Toyota Birfield Upgrades: All About Chromoly, 30 Splines, and Fit For Minis, Pickups, 4Runners, and Land Cruisers

We have a stock Toyota mini Birfield on the left and a 30 spline Longfield on the right. What makes the Longfield better? Read on....

Once you start breaking front axle shafts, you’ll start looking at replacement options. 30 spline aftermarket axles are the most significant strength upgrade you can make to your front axle, short of swapping it out for an upgraded Dana 60. So how is it different from a stock Toyota axle, and how does it fit your truck? Read on for the answers.
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If you're here it's probably because you want chromo Birfs for a specific Toyota. We've got lots of general info here, but our model-specific pages list all the kits available for each model along with their specs. We also include model-specific information on issues you might have, so make sure you read before you buy. If you want to see a breakdown of a stock Birfield along with stock specs, we have that too.

The Chromoly Birfield Master Lists and Model-Specific Notes

There is a wide variety of upgraded Birfield axle kits. Where are the Profields, Smurfields, whateverfields? When chromoly Longfields came out, they killed off several of the competing Birfs as they were refined. The least expensive Birfs in our lists are fully made with all 4340 chromoly steel at the minimum which is a great upgrade over your stock Birfields.

If you mall-crawl, or just don’t really want to upgrade, there are aftermarket stock replacement Birfields available. We don’t know much about these (although we plan to look into it!) other than that there are Japanese replacement Birfs that are good quality.

Note that manufacturers make axles that fit the widest range of vehicles. For you, this could mean that you’ll have to swap some hub parts to upgrade your Birfs since most fitment issues are about axle lengths. We’ve included model specific notes for those applications:

Toyota Pickup and 4Runner Chromoly Birfield Axle Guide and Comparison

Toyota Land Cruiser FJ40/FJ55/FJ70 Chromoly Birfield Axle Guide and Comparison

Toyota Land Cruiser FJ60 and FJ62 Chromoly Birfield Axle Guide and Comparison

Toyota Land Cruiser FJ80 and FZJ80 Chromoly Birfield Axle Guide and Comparison

Birfield strength has increased over the time of their manufacture

As they were made, the solid front axles in Toyotas became stronger and stronger over time. Toyota put the “ball and claw” axleshafts in the first Land Cruisers, then the first Birfields. The most significant strength changes in stock Birfields have probably come from increasing size of the Birfield bell. If you’re wheeling on stock Birfields, you’re likely to break some part of the bell first. The more recently your Toyota 4x4 was built, the stronger its Birfields are likely to be. Big tires, low gears, and hard use will take their toll though, and you’ll want to upgrade your Birfs to something with more splines and stronger materials.

Toyota Axle Strength
Strength Axle
Weakest Early coarse drum (Land Cruisers)
Early fine spline disc (Land Cruisers)
Late fine spline disc (Land Cruisers and pickups/minis)
27 spline aftermarket chromoly Birfield
30 spline aftermarket chromoly Birfield
30 spline aftermarket chromoly Birfield w/300M star
Strongest 30 spline aftermarket chromoly Birfield w/300M star and inner axle

The above chart progresses from small dimensions, weaker materials, and fewer splines to larger dimensions, stronger materials, and more splines. Larger dimensions and stronger materials obviously confer more strength to the Birfield.

What Makes a Birfield Joint Strong?

There are a few factors that are important for your front axle’s strength. It’s probably fair to say that we’ve just about reached the limit in strengthening Birfields. Chromoly/300M front axleshafts have about as much strength as a Dana 60 front axle. As you keep upgrading parts, eventually you move the failure point to another location - this could be your hub, locker, or gears.

The baseline for upgraded Birfields is 4340 chromoly steel. If you’re upgrading your Birfields it will be made of 4340 or 4340 and 300M (a 4340 variant). All upgrade Birfields have chromoly, but note that not all aftermarket axles are chromoly. There are a few stock replacement axles that will help you replace a stock Birfield with a stock equivalent.

Here are the factors that matter in Birfield strength:

Spline Count - MORE IS BETTER

More splines is better. Generally, more splines indicates a larger axle diameter. Splines have mostly been standardized such that if you know the diameter of a shaft, you can estimate the spline count. Assuming all shafts followed the standardized diameter-spline scheme, you would easily know that a shaft with more splines was a larger diameter (Toyotas don’t follow all the standards). More splines results in more surface area for component engagement which makes for a more even distribution of force across the splined surface.

