Why do Subaru Lug Studs Keep Breaking?
The Problem With Subaru Lug Nuts
Occasionally a lug nut gets stuck and the lug stud snaps off when you try to remove it. This is a normal part of auto repair. Sometimes it’s because the car came from somewhere with snow and salt. Or it might be because someone didn’t get the threads lined up right, but then went ahead and tightened the nut anyway. And then there are some in our profession that think “more is better” and apply this philosophy to lug nut torque, causing the lug nut to pressure weld itself to the stud.
What’s not normal is finding lots of studs breaking off. Brrrapp, brrrappp, brrrrrrrapp, crack, rrrrheeee goes the impact wrench as the stud snaps. Crap says the mechanic, but hey, this kind of stuff can happen, so he moves onto the next lug nut.
Brrrapp, brrrappp, brrrrrrrapp, crack, rrrrheeee. “What the hell is going on here!”, curses the mechanic. This isn’t normal and now he’s grumpy. The extra time needed to replace these studs will have a domino effect on the rest of his day and he may need to skip lunch or risk disappointing one or more of his customers. Some people thing that frozen lug studs are an indication that the last person to service the car probably shouldn’t be servicing cars. This may be true in some cases and may not in others.
Here’s the thing. It’s seeming like “bad mechanics” are targeting Subaru lug nuts disproportionally these days. When most of the broken lug nuts are on Subaru cars, one starts to think that maybe the car has some role in the failure.
What’s different about Subaru lug nuts and studs?
Nearly all Japanese cars use 12mm lug nuts and studs. This is the diameter of the threaded section of the stud. Subaru and Nissan do something a little different than the other manufacturers; they use a 1.25 thread pitch. All of the other Japanese manufacturers use a 1.5 thread pitch. Thread pitch is measurement of the peak to peak or valley to valley distance in millimeters. A Subaru stud has thread peaks that are 1.25mm apart from each other. A Toyota or Honda would have thread peaks that are 1.5mm apart.
In one way, the Subaru lug stud should be stronger
Nearly all threads are triangle shaped with a 60 degree angles. Therefore, if the thread peaks are further apart, the thread must be cut deeper into the 12mm rod. So this would mean that if you were to pull on both ends of a 12 x 1.25 stud until is snaps, it ought to take more force than a 12 x 1.5 stud. The thread root (valley) is deeper on a 1.5 stud, so in effect, it’s a thinner diameter rod and weaker.
So, a 12 x 1.25 stud should have more tensile strength than a 12 x 1.5 stud, yet there’s anecdotal evidence that they break more often. Why?
Tensile strength isn’t everything
While the 12 x 1.25 stud is stronger, the threads have some issues.
Metal filings are more common on Subaru lug studs
There are a couple of things I’ve noticed about Subaru lug threads. First, I frequently find metal filings on the stud after removal. This is far less common on 1.5 pitch studs. Bits of metal can “snowball”, and then create pressure and heat which might lead to galling. Generally I’ll just wipe studs off with a rag, not wanting to blow brake dust everywhere with compressed air. I don’t know why the filings are more common on 1.25 pitch lugs, but it may have something to do with the next issue.
Torque is “twisting force”. Nearly all fasteners have a torque specification, and on critical fasteners, mechanics use a torque wrench tighten a fastener to the manufacturer’s specification. Back in the 1980s and early 1990s, nobody measured torque on wheel nuts. In fact the impact wrenches we used back then weren’t all that powerful and it wasn’t uncommon to check all the lug nuts by hand after running them down with an impact wrench, because depending on the power of your impact wrench, the lug nuts might not have been tight enough.
Modern impact wrenches are very powerful. Many can snap a 12mm bolt easily. This became a problem as mechanics bought new guns and started overtightening lug nuts and causing damage to rims and rotors. There was a large industry movement re-educate the old-timers and teach the next generation. Now nearly all mechanics either use “torque sticks” (a device that’s attached to an impact gun to limit its output to a fixed rating) or a torque wrench.
None of this is specific to Subaru. All cars should have the wheel lug nuts torqued. What is specific to Subaru is the 1.25 thread pitch. If the same torque is applied to a 1.25 pitch fastener and a 1.5 pitch fastener, the 1.25 will be tighter, that is, it will have higher clamping force.
There are two issues with torque. First, mechanics may not look up the torque for every car. Second, Subaru’s recent torque recommendations seem a little strange to me, but I’ll allow they may have reasons beyond my comprehension.
Let’s start with the mechanics. In general, torque recommendations are based on 4 things.
- the diameter of the fastener
- the pitch of the fastener
- the strength of the fastener
- unique design situations and the materials being fastened
Since Japanese lug studs are almost all 12mm, and they all hold wheels on, and the ones that are marked are 10.9, the only real difference is the thread pitch. As you might imagine, the torque specifications are all pretty similar: 75 ft/lbs to 85 ft/lbs is pretty typical. Most of the time a mechanic will use a 80 ft/lb torque stick without checking the manual.
