Longfield™ FJ40 30-Spline 4340 Chromoly Axle & Birfield Kit With larger tires and more horsepower being used on the trails these days, upgrades to your axle are more critical than ever.  In a Toyota, one of the common failures are the stock 27-spline birfields and axles.  The solution to this problem is our Longfield™ 30-Spline 4340 Chromoly Axle and Birfield Kit.  These birfields and axles will get you to wherever you need to go and give you confidence and peace of mind to run any trail. The Longfield™ birfields are made out of 100% chromoly and are precision polished to remove any imperfections that may cause stress risers.  The Longfield™ birfield housing is made out of forged 4340 chromoly steel. The axle shafts are also made out of 4340 chromoly steel. The internals of our Longfield™ birfields are made out of 4340 chromoly steel andheat-treated for superior strength.