Root diameter is greater with more splines. Root diameter is measured from the bottom of the “V” between splines across the shaft. More splines result in a larger root diameter of the axleshaft. This would be particularly obvious when looking at the end of 6 spline shaft versus the end of a 27 spline shaft. Splines on the 6 spline shaft have to be deeply cut into the shaft, which reduces the root diameter.

Very small spline counts usually indicate cut splines. Cut splines are weaker than rolled splines. Cutting splines merely removes material from the shaft to make a spline. Rolling splines compresses the metal, aligns the internal structure, and makes it denser and stronger.


There are two materials that are now used for upgraded shafts: 4340 chromoly and 300M.

4340 Chromoly

4340 chromoly steel, also known as 4340 chrome moly steel, is a low alloy steel that has become a standard material for materially upgrading drivetrain parts, including axleshafts, input and output shafts on transmissions and transfer cases, and steering arms. The steel you buy at a metal supply house for general fabrication (i.e. an alternator bracket, transfer case mounts, a bumper) is most likely 1020 steel. While 4340 chromoly will still rust like regular steel, it is much tougher and more durable.

Importantly, its chemical composition makes it extremely suitable for heat treating to make it even stronger. We’re not sure what Toyota axles are made of, but we do know that 4340 chromoly is an upgrade. Most aftermarket axleshafts will use 4340 throughout the entire axle, but you should always read the advertising copy thoroughly to ensure that you are getting what you expect. There are “stock replacement” Birfields out there that aren’t any stronger than stock. They’re fine for mall crawling, but not for heavy wheeling.

4340 chromoly will wear out in certain circumstances because it is softer than stock axle materials. If you run a Toyota with part-time four wheel drive, this is no problem. However if you have full-time 4WD and you put a lot of miles on your truck, you’ll eventually wear out your 4340 chromoly Birfields and they’ll start to click. This means you, 80 Series owner! See the vehicle-specific sections below. Aftermarket upgraded axle sets will use either 4340 or a 4340/300M combo.


300M is a productized, modified 4340 chromoly. Metallurgically, it is different from normal 4340, but it’s important to point out that it is a product produced by a company (Latrobe). As a product, it will be materially more consistent than a metal that just anyone can make. Also, it will, and has been, heavily marketed as a superior alternative to 4340 and has a price to match the hype. Strength and durability-wise, it has the reputation among wheelers for being stronger and springier than 4340 chromoly, meaning that you can twist a 300M shaft and it will return to it’s original shape. RCV is currently the only company using 300M in their axles. They use it to upgrade the inner race and the inner axle in 30 spline axles.


Bell Size

Most shafts use the largest Birfield bell possible (such that you may need to clearance your knuckle ball) and this should help eliminate “Exploding Birfield Syndrome” along with the use of 4340 chromoly. It’s pretty noteworthy that over the course of their manufacture, the bell diameter of stock Birfields increased. A stock 80 Series Birfield bell is quite a bit larger than an early 40’s. No, you can’t fit an 80 bell in a 40 housing.

Toyota Hilux Birfield Bell, Stock (left) and 30 spline Longfield (right)
What makes these different? The Longfield on the left is machined from 4340 and has a slightly large outer diameter.

Cage and Inner Race

The cage and inner race on an aftermarket Birfield will be much bigger. Some will be machined from 300M which bumps up the strength of 30 spline races since they have slightly less material than a race that fits a 27 spline shaft.

Toyota Hilux Birfield Cage and Inner Races, Stock (left) and 30 spline Longfield (right)
The Longfield on the right is much taller and machined from 4340 chromoly. Some aftermarket inner races are made from 300M, which can compensate for the slight reduction in race material with the larger 30 spline hole.


Some Birfields are gun-drilled and have a zerk fitting on the end. This makes them easy to grease. Traditionally, gun-drilled shafts are used to save weight, but that doesn’t matter so much in your 4x4. Gun-drilled shafts are slightly weaker, but we doubt that this is a problem in a 4340 aftermarket Birfield. You’re more likely to have a failure point elsewhere and the increased ease of maintenance means that you’re more likely to lube your Birfields more.

Gun-Drilled Longfield with Grease Fitting
Gun drilling makes the Birfield easy to maintain. Gun-drilled Birfields are offered by RCV Performance and Trail-Gear Longfields.

Inner axle retention

Most aftermarket upgraded Birfields have eliminated the double-ring retention system on the inner axle. On a normal Birfield, separation of the inner axle from the Birfield would result in the inner axle circlip breaking. The circlip keeps the shaft from moving into your carrier. Marlin of Marlin Crawler invented the Martack, a small weld on the splines to eliminate the circlip. The Martack was mostly created to make it easy to remove an inner axle, particularly a broken inner that was now held in place by a stubborn little clip. With the newest aftermarket Birfields, this is no longer necessary.

