Over torqued head nuts

[Iansparky]

hi all
I did something really stupid today, I set my torque wrench to 85 lbs instead of 58 and proceeded to re torque my cylinder head nuts, I realised almost straight away and backed them off, then re torqued to the correct value, arrrr what an idiot,
I have come to the conclusion that I will need to pull the head and replace the gasket but what about the studs, will I have stretched them, being a series 1 4.2 it's the short stud block so there should be no corrosion in the water jacket.
Many thanks ian

[abowie]

Are the head studs new or used?

My (entirely uneducated) view would be that if the studs were new I think they're fine to use. If used I'd replace them.

I would also guess that the head gasket is also fine, but given the relative ease of replacing it now rather than after it leaks I'd replace it now.

I look forward to seeing what other, more educated members think.

[Airborne62]

I would torque them back to 75 and call it a day.
Metal deforms, that doesn't mean it yielded. You have nothing to lose trying.
Just an opinion.

[lowact]

I’d go one step less, leave as is, 58 lb-ft, wouldn’t even change the head gasket.

By torquing to 85 lb-ft you have proven your studs are still good and strong. You will not have stretched them, it would take a lot more than that. In any case, believe it or not, it wouldn’t matter if you had. Stretching beyond the elastic limit makes them longer, doesn’t make them weaker. The criteria is not how long the bolts are, it’s what torque they will hold.

Over-torquing the bolts may compress the head gasket a bit more. Again this doesn’t matter, the criteria is not how thin the gasket is, it is the torque that is compressing/sealing. Note good practice is in any case to re-torque head bolts after waiting a few days for the gaskets to have permanently compressed.

[mtnjag]

If you are within torque limits of the stud size no problem.
If you are well outside the limits then I believe you have compromised the metallurgy and cannot depend on the stud to hold torque. It will loosen. In that case you should replace.
I forget, what are they 7/16" or 1/2" fine, grade 8?
Here's a chart.

https://www.google.com/url?sa=t&source= ... zCbCiMOmi0

[Series1 Stu]

Hi Ian

Some of these replies are quite worrying.

If you were paying me for professional advice then I would say replace the studs and the nuts (and, of course, the gasket). The only way I could possibly advise otherwise is if you could furnish me with all the data relating to the studs and nuts such that I could produce the necessary calculations. The truth is, there are just too many unknowns to be able to give you a definitive answer. What is the stud size, material and grade? How long have they been in service? How much corrosion do they have? Apply the same questions to the nuts too. Were they lubricated?

What we do know is that you have applied excessive torque which means that you have set up excessive stresses in the studs and threads. These are beyond Jaguar design limits and remember that these stresses will probably increase as engine temperature increases and materials expand.

When it comes to stretching the studs, you can only assume that you have exceeded the elastic limit of the material which means it has yielded. Some modern engines use torque-to-yield bolts but these MUST always be replaced if they are removed. This is a clue for us. If your studs have yielded then they should be replaced. We can't say definitively that they have yielded but neither can we say that they haven't.

It isn't an easy subject but Jaguar specified this torque value for good reasons, ignoring it would be folly and could become an expensive problem further down the line.

So, for peace of mind, replace the studs (hopefully, the threads in the engine block are still in good shape) and the nuts and the gasket. It's frustrating that mental aberrations can cause such problems but we just have to take it on the chin and put it right.

[bitsobrits]

[lowact]

Crazy waste to change these studs imo, if they are otherwise in good condition ...
By my calcs 58 lb.ft torque will load a slightly lubricated (K=1.5) 7/16 unf grade 8 stud to 70% of its nominal yield strength (896 MPa).
So 85 lb.ft would have momentarily caused 102% of yield strength. So what? Note, because Ian reduced the torque back to 58 lb.ft the studs are now NOT overstressed. Maybe (?) microscopically longer and harder but not overstressed. They will also not be any weaker, this does not happen at less than UTS (1034 MPa)? I’m curious so tomorrow I’ll get me a nice new bolt and see what it takes to stretch and weaken it …

[bitsobrits]

A permanent elongation of the stud would be accompanied by a reduction in diameter, however slight. But how did that reduction manifest itself? A consistent reduction along the entire shaft? Near the center of the stud? A reduction of the root diameter of one of the threads? In several places along the stud? Who knows? You don't. No one does, and that's the point. Taking a chance with critical fasteners that have been definitively and massively stressed (147%!) beyond their specified load is just foolish.

[mtnjag]

In Torque to Yield bolts the bolt is used once and discarded.
I don't believe Jag specified torque to yield specs or the fasteners would all be discarded after one use and this is not specified. When a bolt is Torque to Yield it is specified as such in a Factory Manual.
if he Torqued to Yield and the fastener was designed for that torque and he were able to maintain the torque he's fine.
But whether he Over Torqued it by a significant degree and stressed the bolt beyond spec or
Torqued to Yield and then backed off torque, to me the prudent action is to replace.

[lowact]

Corrected my post, I'd previously used the wrong values for yield and ultimate stresses for bolts. So Jaguars spec is only 70% of yield, i.e. not "torque-to-yield". As I added, crazy waste to replace these studs, imo.

[tim wood]

If you are going to change your studs then I have some brand new purchased in error never used !

If interested let me know
Tim

( may ease the pain of having to pull the head once more !)

Tim

[Series1 Stu]

Corrected my post, I'd previously used the wrong values for yield and ultimate stresses for bolts. So Jaguars spec is only 70% of yield, i.e. not "torque-to-yield". As I added, crazy waste to replace these studs, imo.

My point is that we don't know the spec of the studs and the nuts so we can't predict what effect the over tightening may have had. Anything else is just potentially dangerous speculation. If we can't be certain about these things then we should err on the safe side and advise accordingly.

102% of yield strength IS an over tightening. The fact that it was quickly released does not mean that no harm was done - it was taken beyond its yield point and into its plastic zone.

Layne, you have misunderstood me. I never suggested that these were torque to yield - why would you do that with studs? What I said was that, if the studs had yielded then they should be replaced, as is the case with torque to yield bolts.

[ysmalkie]

So, we are talking about about an expense of about 250 GBP. What happens if one or more snap later on? What is the possible cost of repair?

[Iansparky]

Thanks for your input, I have decided to replace the gasket and the studs just to be sure, it also gives me a chance to Inspect the head and bores, supposedly the engine was rebuilt 1000 miles ago by the previous owner, incidentally does anybody know how i can tell if the head has been previously skimmed and by how much
Thanks ian

[mtnjag]

Series1 stu, Actually agreed with your post.
I was actually commenting on the post by lowact in #8. He he has since edited #8 and noted so in #11.
I should have done a quote or at least referenced it.