The reason I bring this up is that remember what I said earlier; a 1.25 thread tightened to 80 ft/lbs will have more clamping force than a 1.5 thread tightened to 80 ft/lbs. Could it be that 80 ft/lbs torque sticks are overtightening Subaru lug nuts to the point where they start to gall?
Subaru’s torque specifications
This might seem plausible if you look at Subaru’s old torque specification. For an example, an older 2005 Forester had a specification of 65.8 ft/lbs. Eighty is more than 65, so maybe 80 pound torque sticks are killing Subaru lug nuts and studs. That seems valid until you look at Subaru’s newer torque recommendations. For example, the specification for a 2013 Crosstrek is 88.5 ft/lbs. Keep in mind that both vehicles had the exact same lug stud, part number 28171 AJ000.
Why are the recommendations different? I don’t know. I’ve never considered wheel torque too critical. As long as its even, not so tight it does damage and not so loose that the wheel falls off, it’s probably OK. The main reason we torque lug nuts now is to prevent people from tightening them to 350 ft/lbs with an impact gun.
Could Subaru’s new higher torque specification be the problem? Maybe, but if you look at a standard bolt torque specification chart, a 12 x 1.25 10.9 bolt should be tightened to around 135 ft/lbs, so it doesn’t seem like 88 ft/lbs would be a problem.
Fine threads are easier to cross-thread
Starting a lug nut with a 1.25 pitch is more difficult than starting a lug nut with a 1.5 pitch. If a lug nut is cross-threaded, it will certainly gall and seize. Is this this issue? I doubt it.
The negative characteristics of fine threads
I think the most plausible answer can be found on boltscience.com . If you scroll down the page to the “What are the benefits of fine threaded fasteners over course threaded fasteners” section. They list a series of disadvantages of fine threads. Namely:
1. Fine threads are more susceptible to galling than coarse threads.
2. They need longer thread engagements and are more prone to damage and thread fouling.
3. They are also less suitable for high speed assembly since they are more likely to seize when being tightened.
Galling is the transfer of metal when two metal parts rub together. Frequently we end up running a thread cutting die over the lug stud and replacing the lug nut because globs of metal are stuck to the stud after removal. So that’s certainly true.
More prone to damage and thread fouling. This is matches up with the metal filings we find on the studs.
Less suitable for high speed assembly and more likely to seize when tightening. Impact wrenches are certainly high speed. I’ve had several mechanics report that the lug nut starts to bind on the way on so they’ve had to reverse the lug nut, clean the lug threads, and replace the lug nut. Now, if a mechanic doesn’t take the time to do this, and just runs the nut on even though it starts to get stiff, the next guy may need to snap the stud to get the wheel off.
Does this mean every stuck lug nut is the mechanic’s fault. Well, if he knowingly runs a stiff nut on, then sure. However, it may be impossible to notice galling every time it starts to occur and it’s entirely possible that a mechanic who’s competent and doing the best he can.
How to solve the Subaru lug nut problem
I briefly though about doing 1.5 conversions for anyone who wanted it, but it might cause problems when some poor guy orders a new lug nut based on the model. We could convert to ARP studs, which are much better that stock, but it would be prohibitively expensive. I think the best solution is the same solution people use for stainless steel fasteners, which are extremely prone to galling — lubrication. Anti-seize, oil, grease, whatever. Any of it will prevent galling and reduce wear.
Why doesn’t everybody already do this? Because we’re not supposed to. Lug nuts are meant to be installed dry, and that’s what the torque recommendations are based on. The clamping force of a lubricated fastener will higher than a dry fastener at the same torque. However, I’ve already mentioned that Subaru has issued specifications from 65 ft/lbs to 88 ft/lbs for the exact same part. I’m going to argue that it just isn’t that important.
It also may help to spin the nut slower during assembly. The trigger on an impact gun doesn’t need to be pulled all the way back. A little trigger finesse to reduce the RPM during assembly may help prevent galling. It can’t hurt and it doesn’t take significant additional time to do.
Replacing Subaru Lug Studs
Before a stud can be replaced, the brake caliper and rotor must be removed. Subaru lugs are pressed into the hub. The old stud is removed by hitting it with a hammer. A new stud is then set in place.
On this car, the stud can be removed and replaced without removing the hub, but that’s not always the case. Sometimes there’s no clearance behind the hub and the old stud can’t be removed and the new stud can’t be installed. When this happens, the whole knuckle must be removed and the hub must be pressed out. The pressing operation damages the wheel bearing, so it must be replaced as well.
There no way to press the new stud in when the hub is still on the car, so we pull the new stud into place instead. A stack of washers prevents galling on the hub and reduces friction during installation. We use a plain nut (not the lug nut) to pull the stud into position.
An impact wrench is used to pull the stud all the way into place. Once the splines are “pressed” into the hub, the lug stud cannot spin and the car is ready for reassembly.