birfield joints stronger by welding additional metal to the inner part of the joint. Although the first generations of strengthened Birfields were not always much of an improvement over the stock units, others turned out to be far stronger than stock with the result being that the weak link in the front end was moved to the axle shaft itself.

In a stock set up it, with stock Birfields, it is almost impossible to break the axle shafts in the front end. Now with the stronger Birfields meeting with some success, instead of the Birfield breaking, the axle shaft would break off at the outer end. This would in turn leave the remaining stub of the axle shaft captured inside the Birfield. With the factory method of retaining the Birfield to the axle shaft, by way of a sacrificial wire circlip, it became virtually impossible to remove the broken off axle stub.

Why and how you ask? Let me explain....

Toyota axles use a 1.31" 30 spline shaft at the differential end of things. At the outer end the shaft is 27 splines and proportionally necked down in diameter, thereby creating a weak spot in the axle shaft.

Why the 27 spline section at the outer end?

If, in the unlikely event that an axle shaft does break, it is much better for it to break off at the outer end. If the axle shaft were to break off at the inner end it would become nearly impossible to remove the remaining part of the shaft and with it the differential third member. So, really, the weaker portion of the axle shaft being the outer portion is a good thing and Toyota designed it that way on purpose.

Why did Toyota use a circlip to hold the axle shaft into the Birfield?

There are actually two cirlcips that are on the outer end of the Birfield. The inner clip, being the stronger of the two, prevents the shaft from going too deeply into the Birfield. The outer (and smaller) clip prevents the axle shaft from coming out of the Birfield since when the vehicle is steered there is some limited movement of the shaft in and out. In the stock set up both clips are used and are necessary for proper operation.

What happens if the axle shaft breaks off in an unmodified vs. a modified Birfield?

Birfields can be disassembled when they have not been modified and so the broken stub can be removed. However, most modified Birfields are made by welding a steel ring to the inner edge of the "bell" of the Birfield. When this is done, disassembly of the joint is no longer possible and removal of a broken off axle stub becomes extremely difficult. To alleviate this problem, axle tacking was decided on as a workable solution to the problem.

What is axle tacking then?

Since the outer circlip became an issue, but was also a necessary part, a method of preventing the axle from moving too far out of the Birfield was developed. The other factor is that the inner axle seal rides on a slightly raised portion of the axle shaft and that portion needs to remain in the correct location of oil will enter the knuckle and create a leak... So, two things are at work here. The need to keep the shaft in the correct location in the Birfield and also to keep the seal surface in the correct location for the seal to do its job.

An axle tack, then, is a small tack weld that is placed on the inner portion of the axle shaft so that the splined portion is no longer able to go into the side gear in the differential any further than it should. The tack on the inner portion of the axle neatly takes the place of the outer circlip and solves the problem of broken shafts and removal of the stub from the Birfield.

Tacked axles also have some advantages to those who are not running Birfields that are strengthened in such a way as disassembly of the joint is impossible. When a Birfield is broken on the trail, removal of the remnants from the axle shaft and subsequent reinstallation of the shaft and circlip into a replacement Birfield can be a bear of a task if things are not working in your favour. In almost all cases the (sacrificial) outer clip is broken and a replacement unit should be installed on an untacked axle shaft. The axle tack makes removal and reinstallation of the Birfield onto the axle shaft much, much easier and there is no need to carry spare clips in your trouble kit since they are no longer required.

How to tack an axle:

A small tack weld should be made to the axle on the splined portion approximately 1.5" from the inner end of the shaft. The weld should be ground or sanded flush with the splines and then smoothed with a wire wheel or a file. If the axle tack is left "sitting up" it may damage the inner axle oil seal when the shaft is inserted into the housing.

Problems associated with the axle tack:

Since welding any piece of metal that is not subsequently normalized or annealed leaves a certain amount of stress in the parent metal, a small weak spot may develop in the shaft (creating a nucleus point for a break). So far, I am not aware of this being an issue in axle strength since the outer end of the shaft is still weaker than the tacked inner end.

In the pictures you can see that a piece of copper was used to prevent the tack weld puddle from extending onto the portion of the axle where it is not wanted. Welding up against copper with a GMAW (MIG) operation does not contaminate the weld or create any problems with it, and creates a relatively "sharp" transition point.

Axle tacking is a relatively simple operation that greatly simplifies Birfield/axle shaft removal and reinstallation in the field or on the shop.

John Barron--Victoria, BC, Canada
John_Barron@shaw.ca