400 - “elasticity” in headnuts

[Erwin83]

For lack of another word...
My CB466f turned quite smokey recently. I determined that the head nuts (especially the 4 surrounding cil2, the one smoking) were not torqued down anymore. So either they came loose, or the head came down during break in, and I was too late re-torqueing them.

I replaced the head gasket yesterday and found something that I find strange:
I torqued all nuts in proper order to 20 Nm, next to 25 Nm, finally to 30 Nm.
Now, even after applying 30 Nm, I can still twist the nuts a little if I apply less than 30 Nm of force. You can see the nut AND the stud itself turn a little. They will come back to the original position however. Hence the “elasticity”: the stud twists a little.
This happens on pretty much all head nuts.

Is it normal?

[Nurse Julie]

30Nm is approx 20 ft/lb, which sounds very high. Did you remove the head studs as part of the rebuild?, Have you stripped the threads on the bottom of the studs? have you got the correct size copper washers in place, because if they are too thin, the nut can come down too far on the thread and not hold. Are they standard head studs or stainless steel?

[Erwin83]

16 ftlb is the spec, so I overshot it a bit with 30Nm. Still the flexibility is weird to me.

Nope, studs have not been removed, hence standard stuff.
I have the correct washers 10 thick copper ones, and 2 with the rubber insert.

[Nurse Julie]

16 ftlb is the spec, so I overshot it a bit with 30Nm. Still the flexibility is weird to me.

Nope, studs have not been removed, hence standard stuff.
I have the correct washers 10 thick copper ones, and 2 with the rubber insert.

🤔🤔🤔 As you say, the flexibility is very strange and anything that flexes can't really give a good hold. Unless by tightening to 30Nm has stretched the studs a bit, dont forget the equivalent of 16ft/lb is the absolute maximum torque. I think mine are done at about 12-14 ft/lb.

[Erwin83]

By the way, the spec I found was 16 ftlb for the 350, 18 ftlb for the 400f
So I’m still a little high, but not as extreme.

I guess I’ll leave it for now, let the bike run a couple heat cycles and than do the trick with quarter turn loose and retorque.

[Bryanj]

If the studs had gone past their elastic point they would keep on stretching till they bfoke. The torsional load is what you are really talking about and they are acting a bit like 450 twin valve torsion bars. Did you put oil/lube under the washers/nuts if so you shouldnt have as book doesnt say so and it will alter the torque

[K2-K6]

Agree with above regarding torque max, I've got 30nm as 22.16 ft lb which is quite significantly over spec.

But feel that they should turn as you experience (although mostly that's not generally tried) I think most would if attempted.

The reason for using the washers they have is partly to seal them,  but also that the torque to turn the nut against a washer is lower than that to tighten the nut on the threads else you'd be measuring friction torque at the washer nut interface instead of between the two threads.

As the nut is tightened it's threads compress,  making the thread pitch smaller. But the stud threads are being stretched longer,  the mismatch as it accumulates causes the torque required to keep turning it to rise.  That final figure is arranged to give the correct tensile load on the stud structure.

But as Bryan says,  raise the torqued level too high and you'll risk permanently stretching the stud.

Probably ok to do as you plan to run and reset,  it'll let you know one way or another.

[Erwin83]

If the studs had gone past their elastic point they would keep on stretching till they bfoke. The torsional load is what you are really talking about and they are acting a bit like 450 twin valve torsion bars. Did you put oil/lube under the washers/nuts if so you shouldnt have as book doesnt say so and it will alter the torque

I haven’t lubed anything, but the area is a bit greasy with engine oil.
A torsion bar is exactly what it seems like.

[Erwin83]

Agree with above regarding torque max, I've got 30nm as 22.16 ft lb which is quite significantly over spec.

But feel that they should turn as you experience (although mostly that's not generally tried) I think most would if attempted.

The reason for using the washers they have is partly to seal them,  but also that the torque to turn the nut against a washer is lower than that to tighten the nut on the threads else you'd be measuring friction torque at the washer nut interface instead of between the two threads.

As the nut is tightened it's threads compress,  making the thread pitch smaller. But the stud threads are being stretched longer,  the mismatch as it accumulates causes the torque required to keep turning it to rise.  That final figure is arranged to give the correct tensile load on the stud structure.

But as Bryan says,  raise the torqued level too high and you'll risk permanently stretching the stud.

Probably ok to do as you plan to run and reset,  it'll let you know one way or another.

Thanks. This makes a lot of sense. I know of (one-time use) head bolts that are stretched to a certain lenght by torqueing and turning them, to get that specific load on the head.
The studs obviously work the same, but I’ve never actually seen it like this.
By the way, going from 25 Nm to 30 Nm is not even 1/8 of a turn of the nut. The ‘stretching’ of the stud is therefore pretty minimal, should that come into play.


Anyways, the bike started right up this evening. I let it idle a bit and took her for a little ride.
Smoke and burned oil smell seems to be gone. 

[Rob62]

I just don’t get why you would torque the bolts in the “correct order” but to the wrong spec? If you’re going to use a torque wrench surely you need to check what value to set it to....otherwise why bother, you might just as well use any old spanner....

[royhall]

We're back to this oil don't oil the threads debate. If you did a full rebuild the threads would be dry on assembly, if you did the head on a bike that's run all the threads will be oiled and there's not much you can do to remove it. What's the correct torque for oiled threads because I suspect we are all doing it wrong on repairs?

[Erwin83]

I just don’t get why you would torque the bolts in the “correct order” but to the wrong spec? If you’re going to use a torque wrench surely you need to check what value to set it to....otherwise why bother, you might just as well use any old spanner....

You never make mistakes?
I only have a workshop manual of a 350f, so for the 400 I searched and found a torque spec of 29 Nm. Must have been the wrong bike or the wrong source. 

[royhall]

Your not the only one, I make mistakes constantly. Sometimes repeating the same mistakes over and over. It's the price I pay for getting older, just have to check and double check everything I do. Mistakes happen to everybody at some point.

[Bryanj]

An old bosses saying was

" Him what don't make a mistake don't do bugger all"

[K2-K6]

"The studs obviously work the same, but I’ve never actually seen it like this.
By the way, going from 25 Nm to 30 Nm is not even 1/8 of a turn of the nut. The ‘stretching’ of the stud is therefore pretty minimal, should that come into play."

It's difficult to feel any changes this small, but if you pull this type of thing on a test rig they start a very benign stretch first,  followed by a big bang as failure comes fast, you just can't separate the phases as its such a small dimensional change.

It's that angle change multiplied by the thread pitch that gives more elongation as the stud top travels away from it's base fixing point.  The change is distributed over the entire length though (as Honda designed it properly with waisted central section) but we don't know a yeald figure at which point the material stretches beyond elasticity, making it a guess as to how close you got. 

I'd try them again after running for a while exactly as you've done now "see if they turn at the correct maximum torque rating" and not release/ reset them. If they are still holding torque ok (you can't separate gasket squish from torque release to clearly see) then they should be ok to leave alone.