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bolts sheared off of 2005

2K views 12 replies 7 participants last post by  idosubaru 
#1 ·
hello, it's been a while since i posted, but felt this was important to share:

while driving from live oak, florida to columbia, sc, my left rear wheel came off. luckily (for us), there was stopped traffic ahead, and we diverted to a small highway. on that road, i started hearing a rumbling sound, i stopped to check the tires and they all looked good. a few more miles and off it came. i have the 3.0 vdc limited w/ alloy wheels.

the shop i use replaced the hub, and not just the bolts. a knowledgeable friend said they _could_ have replace the bolts at less expense, but he felt there was a liability issue with that.

the same friend used to work in a machine shop, he said he was certain that the cause of the shearing was over torque on the nuts, and i was a bit more over-zealous at rotations than i needed to (about every 5k mi). i take the car to a local tire shop, and they say they over-tighten the nuts. and they swear by it.

the friend suggested i get the bolts measured to see if the over tightening pulled them out bit by bit, thus causing it to wiggle just a bit and then shear.

if they have lengthened (due to over tightening), can't i just have them replaced,a and not buy new hubs?
 
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#3 ·
thanks, i'll read through this. this is confusing: my knowledgable friend said over tightening caused it, and the shop i use said they intentionally over-tighten (so much so that people complain they can't loosen the nuts). they said they'd had more problems with using torque wrenches.

vs: group consensus by also knowledgable people that under-tightening is the culprit, made worse by mis-information from subaru. whew.
 
#4 ·
Both are legitimate causes for the same failure mode.

Taken as an average across every car on the road, overtorque is probably the more common cause.

Taken in the specific context of 2005 Subaru Outbacks, undertorque is definitely the more common cause.

This doesn't definitively determine which caused your problem. You'll probably never know. The cost of analysis is high.

Moving forward, you've already got the information you need: You know you can't trust the error published in the old service manual, and you've got good advice from your local friend. There is a valid solution between the two.
 
#8 ·
Mostly unrelated but still funny anecdote:

When I was in college, for a semester I had an apartment-mate nicknamed "Jock" for good reason. He just got a '70 Plymouth Barracuda (give or take a few model years - it's been a while) and was trying to take off one of the driver's side wheels and cursing up a storm by the time I got back home. He couldn't get any of the lug nuts to loosen and had already sheared off a couple of the studs. "Hey, Jock... see this 'L' stamped into the end of the lug?" "Yeah?" "That means it's a left-hand thread." "So?" "Left-hand threads are the reverse of normal. Turn them clockwise to loosen them."

MoPar cars of the day used left-hand threads on the left side because normal accelerations would tend to tighten the lug nuts. He was using a four-way lug wrench, by hand, and shearing the studs off on the spot!
 
#9 ·
There's a very rare issue where rear rotors cause the lug nuts to shear off on 00-04 outbacks. Might be worth considering replacing the rotor.

Overtorquing is not a big deal.
Yes they can shear off if they're overtorqued.
It's possible for two people to be correct (or both wrong), by holding either view dogmatically.

"overtorquing is okay" and "overtorquing shears off bolts". both are true...or both can be false due to the ambiguity of "overtorque".
 
#11 ·
I also torque lug nuts by feel. Not a problem yet. I don't feel like I come even close to "breaking studs" torque. Two ways I can see that happening: 1) Impact wrench. I have a friend who torques all lug bolts and nuts with an impact, and he does a lot of work. Said he's never had an issue. But if you have a powerful wrench that's set too high, you're gonna have a bad day. 2) Greased lug studs (allowing for more tension at a given torque). Grease is good for lots of things. Not lug studs.

I'd put my money on undertorque causing shearing, but we may never know.
 
#13 ·
*** Actually LOOSE lugnuts are waaaaaaaayyyyyy more likely than too tight. I've seen loose lugnuts countless times and i've never seen issues with overtorqued unless they shear the stud off immediately.

I've seen lots of loose lugnuts - one or more are loose and eventually cause enough play to work the other ones loose within a few days or week.

Most tire shops have a disclaimer on their paper work "Aluminum rims require a retorque after XYZ miles".
 
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