Reusable Expansion Bolts: Benefits & Risks

Hi!

Hello, what happens if you reuse expansion bolts?
For example, in the large idler pulley of a timing belt drive, can they break?
or become self-employed after a certain period, or experience events like being fired, laid off, or facing other difficulties.


Best regards, FREDY.

Hey!

As the name "DEHN screw" suggests, it is significantly stretched during installation. Furthermore, it is subjected not only to tensile forces but also to rotational movement. Because these screws are exposed to rougher mechanical loads, reinstallation is not recommended, as they can actually break. An almost 100% reliability can only be guaranteed with a brand new screw.

I've seen stripped head bolts that were reused once, but also some that were used multiple times and didn't cause any problems.

However, when it comes to the timing belt, saving money is not a good idea. If that particular screw were to break, the damage would be enormous and far more costly than replacing the screw itself.

Best regards, Peter.

Mostly, these are screws like the ones used for the cylinder head, which should be tightened to a specific torque. If you tighten them again with the same torque, it's no longer guaranteed that the cylinder head will have the correct pressure, which can lead to leaks and cause a lot of work. I haven't heard of any stretch bolts breaking. At most, they might loosen because the proper clamping force is no longer being applied.

Best regards,

Hello,

Besides the increased risk of failure, there is another problem. Expansion bolts expand (as their name suggests) beyond the elastic limit. They are longer after use.
For example, when using this method for installing cylinder heads with pre-drilled holes ("blind bore"), the second time it's used, it may not provide the necessary clamping force.

You might be able to shorten it further for emergency repairs.

Best regards,
Christian.

'Those of us who are a bit older will likely remember the frustrating topic of 'trailing'.'
Cylinder head bolts for new vehicles and after cylinder head/cylinder head gasket replacement.
The change may still be remembered. The use of expansion bolts...
It should avoid doing that, and it does.
I once spent half a day in Switzerland sometime in the 1970s.
I've been searching for a workshop that can replace my head bolts.
follows a Fiat 128, as it requires extremely curved tools.
I needed it and I got a new laptop one day before the vacation.
'I had it. By the way, this was one of the few times I've been in a workshop.'
Otherwise, I usually do almost everything myself!
The reuse of expansion bolts is strongly discouraged.
I would advise against it, unless you have so much money that you can afford to invest in something like ZKD or even more.
I want to invest in ATM machines. If the screws are once stretched (or strained).
they would no longer have their full [function/capacity/effect] upon subsequent use.
Durability. At least the cylinder head bolts are available as...
Reasonable prices on parts!

Hello,

Expansion bolts have a long, thin shaft so that the shaft itself expands and contracts during operation due to temperature fluctuations, rather than the area with the threads. They expand within the range of Hooke's Law (i.e., in the ELASTIC region). Therefore, expansion bolts (such as connecting rod bolts or the ZK bolts previously used in my BMW 02) can be reused without concern.
The multi-head screws used in VAG cylinder heads, while they are naturally also stretched, are NOT stretch bolts, even though you often read that (people just repeat what others say). They are pre-tensioned into the yield strength range during tightening, which means they undergo permanent deformation and should therefore not be reused. Because the plastic deformation range can be relatively wide depending on the steel grade, it might work a second time, but I wouldn't do it.

What I've noticed is that the screws on the diesel heads that I've disassembled so far were very easy to loosen; there's no comparison to the brute force needed to tighten them. The screw is plastically deformed and, due to the constant swelling stress over time, loses its preload. It seems to be one of the common practices used by VW that eventually forces customers into the workshop .


Best regards, Uli.

Uli S. wrote:

What I've noticed: The screws on the diesel injectors that I have disassembled so far were very easy to loosen; there is no comparison to the brute force needed to tighten them. The screw is plastically deformed and, due to the constant swelling stress over time, loses its preload. It seems to be one of the measures commonly used by VW that eventually forces customers into the workshop.

Hi Uli,

This would mean that tightening the head bolts (ZK screws) approximately every ??? engine cycles prevents unnecessary head gasket repairs.
(until the first screw breaks off ).

Hello everyone,

Dynamically loaded clamping connections (e.g., cylinder head to engine block) are preferably designed such that the screw is softer than the clamped components.
This ensures that the preload force (clamping force) of the components changes only slightly due to settling and/or thermal expansion.
In the torque-controlled tightening process, the actual achieved preload of the screw varies within a rather large range, meaning that the spread of achievable preload values is quite significant for the same applied torque.
This is due to various boundary conditions, such as the thread fit, the surface condition of the thread flanks and the bearing surface of the head, the lubrication state of the thread, etc.
In the tension bolts used in the ZK system, the uncertainty of the bolt preload is minimized by tightening the bolts at least to the yield strength or slightly beyond. Depending on the specific conditions, the bolts are tightened either just to the yield strength or slightly above it, up to the point where plastic deformation begins.
When the preload force exceeds the yield strength, the material begins to deform plastically, and the preload force essentially no longer increases (even with further extension of the screw).
This technique achieves a significant reduction in the wide scattering range typically observed with torque-controlled tightening processes, ensuring that defined preload forces can always be generated.

This also suggests that retightening stretch bolts is a game of chance that should be avoided.
Since it is unknown to what extent the screw has already been plastically deformed (elongated), it is also impossible to determine whether further elongation would be dangerous or not.

If a stretch bolt was tightened only slightly beyond its yield point, it could theoretically be tightened further without the risk of breakage. However, in the other case, even a slight additional tightening could lead to premature failure of the bolt.

Conclusion: Always use new screws!

Regards,
Alex.