Toyota Hilux Inner Axle Cage and Inner Race-2, Stock (left) and 30 spline Longfield (right)
You can clearly see how the inner axle is held in place with the Longfield. The circlip on the inner axle shaft rests in the inner race's groove. The stock inner race uses a circlip on one side and a snap ring on the other.

Some Birfields may be manufactured to reduce stress risers and such. If you must have every last ounce of strength, look for those qualities.

The Strongest Birfield

So, putting it all together the strongest Birfield will use the largest possible bell, the highest possible spline count (30 splines), and the strongest materials (300M where possible, 4340 chromoly otherwise). Gun drilling will slightly weaken it but is a big plus for maintenance. Naturally, the strongest Birf will also probably be the most expensive!

Aftermarket Birfield Spline Count: 24/27 Spline vs. 30 Spline

Aftermarket axles come with either stock spline counts or 30 splines.

If you don’t know the difference between a “27 spline” axle and a “30 spline” axle, you can thoroughly read about splines and Birfield components here: The Birfield Parts of a Stock Toyota Front Axle.

The short answer is that your current axles most likely have 30 splines at the hub and differential and 27 splines inside the Birfield joint itself. This is a “27 spline” axle. A “30 spline” axle has 30 splines throughout the axle. 30 spline is stronger.

Terminology clarification:

Throughout this article we’ll refer to 24/27 spine axles. This is because FJ80s have 24 splines stock, and most other Toyotas have 27 splines stock. There are “24 spline” axles, “27 spline” axles, and “30 spline” axles. There is no “24/27” spline axle.

Should I choose 27 spline or 30 spline aftermarket Birfields?

A 30 spline upgrade is really about upgrading your inner axle shafts. Once you have a chromoly Birfield, you’ll start breaking inners. Stock 27 spline Birfields make a lot of diameter changes. 30 spline axles are pretty consistent sized.

Toyota Hilux Inner Axle, Stock (top) and 30 spline Longfield (bottom)
The Longfield inner axle has a very consistent diameter along its length. The stock shaft has a varying diameter.

There is no question that 30 spline Birfields are stronger than 27 spline Birfields. Here are some things to think about:

  • According to Bobby Long, the inventor of the Longfield, he started making 30 spline inners because stock or chromoly inners would still break and destroy the axle’s locker. This is an extremely expensive problem to have.

  • If you break a 30 spline, you need 30 spline parts to replace it, or a complete stock axleshaft. This means that you’re either buying extra 30 spline parts as spares, or using a weaker stock axle as a spare.

  • If you are regularly breaking chromoly axles (or think you will), one tons may be in your future. In the meantime, 24/27 spline aftermarket Birfields might be a better option so that you can always have cheapish spares that can get you off the trail.

Assuming both are made from chromoly, and both are built with the same processes, 30 spline axleshafts are considered to be stronger. However, it is interesting to note that by increasing the spline count, and thus the shaft diameter of the inner axle, the diameter of the hole in the star is increased. This means that a 27 spline star has more material than a 30 spline star. To compensate for this, several aftermarket stars are machined out of 300M, which is stronger than chromoly.

So I can carry stock 24/27 spline axleshafts as spares for my 30 splines?

Yes, as long as you intend to swap the full axleshaft, inner to outer. You can’t connect a 27 spline inner to a 30 spline Birf, or a 30 spline inner to a 27 spline Birf.

How Do I Fit a 30 Spline Axleshaft In My Toyota?

For many cases, you simply remove your old axleshafts and install the new ones. You generally don’t have to swap parts and the upgrade is often a direct swap whether you’re installing 24/27 spline or 30 spline shafts. However, some vehicles require hub swaps or other parts to accommodate different hub splines (like when upgrading coarse splined Birfs with 6 hub splines) or Birfield lengths (upgrading FJ80 non-ABS Birfs). Check our chart below to see what you need.

Do I need to grind the housing?

Sometimes. Since aftermarket Birfields usually use a larger Birfield bell, you’ll simply have less clearance to work with when installing the shafts. Sometimes, clearancing is required to allow full articulation of the Birfield, like when you’re turning right or left. Some people have to clearance and some don’t. If you think you need to grind your axle housing, check our sources at the bottom of this article for clearancing info. The basic procedure is to temporarily install the Birfield without the inner axle, move it around, and grind where it hits inside the knuckle ball.

Adjusting the steering stops

Extreme operating angles caused by improperly adjusted steering stops is probably the number one cause of chromoly Birfield breakage. Stock or chromoly Birfields most frequently fail at extreme angles. Keep your steering stops properly adjusted so that your Birfields aren’t operating outside their intended limits.

