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I'm working on a VW Jetta and have pulled one of the suspension ball joints to replace a drive shaft. The three bolts holding the ball joint to the lower control arm are marked in the Bentley service manual as "always replace."

In the videos and online instructions that I've found for replacing the drive shaft there is no mention of replacing the bolts (they tend to wrap up with "put it all back together and torque to spec" or something similar).

After removing the ball joint, the alignment needs to be checked/re-set, so one question I have is would I expect the alignment shop to also replace those bolts so that its not necessary or economical to replace the bolts before alignment.

The other question I'd love an answer to is why are the bolts "always replace." Does the torquing process (20 Nm + 90º) weaken the bolt, or is it not reproducible? Or something else altogether?

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I cannot tell you directly if the bolts you are using are Torque to Yield (TTY or T2Y) bolts, but if Bently says to replace them, you bet I'd do it. What are three bolts in comparison to the well being of your family and yourself, not to mention those around you should any of these bolts fail?

As for T2Y bolts, here is what Fel-Pro says about them:

T-T-Y head bolts are engineered to stretch within a controlled yield zone. Once they reach this zone, they are designed to spring back to provide a more precise level of clamping force. This stretches the bolts into their elastic range, and in some cases, the stretching approaches the bolts’ elastic limit, permanently stretching it. Once the yield zone is reached, the clamping force will be more consistent. Getting the to the precise yield zone (for maximum clamping force) is accomplished by tightening bolts to a certain torque spec, then turning the bolts an additional number of degrees.

It goes on to say:

Since T-T-Y bolts are designed to stretch, reusing them can cause improper or uneven torque and clamping force. Stretched bolts can damage threads in the engine, especially on aluminum blocks, and since the bolts are weakened, they may break if retorqued. You should always replace the bolts.

And yet another paragraph:

It is very important to follow the proper torquing sequence and specifications when installing T-T-Y head bolts. Always clean the threads where the bolts pass through, any thread damage, corrosion or rust will create excessive friction giving you a false torque reading, robbing you of valuable clamping force. Using engine oil, lightly oil the threads and under the heads/washers on T-T-Y bolts unless otherwise specified by the vehicle service manual. Clean, oiled threads prevent binding, allowing for accurate and consistent torquing. Be careful not to over-oil the bolts, especially if they are threading into a blind hole. Too much oil will hydrolock the bolt and give false torque readings.

  • 2
    The bolts are definitely getting replaced, I was hoping to learn two things: 1) was there a sensible reason that the procedures didn't strongly urge replacing the bolts (or include them in the list of necessary parts), it seemed like one reason would be that they would have to be replaced as part of the alignment as well so there wasn't a need to replace them right away; 2) the reason why some bolts are considered "always replace." I really appreciate you taking the time to add the information and link from Fel-Pro. – dlu Jul 3 '16 at 19:20
  • It's really important to replace the T2Y bolts. If you are worried about the waste, if you put the old ones back in and take a set of new ones in for the shop to replace when you get the alignment done, I'd doubt you'd have any issues. It's just don't create an issue for yourself in the process. – Pᴀᴜʟsᴛᴇʀ2 Jul 3 '16 at 19:38
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The other question I'd love an answer to is why are the bolts "always replace." Does the torquing process (20 Nm + 90º) weaken the bolt, or is it not reproducible? Or something else altogether?

For a conventional bolt, ignoring the torque required to overcome friction when you tighten it, there is a linear relation between the amount of torque and the clamping load in the bolt. If you double the torque, you get double the clamping load. But the actual clamping load is not very well defined, because the amount of friction is unknown - it depends on the surface condition of the parts, any lubrication applied to the parts, etc.

T2Y bolts work in a different way to give an accurately controlled clamping load. When you tighten the bolt, it is deliberately overstressed so that the bolt permanently stretches. For this to work properly, it depends on two things: you need to apply enough torque to permanently stretch the bolt, but not so much that you over-stretch it, which would weaken it or even break it.

The specified torque load (20 Nm for your bolt) is enough to get "close" to permanently stretching it, even if there is some friction in the joint. Actually the torque level is not too critical - the main reason for specifying a value is to make sure that you have taken up all the slack in the joint before the next step.

The second part of the operation, turning the bolt through a specified angle (90 degrees) is the critical part that ensures it really is stretched - but not by too much, because the maximum stretch can't be more than 1/4 of the thread pitch, if you turn the bolt by a 1/4 of a turn.

If it is installed properly, the clamping load will depend only on the diameter of the bolt and the material it is made from, and not on uncontrolled factors like friction.

The stretch is permanent - if you measure the length of the bolt, install it and remove it, and measure it again, it will be longer than before.

If you re-use the bolt following the same torqueing procedure, you will stretch it by another quarter of a thread pitch - i.e. the total stretch will be twice as much as it was designed for. That may not have much effect on the clamping load (indeed it might increase it rather than decrease it), but the stretching changes the structure of the material in the bolt, and it will be more likely to crack and/or break.

In fact, stretching the bolt with a 90 degree turn, releasing the stress, and then stretching it again would be likely to cause more damage than one continuous 180 degree turn.

So the take-home message is: these bolts are for single-use only!

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The actual reason to replace the bolts isn't specifically the bolts, it's the nuts. These are self locking (i.e. Nyloc) style nuts and after they've been removed and refitted, you can't be sure that they'll self-lock effectively.

When purchasing a new balljoint, replacements for these are almost always provided. Your local dealership should charge you no more than a few pence for these so I personally wouldn't risk their re-use.

  • Somebody edited with something that belongs to a comment: "The above statements are wrong. This has nothing to do with reusing nylon insert lock nuts. The issue is the torque to yield bolts, which are designed to stretch when correctly torqued. They are one-time-use-only bolts and replacement is recommended by the manufacturer. (I did it this way because I do not have appropriate status on this site to make a comment. But this needed to be corrected so future users aren't confused.)" -- I rejected the edit. – juhist Sep 13 '17 at 16:35
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So it sounds like VW uses three different kinds of "always replace" bolts:

  1. Torque-to-yield (TTY or T2Y) bolts which must be replaced because they are engineered for one-time use. Reuse of these is risking failure,
  2. Bolts treated for corrosion resistance (often with a green color), these are being replaced to ensure that the corrosion resistance is not compromised, and
  3. Bolts that are treated with an anti-loosening coating (like LockTite).

I suspect that there may also be bolts that are combinations of these three. I suppose if you know for sure which of these categories your bolts fall into you could make an informed decision about 2 and 3 (how large the risk of corrosion is on 2, and using a thread locker on 3), but that is hard to know and VW doesn't differentiate. All in all it seems wiser to take them at their word. In my case there is a reasonably local (one day by UPS Ground) source for the bolts and they cost $5.00 for the set. That seems like a small price to pay for the security in knowing that the bolts will be right. Since the ball joint bolts don't have the obvious reduced shank that many TTY bolts have and since mine had a green coating, it may be that the writers and video makers who didn't recommend replacing them weren't completely irresponsible…

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