SOHC.co.uk Forums > CB350/400
Head + Rocker cover torques...
Bryanj:
Thanks Julie, Teri, my beloved wont have it any other way!!
Laverda Dave:
Good news Bryan and thanks for the update.
A friend of mine had a modern MG (the one with the K series engine). Like all these engines the head gasket blew. The garage changed the gasket but not the stretch bolts, the gasket blew again a coupe of weeks later. To do the job properly cost £1,100!
K2-K6:
That is good news Bryan, hopefully after the "rebore" you'll get no more trouble :)
The technical points:- as you say Dave, the bolts are a real problem if corrosion gets into the threads. The zinc plating is the part that protects them originally, but as we all know when exposed to salty winter roads it will strip that off over time. The zinc is sacrificial so that when you see them going white in early stages of their life you are observing that process in operation. Once gone though, it starts on the components to give that siezed stage that can destroy the parts. Using something to avoid this is sensible as you do with coppaslip, just have to be aware of the effect on measured torque.
Bryan picked up on scale with things like this. it seems to trip up the unaware in not acknowledging the very small scale that is involved, many just don't believe that they are working with such small numbers, for both tolerancing and torque settings.
To elaborate on the cam cover bolts Simon, yes it should be true that if never over torqued they should be ok for most eventuality. They have more to them than is generally appreciated I believe. I mentioned earlier about the torque on the threads, what Honda have done on these though is to design in a wide head/flange on the bolt. The increase in the surface area is the principal element in how the torque is built up. If tightened correctly it'll reach the measured specification just from this surface and not overstress the thread area. As mentioned, the joint face is not compressed like a gasket, so once the two metal components come into contact that is job done, the bolts just need the torque setting to stop them backing out during use.
It's one of those assemblies in which some very subtle elements combine to give an integrated whole component.
Retightening head bolts, it's one of those things that I don't think can be answered definitely as to method, unless the original manufacture gives a prescribed routine.
Faced with trying to retourque them means it's already compromised the gasket to some extent, else it would not be required. Agree with Bryan in releasing first then tightening again to get a correct reading.
I'd do them one at a time so that the head casting is not released and in the original pattern of engine build if doing them all.
It's one of those things that you'd hope to get it to seal, but may have to accept it needs replacement if not effective.
That K-series engine Dave, as far as I remember they have bolts that go right through the whole assembly and hold crank main, pass through block, then the head to squeeze the whole assembly. It's quite a clever design, but like many things considered in that way, sometimes mis-understood.
They are definitely "stretched" bolts, in that they are pulled during tightening past elasticity into primary yeald to set them correctly and need replacing if taken apart.
Great irony of those engines is it's not this construction that causes the fault in the first place, but often gets blamed. The overheating usually originates with failure of water system sealing to the inlet manifold as I understand it. They run a fairly small coolant volume and will readily overheat because of this problem, by which time the head gasket is compromised.
Laverda Dave:
Great explanation Nigel.
Yes, the K series was known as 'ladder construction' I believe. I remember Suzuki made a big thing about it a few years ago with the GSXR engine even though most motorcycle engines are built using this method!
I've just bought a new Sealey 1/4 torque wrench as I think my SP one is out of calibration after 25 years!
Bryanj:
All you need to check calibration is a vice a rule and a spring balance-------and before sombody tries to correct me there is only one ruler in this country and she lives in a big house at the end of The Mall in London!!
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