Really, if you’ve never adjusted your stops, go do it right now. If you install new Birfields, steering stop adjustment should be part of the installation since the adjustment will be partly based on the maximum turn angle of your new chromoly Birfields.

If you use your Toyota 4x4 long and hard enough, you’ll deform your stock steering stops. You can beef them up by welding a nut or bolt inside them, or by replacing them altogether. It’s better to add material to the inside of the stop, rather than merely capping the top and bottom.

If I break a 30 spline axle, what can I swap in?

If you break a 30 spline, you can swap in only 30 spline axle parts or a complete 24/27 spline axleshaft (including the outer and inner axleshaft). Check our below table for parts swapability. If you always carry spares, or you pool spares with a group on trail rides, you may want to keep this in mind.

The chart is pretty simple and we assume swaps for the same model/same model year. We mostly want to indicate how parts fit together regarding spline counts. Remember that inner axles and Birfields have changed lengths across models. If you run 24 spline axles, pretend that 27 is 24.

Can I combine parts from different manufacturers?

We do not recommend that you try to mix and match axle parts from different manufacturers. If you check out the model specific guides, you’ll note that inner axle lengths often vary from manufacturer to manufacturer. We have not compared inner races (yet), but this suggests that the c-clip retention grooves are located in different places in the inner race. Since splines are standardized, we would expect that most 30 spline inners will fit most 30 inner races. However, the wrong combination could potentially result in either parts grinding (axleshaft too long) or reduced spline engagement at the carrier (axleshaft too short). Both of these scenarios should be avoided.

You shouldn’t rely on an aftermarket axle to be a trail spare unless you know it will fit in your axle housing. Read the section on axle housing clearancing.

Do I need extra parts to change Birfields?

If you’re just replacing your existing axle with the same thing, you should consider carrying some small spare parts:

  1. inner axle seal (can get mangled in a break)
  2. Inner axle inner snap ring (can get mangled in a break or if you need to remove)
  3. Inner axle outer circlip (may break on axle separation, especially with stock axles)

Aftermarket axles will not need the inner axle inner snap ring.

Identifying Used Aftermarket Axles

Good luck! Aftermarket Birfield designs evolved over the last few years quite a bit. There have been Newfields, Profields, Marfields, Longfields, and probably a few others. The earliest versions were certainly weaker than the current offerings. If you’re buying used, you need to do research. Ask for receipts from the seller and try to find out the original date of sale or manufacture. You can then contact the manufacturer to find out what version of the axle your seller is offering.

If you pull off the hub and see a 30 spline end it means...NOTHING! All axles except early Cruisers will have a 30 spline outer. This doesn't tell you anything much.

Most axles today are fairly visually different between manufacturers.

Serial numbers

If you can look at the axleshafts, they will usually be marked on the bell with a brand and may have a serial number. If you can contact the manufacturer, they should be able to tell you about the Birfield.

Removing the locking hub/axle flange

If you pull the locking hub/flange off a front axle assembly, you can see the end of the axle shaft. Current Longfields and RCV axles are gun-drilled and have a zerk fitting on the end. RCV axles are also black oxide treated (OR nickel-plated), but this might be hard to ID without something to compare to.

27 vs. 30 spline

With the full axleshaft out, you’ll be able to know the spline count just by looking at the inner axleshaft. Right before it enters the Birfield, a 27 spline shaft will taper down in diameter to be the proper size for 27 splines. A 30 spline axle has a consistent outer diameter along its entire length.

If you’re buying a complete axle assembly, there is almost no way to know whether you’re getting what the seller says short of pulling the axleshafts.

Measurements: Stock 27 Spline Toyota Mini Birfield vs. 30 Spline Mini Longfield

We took some measurements of stock and upgraded Birfields. Both would fit the same Hilux axle. Please note that we took some slight liberties with these numbers for illustrative purposes and assume that all measurements in the thousandths are within +/-0.003”.

Stock 27 Spline Toyota Mini Birfield vs. 30 Spline Mini Longfield
Stock 27 Spline Longfield 30 Spline
Birfield length 9 3/8" 9 1/2"
Inner axle length 15 5/16" 15 1/8"
Birfield bell outer diameter 3.550" 3.660"
Hub spline diameter 1.283" 1.283"
Shaft next to hub spline diameter 1.239" 1.283"
Inner axle inner spline diameter (at star) 1.158" 1.283"
Inner axle diameter at center 1.203" 1.283"
Inner axle diameter - before carrier splines 1.212" 1.283"
Carrier spline diameter 1.283" 1.283"