SOHC.co.uk Forums > CB350/400

400 - “elasticity” in headnuts

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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:

--- Quote from: Bryanj on July 08, 2020, 08:57:39 PM ---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

--- End quote ---

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:

--- Quote from: K2-K6 on July 08, 2020, 09:34:09 PM ---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.

--- End quote ---

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.  8)

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.... ::)

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