Why do manufacturers use nonstandard bolt head types? Is it just to make it more difficult to the DIY-er to get away from visiting a dealership? It was already too much work to dig down to the cylinder heads on my Tacoma, just to find out that the head bolts are not regular hex heads. They look like triple square but I just learned they are actually not and are double hex instead. Why all of this seemingly unnecessary confusion ? Is there a practical reason to use something other than standard that most people have tools for?

enter image description here

  • 1
    As a side note, this website says that Allen head (or standard hex key) wrenches will work on the double-hex bolts. Commented Jun 29, 2015 at 16:38
  • have you tried, @Paulster2 ?
    – amphibient
    Commented Jun 29, 2015 at 16:41
  • I'm not sure that the site says that, @Paulster2
    – amphibient
    Commented Jun 29, 2015 at 16:43
  • I haven't ran into these personally, but would use the Allen wrenches (or Allen sockets) in a heartbeat to pull them out. It's the triple squares which would screw me over. Commented Jun 29, 2015 at 16:54
  • i fear stripping...
    – amphibient
    Commented Jun 29, 2015 at 17:06

5 Answers 5


From what I understand the answer to this very simple and logical.

When someone might be attempting to DIY a fix and have a particular socket that fits a nut, they may just turn it not realizing what loosening the bolt may do. As a result there are less accidentally loosened head bolts. If you are a DIY person, which I am, I don't have a real issue with the specialty bolt. If I don't have the tool already, I'll get get after I purchase the vehicle.

  • 1
    "accidentally loosened head bolts" - I don't think many people loosen an engine head just by accident. ;-) I think it's just making it slightly more difficult for the casual tinkerer - and slightly more expensive for the serious DIY person. These tools do not tend to come cheap, at least when first introduced to the market. Remember way back when the Torx started to appear? Now they are cheap, but not back then.
    Commented Jun 29, 2015 at 16:52
  • Indeed, much the same experience in a completely different field (computers).
    Commented Jun 30, 2015 at 5:45

Having had to work in tight spaces, I say it's very useful to have a bolt which you can turn in less than one-sixth turn. What I mean is, that bolt can be turned with a common hex tool, but sometimes you don't have the freedom of movement to rotate it 1/6, this way you can find the position that suits you best.

Hope I've been clear enough, sorry if I have not.


Those are engine head bolts and need to be torqued precisely, in order to last their useful life. Since they are located in an awkward location and have to be assembled in a factory, I suppose they are using double-hex heads to allow for greater torque, despite the complexity of having different tooling. Otherwise they would use simple outer hex bolts.

Such bolts should not be reused after removal as they stretch out a bit when properly torqued. If you reuse the bolts, they may fail sooner.


well it depends on several things :

1- the manufacture assembly arm and it's standards ( it may be fixed for some strict type of bolts) or the double hex bolts is easier for the machine to mesh with

2- the space around the bolt head

3- specific mechanical design ( stress , strain , stiffness , high Temp duty )

best regards,
I'm a specialized Mechanical design Engineer


These appear to be Torx bolts, and require Torx tools to remove and replace them


You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .