SOHC.co.uk Forums > CB350/400

400 - “elasticity” in headnuts

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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:

--- Quote from: Erwin83 on July 08, 2020, 07:35:05 PM ---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.

--- End quote ---
🤔🤔🤔 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.

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