SOHC.co.uk Forums > CB350/400

Head + Rocker cover torques...

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Lobo:
Guess one for the builders here, Gents...
Slight oil weep on the ‘400, emanating within the fins / RHS. The engine was rebuilt about 500miles ago, and so figured I’d simply re-torque the head.
Which is what I’ve now done...
Have to say, and using ‘Clarkson parlance, “poo was coming out” as I slackened, and re-torqued each to the book figure (15 ft-lb). Fingers crossed the slight weep has been remedied.
The Rocker cover however, was a different matter, and I noted at least 3 thread inserts already in place. And whilst I couldn’t bring myself to use the book figure this time (6 ft-lb)... bloody #12 (the last bolt!) failed to tighten... even to my calibrated elbow.. and well below 5ft-lbs.
So... that thread has an insert now, and job fettled... til next time.
Q - any comments here wrt using book figures on 40 yo engines which may have indeed been apart more than once.  And if so... given how soft these heads are... what are you typically torquing to? There seems to be a finite number of times these motors can be taken apart and put together!

Bryanj:
Never used torque on the 6mm bolts just "feel" but all critical bolts(head,rods,mains et) need to be torqued and i always use the highest number and never had a problem

Harold 400/4:
Lobo. I have exactly the same issue with my 400/4. Once the rocker cover is off, can you get at all the bolts to tighten down without dismantling camshaft etc etc? I was planning just to torque up, but see you slackened off before torquing? What is the reason for this?? Many thanks
H

Lobo:
Hi Harold,

Yep, you can easily access all 12 bolts to torque the cylinder head. Spark plugs 2 & 3 will need to be removed, as will a couple of oil spray bars. (They just lift out). You might want to stuff a lint free rag abouts the cam chain sprocket tunnel area, as anything dropping down there will surely spoil your day.

Note there’s a gradual tightening sequence up to full torque... basically working from the centre of the head (and indeed rocker cover) out.

Didn’t see it written anywhere.. but you probably want to consider loosening all the tappet adjusters to allow the rocker cover to naturally fit onto the head prior to buttoning it all back up.

You’re asking the wrong person here as to whether you need to ‘slacken’ each head bolt first - my logic was that ‘stiction’ would possibly screw the process, and that a gentle continuos tightening to required torque would give a more accurate setting.

Thanks for the comment Bryan... fits in with my gut feeling given the head’s made of something akin to butter...

 Btw - took it out for a run this arvo... and so far all looking good. Phew..

K2-K6:
Reference the bolt/fixing integrity for rocker cover.

From an engineering point of view I don't feel that they wear out, as such.  It's the case that the steel bolts can very easily exceed any torque aluminium castings can tolerate.

In tightening them,  if the torque used succeeds in fracturing the threads ( in essence detaching the thread form from its root anchored to the surrounding metal)  then they are scrap from that point onwards. It doesn't matter if it's the first, or many times its been used.

That joint is unusual in how it's viewed from a technical point.  Using its two faces to provide the tolerancing for cam bearings,  it only has to be tightened until the surfaces touch by which time the o-ring shouod have long been compressed into its sealing function. The remainder of tightening after this point is just to provide a "shakeproof" torque on the bolts. It really doesn't need much to do this.

There's anther factor that's not really considered,  which is lubrication of the bolts and the affect it has on torque rating. I think Bryan made this observation about whether to lubricate the thread's some years ago (that was in relation to head studs, that I remember).
As I understand it,  thread torque used is given for a dry thread UNLESS a lubricant is specified.
If you add a lubricant to this equation it requires you to use LESS measured torque to achieve the same clamping force, but the load on each thread goes upwards toward failure. Torque is just measuring friction and has to be qualified for the friction in place.  Change the friction,  and you change the measurement of torque as it's rotary and specifically doesn't measure the clamping force.

It follows that if you use oil and maximum torque specified,  then you'll come closer to the materials capabilites to resist failure.

The general interpretation would be to reduce torque by 10 to 15% indicated by figures I can find. 

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